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With a planisphere, or star wheel, you turn a disk to set your time against your date. The edge of the star map then represents the horizon all around you at that time. Some planispheres come with extra features. David Kennedal's Precision Planet and Star Locator, shown here, includes settings to build in corrections for daylight saving time and your longitude, as well as a marker on the sky map that can be dialed to any right ascension and declination. The most important aspect of a planisphere, however, is the clarity and realism of its star map. Among the many devices on the market, the Precision Planet and Star Locator and The Night Sky by David Chandler take the prize in this regard. All Sky & Telescope photographs with this article are by Chuck Baker.

THE MOVEMENTS of the stars have taxed the human intellect  throughout the ages -- from ancient Babylonians seeking to predict sky events, to Greek philosophers wrestling with the structure of the universe, to beginning amateurs today trying to point a new telescope at the Andromeda Galaxy.  

At first, the turning of the celestial sphere perplexes everyone who takes up skywatching. Sooner or later the picture snaps into place and the whole setup becomes obvious. But those who think the sky's motion is inherently simple should try explaining to a beginner why every star follows a different curved path across the sky at a different speed. And why do some stars move from west to east while most move east to west? Can you explain why some constellations turn somersaults during the night while others just tilt from side to side? 

To bring the sky's motion down to Earth, astronomers for millenniums have built little mechanisms that duplicate it. A working model not only illustrates how the sky turns but can help locate objects at any given time. The simplest sky model is a planisphere.

Untold numbers of these star finders have been designed and published in the last century. Even the most experienced observers rely on them, especially at unfamiliar hours of the night. The word "planisphere" simply means flat sphere. It incorporates a map of the sky that pivots at the celestial pole. As the map revolves around the pivot, it slides under a mask that represents your horizon. Turning the map mimics the apparent daily motion of the sky, complete with risings and settings at the horizon edges.

The basic idea was used in ancient Rome. The architect and engineer Vitruvius, writing around 27 B.C., described a star map engraved on a solid plate and a horizon mask that rotated over it to show the risings and settings of celestial bodies. A water clock turned the mask once a day to keep up with the sky. Nearly two centuries later, Claudius Ptolemy analyzed the map projections used for such devices in his treatise Planisphaerium.

By the 4th century A.D. a version known as the planispheric astrolabe was in use. Its star map was a skeletal metal framework sliding over a solid plate engraved with the observer's horizon. Medieval Arabs and Persians refined the astrolabe to a peak of versatility and beauty. Some of these ornate "mathematical jewels" made their way to Europe, where they were prized as almost magical. "All the conclusions that have been found, or might be found in so noble an instrument as an astrolabe, are not known perfectly to any mortal man in this region," wrote Geoffrey Chaucer in 1391. By the end of the Middle Ages astrolabes were the universal trademark of astronomers and astrologers. 

Modern planispheres are direct descendants of the astrolabe, such as this one made in Nuremberg, Germany, in 1532. The ornate scrollwork supports 27 points that form a rudimentary star map; each point is labeled with the name of a star or constellation. On the plate under them are lines marking the local horizon, altitude, and azimuth. The sky is portrayed backward, right for left.

Astrolabes were commonly used to sight on the Sun and stars to tell time. The invention of accurate clocks allowed the procedure to be reversed. If you knew the time, you could use this kind of device to find stars. And that is how planispheres have been employed ever since. 

Using a Planisphere
In principle nothing could be simpler. You turn a wheel to put your time next to your date, and presto, there's a custom-made map of the stars that are above your horizon for that moment. The edge of the oval star map represents the horizon all around you, as you would see if you were standing in an open field and turned around in a complete circle. The part of the map at the oval's center represents the sky overhead -- much like the all-sky constellation map in the center of each month's Sky & Telescope.

In practice, several complications can throw beginners off. The worst is that a planisphere's map is necessarily small and distorted. It compresses the entire celestial hemisphere above and around you into a little thing you hold in your hand. So star patterns appear much bigger in real life than on the map.

Moving your eyes just a little way across the map corresponds to swinging your gaze across a huge sweep of sky. The east and west horizons may look close together on a planisphere, but of course when east is in front of you west is behind your back. Glancing from the map's edge to center corresponds to craning your gaze from horizontal to straight up.

There's only one way to get to know a map like this. Hold it out in front of you as you face the horizon. Twist it around so the map edge labeled with the direction you're facing is down. The correct horizon on the map will now appear horizontal and match the horizon in front of you. Now you can compare stars above the horizon on the map with those you're facing in the sky.

Then there's the distortion issue. On a planisphere designed for use in the Northern Hemisphere, constellations in the southern part of the sky are stretched sideways, taffy-like, making it hard to compare them with real star patterns. This problem does not exist on a well-designed map for fixed dates and times, such as the one in the center of each month's Sky & Telescope. Some planisphere designers have come up with a partial solution. David Chandler's planisphere The Night Sky presents two maps, one on each side. One minimizes distortion north of the celestial equator, the other south of it. Just flip it over for the best view.

A further complication is that a planisphere works correctly for only one latitude on Earth. Most today are made in several editions, each for a particular latitude. 

Then there's the matter of daylight saving time. When this is in effect (from the first Sunday in April to the last Sunday in October in most parts of the United States), remember to "fall back" to standard time by subtracting an hour from what your clock says before you set the planisphere's dial.

Actually, planispheres don't employ standard time either, but rather local mean time. The difference, which depends on where you live in your time zone, can amount to a half hour or more. Instructions for finding your local mean time correction are in the text following the Skygazer's Almanac on page 76 of the January 1997 issue of Sky & Telescope. (available separately as a reprint.) Fortunately, even a half hour one way or the other doesn't really matter for most star finding. 

In fact, if you just want to know which constellations are up and where they are, a planisphere's limitations can largely be overlooked. It's remarkable that such a simple working model of the sky can work so well.

Alan MacRobert is an Associate Editor of Sky & Telescope magazine and an avid backyard astronomer.

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The most common equipment hassle that observers face at night is water on the telescope, which comes as a surprise to newcomers who expect things to stay dry in clear weather. Unfortunately, the steadiest, sharpest telescopic views are often had under precisely the atmospheric conditions that cause dew to form. At the eyepiece you first notice dim stars and galaxies becoming harder to see, then bright stars develop fuzzy halos -- and a check with the flashlight reveals wet haze coating the optics. In severe cases the whole telescope may be soaked. Wiping never helps; more water condenses the moment you stop. At this point many observers pack up, defeated.

However, you can keep your lenses and mirrors crystal clear in even the heaviest dewing conditions. You just need to understand the enemy and take effective countermeasures.

Dew does not "fall" from the sky. It condenses from the surrounding air onto any object that's colder than the air's dew point. The dew point, often mentioned in weather broadcasts, depends on both temperature and humidity. When the humidity is 100 percent, the dew point is the same as the air temperature. At lower humidity, the dew point is below the air temperature. If it's below freezing, you get frost instead of liquid water.

An example of dew physics occurs when you take a bottle out of the refrigerator. If the bottle is colder than the air's dew point, it drips with condensation. Your telescope is the bottle.

"But my telescope can't get colder than the air!" a new Schmidt-Cassegrain owner once told me. "It was warmer than the air when I brought it outdoors. The Second Law of Thermodynamics says that can't happen!"

If only life were so simple. Objects do try to come to the same temperature as their environment and then stay there, as the Second Law says. But they don't exchange heat just with the air around them. They also exchange heat with objects at a distance by radiation. That's why the Sun can feel warm on your skin even though it's 93 million miles away. At night the heat flow goes in the opposite direction. The effective temperature of the dark night sky is just a few degrees above
absolute zero, and a telescope in an open field is exposed to a whole celestial hemisphere of this cosmic chill.

The first line of defense against dew, therefore, is to shield your optics from as much exposure to the night sky as is feasible. The traditional dewcap extending beyond a refractor's lens often serves this purpose well enough to keep the lens dry. The longer the dewcap, the more likely it is to work. One of the nice things about a Newtonian reflector is that its entire tube acts as a dewcap to shield the mirror in the bottom. An open-tube reflector, however, needs a cloth shroud around its open framework to gain this benefit. The cloth itself, of course, will get wet on its sky-facing side.

The worst dew problems appear on exposed parts that are thin (or have low heat capacity) and rapidly radiate away their warmth. Schmidt-Cassegrain corrector plates are notorious for dewing; so are Telrad sights with their exposed glass. A dew shield is reportedly the first accessory that Schmidt-Cassegrain owners most often come back to buy.

You can easily make your own. A piece of tough 5/8" foam rubber, the kind sold in sporting-goods stores to go under sleeping bags, makes a dew shield that's cheap, durable, and very lightweight. The foam is an excellent insulator, for maximum effectiveness. If you're concerned that the cap might vignette the image (block some starlight near the edges of the field of view), you can cut the foam so it flares open at a very slight angle. A 3° opening angle should allow a 3° unvignetted field of view.

As a rule of thumb, a dewcap should be at least 1½ times as long as the aperture is wide. A side benefit is that the cap also cuts down on stray light getting into the telescope.

Eyepieces too are prone to dewing. Warm radiation from your face slows the process, but humidity from your eyeball and breath speeds it up. A tall rubber eyecup, the kind that extends above the eye lens all around, not only blocks stray light while you're observing but acts as a miniature dewcap when you're looking away.

The same principle works on large scales. Early on a clear morning, have you noticed grass in the middle of a field white with frost or dew while grass near a tree has none? The tree is a giant dewcap, and it can work for you too. If you'll be looking at only one part of the sky, it's nice to have trees around and behind you. Not just your telescope but your charts and accessories will stay dry longer.

Trees also reduce wind problems, but a slight breeze is a good thing. Radiational cooling is slow and inefficient compared to heat transfer with the surrounding air, so even the mildest breeze will keep your telescope nearly up to air temperature.

Then there's the observing umbrella, not a widely known accessory but one that works. A beach umbrella blocks the chill of absolute zero the same way it blocks the heat of the Sun. It can help shield all your gear and keep the chill off you too. On a still night a thermometer under an umbrella can read more than 10° Fahrenheit higher than when it is exposed to the open sky.

The Heat is On
There will be times and places where none of this is enough. You then have no choice but to warm your optics, usually electrically.

A 120-volt hair dryer, used gently from a distance so it doesn't overheat the glass and warp it, will blow off dew for perhaps five minutes. Then you have to use it again. And again. A 12-volt auto windshield defogger gun is somewhat less effective. A better way is to apply a little heat continuously. Heated dewcaps that run off a 12-volt battery are available (see "The Kendrick Dew Remover System" for a review of one such system). Or with just a little electrical know-how you can make an antidew heater to any size, shape, and specification you want. Here are the details.

Warmed optics can have unexpected benefits. Dew works its first subtle evils before you notice anything. The late Walter Scott Houston used electric warmers on both the objective and eyepiece holder of his 4" refractor. When he turned off the power, the telescope could lose a whole magnitude of light grasp before the objective actually looked dewy.

"Even on nights when dewing is not noticeable," Houston wrote, "the star images seem better with the heaters on than without them!" This may be because, contrary to what you might think, gentle heating keeps a telescope close to the temperature of the surrounding air. After all, the whole idea is to stop it from growing colder than the air.

Not-So-Cold Storage
The most destructive dewing happens when a telescope is in storage. No telescope should be closed up and put away until it is thoroughly dry. Water with nowhere to escape, or condensation that forms and evaporates repeatedly in a sealed environment over months and years, may attack optical coatings and ultimately etch the glass itself.

How, you may ask, does water get into an airtight space that was dry when you sealed it? The answer is it was there all along. Air contains water vapor, and if your telescope gets colder than what the dew point was when the air was sealed in, water will condense. This is why so many puzzled telescope owners discover water stains on the inside surfaces of their corrector plates and refractor lenses.

Several approaches can prevent this. Don't move a sealed telescope from warm to cold storage. In fact, sealing may be a bad idea altogether. The best telescope covering is cloth, which will "breathe." It keeps dust off but lets water vapor out. And you might want to leave the eyepiece holder covered only with cloth, just enough to keep dust and spiders out.

The worst problems occur when a warm front of humid air blows in after cold weather, as often happens in early spring.  Everything cold gets drenched. A cloth wrap may be the best defense here too; it will greatly reduce the amount of humid air that can flow over cold parts.

The usual advice is to store a telescope at the outdoor temperature to minimize tube currents when you set it up. But this old rule may need modification. Keeping the telescope a little warmer will tend to thwart condensation. An enclosed porch or attached garage may provide the extra few degrees you need. And really long-term storage should probably be inside your living space. Never leave a telescope in a damp basement or garage or, as a rule of thumb, any place where tools grow rusty.

You can take active countermeasures too. A 4- or 7-watt light bulb inserted into a blanketed telescope makes a nice low-power heater. Position it just below or right next to the glass, or else you may merely drive off water from other parts of the tube that will condense onto the cold optics. Running the bulb continuously will cost about a dollar per watt per year. You might turn it on only in the damp season, or attach it to a humidistat switch.

Silica gel desiccant will dehumidify the air in a tightly sealed enclosure. I keep a ¾-pound bag in plastic webbing attached to the inside of one of the tube caps of my 12.5" reflector. Every month or two when the bag's indicator slip turns from blue to pink, I heat the bag in a toaster oven in my observatory to drive off the collected moisture. The more tightly you seal your tube or storage case, the less often you'll have to do this. Silica gel is available from many sources.

Water can be an insidious enemy for astronomers, but a little knowledge will keep it permanently at bay.

Alan MacRobert is an Associate Editor of Sky & Telescope magazine and an avid backyard astronomer.

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THE DEEP BLUE sky of a frigid late afternoon in winter sets an astronomer's pulse to running -- or so it always has mine. Night comes early. The arctic air shows no sign of haze or humidity, promising the darkest skies of the year. Studding the icy blackness will be such bright riches as Orion, Canis Major, Gemini, Auriga, Perseus, and Cassiopeia.

And yet I hear amateurs say their scopes are "in storage;" that this is the season one reads about astronomy rather than practices it. Do these people shiver too hard to keep a steady eye? Do they think Orion can be viewed only through the pain of frozen fingers and toes? In fact you can enjoy winter nights comfortably for hours on end if you dress properly and heed a few cold-weather tips that everyone should know.

Clothing
The first principle of cold-weather dressing is to trap layers of warm air near your body. Studies by the U.S. Army have found that "dead air space," air held in place by tiny fibers, is the only effective body insulator. It doesn't really matter what the fibers are, whether thrift-shop cotton, finest goose down, or exotic synthetics -- only how many inches you put on.

Of course some insulators are lighter than others, per inch of dead air space provided, and have other desirable properties. Vigorous hikers and skiers need light, flexible materials that wick perspiration away from the skin so it can evaporate without leaving a clammy, cold feeling. Special winter outfits are designed for these needs. Skywatching, on the other hand, is hardly athletic. So you can do fine by piling on layers of ordinary clothes that are already around the house.

What matters is how you wear them. Many thin layers are often better than a single thick one. Remember, you want to trap air. The outermost layer should be windproof to keep cold air out. It should also have elastics or ties to close off the waist, sleeves, and the face of a parka hood.

The second principle is to cover your whole body evenly. Three sweaters and a down parka won't keep you warm if there's nothing on your legs but blue jeans. Long underwear and an extra pair of pants -- perhaps heavy wool hunter's pants or insulated snow overalls -- are just as important as a coat. Pajama pants make good "long underwear." Two pairs of pajama pants are better. Your neck and head are major areas of heat loss, so a thick, warm hat and scarf or a thick parka hood are essential.

Where different items of clothing meet at ankles and wrists, prevent bare spots by interleaving the layers. Pull your inner socks up over the legs of long underwear, your pants down over the socks, and your outer socks up over the pants. Whenever it's mildly chilly it helps just to tuck your pant cuffs into the tops of your socks to keep cold air from blowing up your legs.

The third principle is to protect your extremities. Fingers, ears, toes, and nose freeze first. Good footgear is crucial. Your boots should be heavily insulated, but since you won't be scrambling up rockslides they needn't be rugged. Many observers swear by the large, puffy snow boots ("Moon boots") used by snowmobilers. Much warmth is lost from the feet to the ground by conduction through the soles of ordinary boots, so an insulated bottom liner or insole will help. Boots should be large enough to allow you to wear heavy wool socks over your regular socks without any feeling of tightness. Circulation to hands and feet must be kept completely free; anything that feels tight will soon feel frozen.

Protecting fingers is a problem because you have to manipulate eyepieces, charts, pencils (pens freeze up), and so on. One strategy is to wear thin skier's gloves inside loose, more heavily insulated mittens. The mittens can come off briefly as needed. My little finger is always the first to turn painful unless I keep it in the same finger of a glove as my ring finger; then it's no problem. Better alternatives are shooter's mittens with flap-covered slots that allow you to stick your fingers out. You might make cuts in the fingertips of an old pair of gloves.

A ski mask with holes for your eyes and mouth protects the face, if you don't mind looking like a terrorist. Don't use the kind with no mouth hole; your humid breath will come out the eyeholes and fog the eyepiece. 

Since you'll be standing still, dress for 20° to 30° Fahrenheit colder than the actual temperature. Studies for Canada's National Research Council indicate that this is the clothing-requirement difference between walking briskly (what most people normally do outdoors in winter) and standing still for long periods.

Eat, Drink, and Act Merry
You can prolong your time in the cold by eating a good meal beforehand and by nibbling carbohydrates, which raise blood sugar and provide heat energy. Hard candy is convenient, but too many sweets can cause a sudden jump in blood sugar followed by an equally abrupt crash. A sandwich gives a steadier lift.

A thermos of hot coffee may feel comforting, but caffeine restricts circulation in the extremities. So does tobacco. A thermos of hot cider or other sweet drink will be better for you. Avoid alcohol; it not only reduces night vision but makes you lose heat by dilating capillaries in the skin.

Once any part of you gets cold, warming it is very hard without an external heat source. So as soon as something begins to feel chilled, run in place for a while or do some jumping jacks. You produce several times more heat during mild exercise than at rest, and good circulation will carry this heat all the way to your toes and fingertips.

Elderly and very thin people have lower metabolism (production of body heat) and are especially vulnerable to cold. Women produce less heat on average than men. People with good muscle tone generate more, even at rest. Vigorous exercise raises anyone's metabolism for up to six hours afterward, so late afternoon or early evening would be a good time for a workout. Beware of exhaustion, however, which leaves you prone to rapid chilling.

A little-known cause of chills, headaches, and ill feeling in winter is dehydration. You lose a lot of water breathing dry winter air, while cold depresses the thirst mechanism so you don't drink enough. When the body runs low on water it conserves fluid by reducing circulation to the extremities, which means your hands and feet freeze quicker. Guzzle water before going out under the stars.

Safety
Cold can kill. If you observe from a remote, lonely site in winter, think through the entire chain of events that will happen if your car won't start. Have you told someone where to come looking for you if you don't show up by breakfast? How will you keep warm until then?

Car batteries lose power in the cold. Even if you normally run your equipment off the car battery and have enough juice left to start the engine, don't assume you can do this in unusually low temperatures. If in doubt, run the engine for five or 10 minutes per hour while observing. This is a good reason to power your equipment from a separate 12-volt battery -- which in an emergency might recharge the car battery enough to get you started.

Any car that is driven in cold rural areas should carry certain emergency items under the seat: extra old sweaters or blankets, an extra hat or two, hard candy, matches, and candles. A candle in a car will provide much warmth if you huddle over it, but open the window a crack. If the engine willstart but the car won't move, check the gas gauge. Conserve gasoline by running the engine and heater only 10 minutes or so out of each half hour or hour. If the area is snowy, check that the tailpipe is clear so exhaust won't be trapped under the car. Police report that exhaust poisoning is a major cause of death for motorists whose cars get stranded in deep snow.

No matter how thirsty you get, never eat snow when in danger from cold. Snow requires so much body heat to melt that rapid hypothermia can result. Instead, melt it in a hubcap over your candle. Times like these make a cellular phone or mobile ham radio look mighty good.

Another piece of advice from cold-weather state police: don't leave your car. It provides by far the driest, most windproof, most comfortable shelter you could possibly devise in the wilds. It's also highly visible. Poor judgment leading to gross stupidity is a classic effect of oncoming hypothermia. When things look bad it would be very easy to walk a half mile down the road from your car at 3 a.m. in hopes of finding the house that you think maybe you passed, become disoriented, and forget to turn around. If you stay in your car, sooner or later -- whether in hours or days -- someone will always come.

Such emergencies, of course, are unlikely. With a little planning and common sense, winter nights under the stars will be as pleasant as any -- as well as darker and more exciting.

Alan MacRobert is an Associate Editor of Sky & Telescope magazine and an avid backyard astronomer.

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Atmospheric dispersion, distortion, absorption, reddening, and refraction seen all at once. The limb of this setting Sun is distorted into horizontal ridges by layers of different-temperature air. A fragmentary "green fringe" floats on top, the result of dispersion placing the blue and  green images of an astronomical object slightly higher than the yellow and red. The Sun is flattened to an oval shape by the greater atmospheric refraction at the horizon, where the thick air also absorbs and reddens the light more. Marc J. Coco of Redondo Beach, California, used a Celestron 8" telescope for this photograph on Kodachrome 64 film.

NASA SPENT $2.1 BILLION to escape from poor atmospheric seeing; that's what it cost to put the Hubble Space Telescope above the atmosphere. Backyard observers on a smaller budget, however, need not despair of improving their fuzzy, shimmering views. You can avoid the worst effects of atmospheric turbulence by understanding its nature and learning a few tricks.

Viewed at high power from the bottom of our ocean of air, a star is a living thing. It jumps, quivers, and ripples tirelessly, or swells into a ball of steady fuzz. Rare is the night (at most sites) when any telescope, no matter how large its aperture or perfect its optics, can resolve details finer than 1 arcsecond. More typical at ordinary locations is 2- or 3-arcsecond seeing, or worse.

It's not hard to understand why. The usual definition of a "good" telescope is one that keeps all parts of a light wave entering it nicely squared up to within ¼ wavelength accuracy by the time the wave comes to focus. But that same light wave, in traversing just six feet of air inside a telescope tube, is retarded by about 800 wavelengths compared to where it would be if the telescope contained a vacuum. Clearly the air is an important optical element, and it had better affect every part of a light wave equally. If the refractive power of the air down one part of the telescope tube differs from the rest by more than just one part in 3,200, the ¼-wave tolerance will be breached. Such a change results from a temperature difference of just 0.1° Celsius.

Add the miles of air that the light wave traverses before it even gets to the telescope, and it's a wonder that we can see any detail beyond the atmosphere at all.

The air's light-bending power, or refractive index, depends on its density and therefore its temperature. Wherever air masses with different temperatures meet, the boundary layer between them breaks up into swirling ripples and eddies that act as weak lenses. You can see this where hot air from a fire or a sunbaked road mixes with cooler air; the heat waves are astronomers' poor seeing writ large. Our windy, weather-ridden atmosphere is almost always full of slight temperature irregularities, and when you look through a telescope you see their effect magnified.

Much of the problem, however, arises surprisingly close to the telescope, where you can take control of it to reduce it.

Inside the Scope
Seeing problems are often at their worst a fraction of an inch from the objective lens or mirror. If the objective is not at air temperature, it will surround itself with a wavy, irregular, slowly shifting envelope of air slightly warmer or colder than the ambient night. So will every other telescope part. Therefore, give the telescope time to come to equilibrium with its surroundings. Amateurs soon learn that the view sharpens within about a half hour after bringing a telescope outdoors. The full cool-down time for a large, heavy instrument may be much longer. It pays to set up early.

Usually the telescope is too warm, especially if it is stored indoors to prevent destructive dampness from condensing inside it during weather changes. But sometimes the opposite happens. Whenever a telescope begins to collect dew or frost, you know that it has grown colder than the air via radiational cooling. In this case gentle heat not only prevents dew but also keeps the scope closer to the air temperature -- and thus may sharpen its resolution.

"Tube currents" of warm and cool air in a telescope are real performance killers. Reflectors are notorious for tube currents, but closed-tube Schmidt-Cassegrains and refractors can get them too. Any open-ended tube, amateurs these days tend to agree, should be ventilated as well as possible. Suspending a fan behind a reflector's mirror has become a popular way to speed cooling and blow out mixed-temperature air.

It's easy to check whether tube currents trouble your images. Turn a very bright star far out of focus until it's a big, uniform disk of light. Tube currents will show as thin lines of light and shadow slowly looping and curling across the bright disk.

Near the Scope
Some seeing problems arise just a few feet in front of the telescope. Obviously, try to keep your breath and body heat out of the light path. This is one reason to put a cloth shroud around an open-framework tube.

A telescope's immediate surroundings should have low heat capacity so they don't store up the warmth of the day. Grass and shrubbery are better than pavement. The flatter and more uniform the greenery the better. Heated buildings are disasters of poor seeing, especially if you find yourself looking over a chimney.

If you build an observatory, make it of thin materials that cool quickly: plywood or sheet metal, not masonry. Paint it white or a very light color to reflect solar heat, and ventilate it very well. A thick rug belongs on the floor. A roll-off roof that opens the whole room to the sky provides quicker cooling and better seeing than a dome with its chimneylike slit. If you insist on a dome, it's a good idea to install a large fan in one wall to suck air down through the slit past the telescope, just as professional observatories do. It's widely considered a poor idea to attach an observatory to a heated house unless you resign yourself to low-power work. At least put it on the upwind side.

Much poor seeing hugs the ground, so an elevated observing platform is a good idea if you can manage it. A scope is likely to show the stars and planets more sharply if you can get it up just a few feet closer to them.

High-Altitude Seeing
Now we come to the unavoidable heart of the problem. There's not much you can do about the air thousands of feet up. But you may be able to predict when and where it will be smoothest.

Telescope users recognize two types of seeing: "slow" and "fast." Slow seeing makes stars and planets wiggle and wobble; fast seeing turns them into hazy balls that hardly move. You can look right through slow seeing to see sharp details as they dance around, because the eye does a wonderful job of following a moving object. But fast seeing outraces the eye's response time.

An old piece of amateur folklore is that you can judge the seeing with the naked eye by checking how much stars twinkle. This often really does work. Most of the turbulence responsible for twinkling originates fairly near the ground, as does much poor seeing. But high-altitude fast seeing escapes this test. If the star is scintillating faster than your eye's time resolution (about 0.1 second), it will appear to shine steadily even if a telescope shows it as a hazy fuzzball. 

Astronomers often talk of "seeing cells," air-eddy lenses ranging in size from millimeters to a few meters wide that swarm through the sky. These eddies originate wherever air masses rub past each other -- either horizontally in winds, vertically by convection, or both. Sometimes, when watching an extended object like the Moon or a planet, you can focus on a horizontal layer of "shear turbulence" a few thousand feet high. The ripples sharpen up when you turn the focuser slightly to the outside of infinity focus (eyepiece farther from the objective). This is the signature of an inversion layer in which a mass of warm air flows across cooler air below. The actual temperature difference may be very slight.

Large or slow-moving eddies cause slow seeing, but they don't stay large forever. No matter what size the eddies are when they originate, they break up into smaller and smaller ones. When these become as small as roughly millimeter-scale, they finally die out and dissipate their energy as heat via the air's fluid friction (viscosity).

This complex situation belies an often-repeated piece of astronomer's lore: that seeing cells are 10 centimeters (4 inches) in size. In fact they come in all sizes. But cells in this middle range do have an important property: they affect a large telescope more seriously than a small one. If you have a 4" scope, 4" and larger cells passing through its line of sight will make an image shift around while staying relatively intact. The same cells passing in front of a 12" aperture will superpose multiple images at once, making a fuzzy mess.

This fact has led to another piece of folklore: that when the seeing is bad, a large telescope shows less detail than a small one. Therefore, supposedly, you can improve the view by stopping down a large aperture with a cardboard mask.

Technically there is a bit of truth in this, as mathematical analysis of seeing has shown, but in practical terms the improvement is slight to nonexistent. I have never seen an improvement by stopping down a telescope when the problem was poor seeing. The most that can usually be said is that on a really rotten night, large- and small-aperture views will be equally poor. Even then, if you constrict the aperture you miss the chance for the momentary high-resolution views that the full aperture will provide if the air briefly steadies.

There are unrelated reasons why you may indeed see more in a stopped-down telescope, most of them bad. Maybe your eye was dazzled by a too-bright planet; in that case an eyepiece filter would solve the problem better. Maybe the aperture stop is masking optical errors in a flawed objective, or maybe it's just allowing a mediocre eyepiece to perform better by increasing the telescope's f/ratio. Poor collimation is also less damaging when the f/ratio is increased. On a reflector or Schmidt-Cassegrain, an off-axis mask does give you the advantage of a clear aperture. A clear aperture, mathematical analyses have shown, is slightly less affected by atmospheric turbulence than an obstructed one.

In Search of Steady Air
The seeing quality depends on the weather, but not by simple rules that apply everywhere. Poor seeing does seem more likely shortly before or after a change in the weather, in partial cloudiness, in wind, and in unseasonable cold. Any weather pattern that brings shearing air masses into your sky is bad news. Good seeing, some observers claim, is most likely when a high-pressure system settles in to bring clear skies for several days running. Keep a seeing-versus-weather log for your locality, and you may discover correlations that will become your key to sharp viewing.

Seasonal patterns are more predictable. The seeing is often mediocre in the cold months over the northern United States and southern Canada, when the high-altitude jet stream flows above these latitudes. The very best seeing often comes on still, muggy summer nights when the air is heavy with humidity and the sky looks unpromisingly milky with haze. Some astronomers claim that a blanket of industrial smog steadies the air as effectively as summer humidity -- or rather that it accompanies the same tranquil air masses that are conducive to fine seeing.

Time of night also plays a role, but again there are few universal rules. Right after sunset the seeing is apt to be excellent, so start your planetary observing as soon as you can find a planet in twilight. The seeing is apt to deteriorate before dusk fades out. Some observers find that their seeing improves after midnight; others say it goes to pieces. This depends largely on local topography; observers in valleys might get worse seeing as the night goes on and cold air pools in the valley. Late dawn may be another excellent time.

For observing the Sun (use an astronomer's solar filter!), the best time is early morning before the Sun heats the landscape. The very worst seeing of the 24-hour cycle comes in the afternoon.

Geography is critical. Smooth, laminar airflow is the ideal sought by observatory siting committees worldwide. The best sites on Earth are mountaintops facing into prevailing winds that have crossed thousands of miles of flat, cool ocean. You don't want to be downwind of a mountain; the airstream breaks up into turbulent swirls after crossing the peak. Nor do you want to be downwind of varied terrain that absorbs solar heat differently from one spot to the next. Flat, uniform plains or gently rolling hills extending far upwind can be almost as good as an ocean for providing laminar airflow. You may learn to predict which wind direction brings you the smoothest air.

One easy countermeasure when observing bright objects like the Moon and planets is to use a color filter. Different colors seem to shimmer out of phase with each other in the seeing (the reason stars twinkle in colors), and in a telescope this contributes to the general fuzzing up. The blue image of a planet may align with the yellow image one instant and separate from it the next. If you isolate just the yellow light, for instance, the planet will often appear to quiet down noticeably, at least in a small aperture.

A color filter is especially useful when you're aiming at altitudes lower than 45° above the horizon. The seeing is always worse at low altitudes because you're looking through more air. In addition, you face more atmospheric dispersion. This is the smearing out of a celestial image into a short spectrum with blue on top and red on the bottom. Even as high as 60° up, the blue component of an image appears 0.9" above the red. The difference is 1.5" at 45°, 2.5" at 30°, and 5" at 15°. Your eye is fairly insensitive to light at the extreme ends of the spectrum, so dispersion really doesn't look quite as bad as this. Still, filtering out all but one color in a swarm of chromatic aberration will sharpen your view. In the summer of 1994 I found a yellow or orange filter invaluable for following the comet-crash spots on Jupiter as the planet sank each evening toward the horizon.

Atmospheric problems get worse the lower you look. A star 15° above the horizon will be enlarged twice as much by atmospheric turbulence as one at the zenith, regardless of whether the seeing is good (defined here as 1" star images overhead) or poor (4"). Atmospheric dispersion elongates a star into a colorful little spectrum; at very low altitudes this overtakes even poor seeing as a cause of blurry images. Courtesy Andrew T. Young.

Mostly, though, beating the seeing is just a matter of patience. Keep watching, and intermittent good moments may surprise you. One reason why experienced observers see more on the planets than beginners is that they simply watch longer, ignoring all but the steadiest moments. Moreover, the seeing can change as radically from minute to minute as it does from second to second. When that perfect minute comes along, the dedicated observer is the one most likely to be there at the eyepiece to catch it.

A Scale of Seeing
Amateurs have long recorded the seeing quality in their observing logbooks on a rather subjective scale of 1 to 10, with 1 hopeless and 10 perfect. People's ideas of what the numbers mean are likely to differ. In the interest of uniformity, here is the scale in its early form as described by Harvard Observatory's William H. Pickering (1858-1938). Pickering used a 5" refractor. His comments about diffraction disks and rings will have to be modified for larger or smaller instruments, but they're a starting point:

1. Star image is usually about twice the diameter of the third diffraction ring if the ring could be seen; star image 13" in diameter.

2. Image occasionally twice the diameter of the third ring (13").

3. Image about the same diameter as the third ring (6.7"), and brighter at the center.

4. The central Airy diffraction disk often visible; arcs of diffraction rings sometimes seen on brighter stars.

5. Airy disk always visible; arcs frequently seen on brighter stars.

6. Airy disk always visible; short arcs constantly seen.

7. Disk sometimes sharply defined; diffraction rings seen as long arcs or complete circles.

8. Disk always sharply defined; rings seen as long arcs or complete circles, but always in motion.

9. The inner diffraction ring is stationary. Outer rings momentarily stationary.

10. The complete diffraction pattern is stationary. 

On this scale 1 to 3 is considered very bad, 4 to 5 poor, 6 to 7 good, and 8 to 10 excellent.

Alan MacRobert is an Associate Editor of Sky & Telescope magazine and an avid backyard astronomer.

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Light pollution is the bane of amateur astronomy -- but you can see a lot even through the worst of it. Refusing to be defeated, John Starr observes Jupiter over the Teapot star pattern of Sagittarius from his roof in Los Angeles. (Photo by John Starr)

For amateur astronomers these are the best of times and the worst of times. Never have such large and sophisticated telescopes and such powerful accessories been so readily available at moderate prices. Never has so much celestial information been available at the flip of a page or click of a mouse. But never have so many people lived under such awful skies. Many Sky & Telescope readers can follow the motion of Pluto in 3000 A.D. on a screen, but cannot step outdoors and find Polaris through the light pollution.

This paradox will grow ever more extreme as equipment improves and dark skies retreat. The future of amateur astronomy is, perhaps, a microcosm of the rest of the world's future: better technology in a poorer environment.

Which of these two trends will outrun the other and become dominant is anyone's guess, in amateur astronomy as in the larger world. One result, however, is already becoming clear. The environment is public -- but equipment ownership is private. The stars belong to everyone, but access to them is becoming privatized. Many once-common celestial sights already require expensive instruments or the money and time to travel to distant, unspoiled locations.

No matter what the future holds, however, some observers will never let anything stop them.

These are people who set up telescopes in city lots and observe with blankets draped over their heads to block streetlights, while keeping an ear out for muggers. These are people who spend a year examining bleary star images through an apartment window and come away with a sheaf of variable-star light curves. These are people who time the instants when stars are occulted by skyscraper walls and determine the rate of precession of the Earth's axis.

"Normal" observers who have (or travel to) decent skies tend to regard such enthusiasts as crankish inhabitants of an unimportant amateur-astronomy backwater. They are wrong. As the world grows more densely populated, urbanized, and brightly lit, city observers are the vanguard exploring trails to our future.

Rooms With A View...
Years ago I discovered the unexpected possibilities of city observing after moving into downtown New Haven, Connecticut. I assumed that skywatching would cease to be part of my life. But our garret apartment had a plastic bubble skylight over the kitchen sink, and one night, just for laughs, I tried looking through it with 7×50 binoculars. Amazingly, I could make out some stars.

Observing anything under these conditions seemed so remarkable that I did some experimenting. I arranged a way to stand on a stool with my head in the bubble surrounded by light shields. Using a variable-star chart for Mira published in Sky & Telescope, I discovered that my limiting magnitude with the binoculars was as faint as magnitude 8.6. I even spotted Mira near its minimum light.

Star images were distorted by the plastic bubble, it was true, but they were there. Surely lots could be done with a limiting magnitude this faint! In the following months I explored the binocular sky more carefully than I ever had before. I would pick a small area of sky and research everything about it -- the distances and spectral types of stars, interesting objects to try for -- and draw little maps. I set up a writing board in the bubble and arranged to rest comfortably there. It was my own little world, with my feet on the stool and my head in the stars. I followed the monthly pulsations of T Monocerotis, the unpredictable quiverings of Y Tauri, and the nightly creep of asteroids. Binocular double stars could be surveyed at leisure, and I spent long periods mapping everything I could see in Orion's Sword. I identified scores of features on the Moon.

Unexpected benefits began to appear. Restrictions on your observing impose discipline; rather than aimless sightseeing, I had to do desk work with maps and catalogs beforehand to develop good projects. This turned out to be the key to rewarding astronomy. The inside surface of the bubble proved to be a rock-solid "mount" for the binoculars when they were pressed against it face-on. The beautifully steady views made up for the plastic's poor optical quality and the gray film of pollution coating its outside. And it was a new experience to be in shirt-sleeve warmth examining Orion, while an icy winter wind screamed by inches away.

The lesson was clear. Desk work, steady optics, and comfort make for fine sky-watching in the worst environment.

If so much can be done in the city with binoculars, a telescope offers much wider possibilities. The Moon and planets show every bit as clearly to the urban astronomer as the rural one. True, a city is full of heat sources that can cause atmospheric turbulence and degrade the seeing. But often the city haze actually seems to steady the view.

Jim Phillips of Charleston, South Carolina, was one of many city observers who answered a request in Sky & Telescope for their stories. Phillips dedicates his observing time to the Moon and planets. "Quite honestly," he writes, "I am amazed at how many nights of good seeing I get from my 'middle of the city' location." He uses a custom-built 8" f/13 refractor in a roll-off-roof observatory behind his house. "I realized long ago that I was likely always to live in or near a city, and, after great thought, concluded that rather than a portable telescope I could take beyond the lights, I would prefer an observatory in a light-polluted area. To me the advantages of having an observatory with charts, books, and catalogs handy, and the ability to begin observing within minutes, far outweigh the light pollution and partially obstructed sky.

"I have split doubles at or near the theoretical limit of my telescope. Detail on Jupiter is excellent, as is detail on the lunar surface." Phillips ended up taking over the revived Lunar Dome Survey of the Association of Lunar and Planetary Observers.

It doesn't take such a powerful telescope to overcome viewing problems. Robert W. Bethune of Grand Rapids, Michigan, used a 3 1/2" Questar for nearly two years "from the heart of Seoul, Korea, a city of many millions with both smog and light pollution. The compound where I lived was heavily lit, with several streetlights nearby plus bright lights on the walkways and in windows." Nevertheless, he writes, "the Moon was of constant interest. This past summer offered excellent opportunities to study the Jovian and Saturnian systems. There are a number of variable stars, double stars, and even nebulae that can be observed with practice through heavy pollution. My greatest success was timing the bright-limb occultation of an 8th-magnitude star during the recent Pleiades passage."

Bethune offers several tips: "One can be aggressive about certain lights. I dealt with offending sidewalk lights by simply opening them up and loosening the bulbs, remembering of course to restore them afterwards.

"It helps to retire late and rise early. Even in the biggest of cities, things slow down in the dead of night. Planning ahead also helps. One becomes skilled at digging into sky calendars and planispheres for events that will take place within one's limited hunk of sky. The most important lesson is to accept the limits of the situation, after careful experiment and investigation have revealed what they are."

Pointing a telescope out a window is supposed to be utterly taboo for anyone who aspires to the title of amateur astronomer. Temperature differences between outdoors and indoors are supposed to destroy the steadiness of the seeing. This is certainly true at times -- but not always, as Michael Boschat of Halifax, Nova Scotia, discovered. "For planetary work I stick my 3" refractor out the front window, and at times the images are so steady I can use 200 power. I did so for Mars last summer and saw markings even though the planet was low in the sky over a neighbor's roof."

Then there's always the Sun. Sol Steinberg is a retiree living in a garden apartment complex near Camden, New Jersey. "My windows face east, into the 'garden' surrounded by 11 buildings." Using an inexpensive 3" reflector on a tabletop mount, he has photographed the Moon and followed the satellites of Jupiter and phases of Venus. "The Sun has become my regular morning subject," he writes. "The 55-mm eyepiece projects a bright image on my bedroom ceiling, but nothing equals direct viewing with a filter." He began following the comings and goings of sunspots. "A new group appeared yesterday, and this morning two of the spots have clearly become arcs -- a new and exciting phenomenon to me."

...And the Deep Sky Too
Nebulae and galaxies, with their low surface brightnesses, are hit hardest by light pollution. Therefore many urbanites give up on them before really trying. One who refused to be defeated was Jenny Worsnopp near New York's Lower East Side. "Manhattan is the worst any amateur astronomer has to live with," she wrote. "But my great love is deep-sky objects. What to do?" The answer was to go out and give it her best shot.  

From her apartment's roof Worsnopp had an open view of the sky as well as such luminous spectacles as the Empire State, Chrysler, and Citicorp buildings. The last throws a brilliant fan of light toward the zenith as some architect's twisted notion of art. Nevertheless Worsnopp logged 46 of the 110 Messier objects from her roof using a 6" f/8 reflector. "The open clusters and bright globulars are visible even in the early nighttime," she wrote. Many buildings turn off their decorative lights at midnight, "and after that, the sky is ours. Sort of.

"I recently got a pleasant surprise. I went to the roof at 12:30 a.m. and found the galaxies M104, M66, and M81 -- and, amazingly, M97," a large, dim planetary nebula that can be difficult under any circumstance. "I used an OIII nebula filter and averted vision."

Such feats do require practice and skill, not to mention lengthy star-hopping from a naked-eye starting point that may be tens of degrees away. "City observing," Worsnopp comments, "is good training for those exotic objects that we all want to see from better sites. City observers don't look in their finders and see the Messiers glowing, waiting to be centered; we have to find the spot exactly, so it makes us good map readers. I guess my feeling is that if I can see it here, you, no matter where you are, can see it too."

Worsnopp's tally is surpassed by that of David H. Frydman of London, England. "I have observed from cities for 30 years," he writes. Using a refractor "with a 5" f/5 Jaegers objective, I have observed 350 of 600 deep-sky objects that I know are possible (excluding double stars), most of them many times." He offers a number of pointers:

"There is a window with a radius of 25° from the zenith where faint objects are well seen even in London pollution. Every effort should be made to observe within this window, or at as high an elevation as possible.

"Exclude as much local light as you can. Observe after 11 p.m., choose the most shielded site, and, if necessary, put an open box over your head and telescope. 

"The best conditions are after rain and in high winds, as clear country air is blown over the city. During gales I have seen the Veil Nebula, the Owl Nebula M97 including its dark 'eyes,' and enormous detail in M33. Normally the first two are quite invisible in the 5" refractor."

Highly detailed charts, Frydman stresses, are essential. "You see one or even two magnitudes fainter if you know exactly where an object is and keep waiting until it comes into view."

The visibility of a deep-sky object in light pollution depends much less on its total magnitude than its surface brightness. So when looking through catalogs for promising targets, seek those that combine brightness with small size.

Many tiny planetary nebulae have quite high surface brightnesses. Alister Ling published a list of 60 "planetary nebulae visible from the city" in the now-defunct Deep Sky magazine (Summer 1986 issue). Many of them are practically starlike and require very high power to resolve -- not to mention excellent charts to identify them in the first place. Ling gives several tips for distinguishing them from stars. One method turns bad seeing to an advantage: a tiny planetary twinkles less than a star, for the same reason a planet twinkles less (both have appreciable disks). Another tip-off is an unstarlike greenish or bluish tint.

The best "city planetaries" in Ling's list that are larger than 15 arcseconds across and brighter than magnitude 10.0 are not widely known: NGC 1535 in Eridanus, NGC 3242 in Hydra, and NGC 6826 in Cygnus. Clearly this is a big open field to explore.

A more recent article on observing high-surface-brightness planetary nebulae in the city was published by Donald R. Ferguson in the April 1995 Sky & Telescope, page 96. Ferguson includes a list of 18 of these objects that he collected with a 3.5" telescope from within "the vast urban sprawl of Houston."

Nebula Filters
City and suburban observers gained a new claim to the deep sky when nebula filters were developed in the late 1970s. These function on a straightforward principle. Emission nebulae give off light at narrow wavelengths that differ from those of sodium- and mercury-vapor streetlights. By using a multilayer interference filter, the spectrum of visible light can be cut finely enough to separate these wavelengths. The result is a much darker sky, somewhat dimmer stars and galaxies, and only slightly dimmer planetary and emission nebulae. This enhanced contrast can, in many circumstances, more than make up for the relatively small amount of light lost from the nebula, and so it stands out more clearly.

These filters do not bring country skies to the city, but they do help. One technique for detecting nebulae, especially tiny planetaries, is "blinking" with the filter. Hold it at the eye and move it rapidly in and out of the line of sight; a nebula will blink relative to the surrounding stars. Alternatively, blinking can be done by tilting the filter back and forth while looking through it, since it loses its effectiveness when at an angle.

Several nebula (or "light pollution") filter designs are available. They use somewhat different strategies for different types of objects and conditions. A detailed review of them is in the July 1995 Sky & Telescope. 

The CCD
The biggest promise that technology holds out -- for those who can afford it in both money and time -- is the CCD camera. By 2000, CCD (charge-coupled device) cameras had taken over and vastly expanded high-end amateur astronomy, and their prices are declining every year. A CCD camera has two enormous strengths. First, the CCD chip is many times more sensitive to light than either your eye or photographic film. Second, it feeds a digitally recorded image from the telescope directly into your computer, where the image can be enhanced, analyzed, measured, and manipulated.

The most important manipulation is the ability to subtract away an extremely light-polluted background, as if by magic, with hardly any loss of data. An 8" telescope can now record 15th- or even 16th-magnitude stars in the worst city light pollution or moonlight. This is several times fainter than the same telescope can show stars to the eye under black, mountaintop conditions!

Drawbacks to CCDs include the very small field of view, the difficulty of aiming this field where you want, and problems of focusing. The equipment may be temperamental; the telescope mounting must be as rigid and controllable as for long-exposure astrophotography. And, of course, you're looking at a computer screen, not stars. It has been said that CCD astronomy is about working with equipment and computers, not skygazing.

The most important advance that CCDs represent is the science that can be done with the recorded images. For much of the 19th century, amateurs were almost on a par with professional astronomers in terms of the useful science they could do. Then amateurs fell very, very far behind -- but now CCD cameras in dedicated hands are making up some of this lost ground. Amateurs are discovering asteroids in great numbers, performing professional-quality variable-star studies, detecting the 19th-magnitude optical afterglows of gamma-ray bursts near the limits of the observable universe, taking spectra of stars and galaxies, imaging the planets more finely than was once thought possible, and much more.

No machine, however, will ever replace the simplicity and delight of examining the stars directly, as a part of living nature.

Duck and Cover
"Light pollution" is the glow in the sky itself. It should not be confused with local lights that shine directly into the observer's eyes.

Local lights are more aggravating but easier to defeat. Many observers have cooperative neighbors who turn off outdoor lights on request. A good way to break the ice on this issue is to offer views through your telescope.

If you can't observe in the shade of trees or walls, you might rig a tarpaulin to shield your site. Max Wyssbrod lives in Lucerne, Switzerland, which he calls "the brightest country in Europe." His "cloth observatory" consists of four aluminum poles 10 feet long that fit into tubes cemented into the ground in a 10-foot square. The four walls are black cloth; guy ropes add stability. The whole rig, along with an 8" Schmidt-Cassegrain telescope, takes 15 minutes to set up.

Another strategy is to shield only your eye and the back end of the telescope. An old-fashioned photographer's black cloth or equivalent, or a cape that can be thrown up over your head, does the trick. Any telescope in bright local lights should also have a long dewcap or side shield to keep the light out of the tube. Eyepieces should have rubber eyecups.

"I use a black hood, and blinders I made from cardboard fitted to each side of my face," writes Charles Haun of Morristown, Tennessee. "This works quite well."

Hiding under cloth and wearing blinders may seem an ignominious way to experience the glories of the cosmos. But such is the garb that amateur astronomers shall increasingly wear as they march bravely into the future.

Alan MacRobert is an Associate Editor of Sky & Telescope magazine and an avid backyard astronomer.

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The Stellar Magnitude System By Alan MacRobert

Most ways of counting and measuring things work logically. When the thing you're measuring increases, the number gets bigger. When you gain weight, the scale doesn't tell you a smaller number of kilograms or pounds. But things are not so sensible in astronomy, at least not when it comes to the brightnesses of stars.

Star magnitudes do count backward, the result of an ancient fluke that seemed like a good idea at the time. Since then the history of the magnitude scale is, like so much else in astronomy, the history of increasing scientific precision being built on an ungainly historical foundation that was too deeply rooted for anyone to bulldoze it and start fresh.

The story begins around 129 B.C., when the Greek astronomer Hipparchus produced the first well-known star catalog. Hipparchus ranked his stars in a simple way. He called the brightest ones "of the first magnitude," simply meaning "the biggest." Stars not so bright he called "of the second magnitude," second biggest. The faintest stars he could see he called "of the sixth magnitude." This system was copied by Claudius Ptolemy in his own list of stars around A.D. 140. Sometimes Ptolemy added the words "greater" or "smaller" to distinguish between stars within a magnitude class. Ptolemy's works remained the basic astronomy texts for the next 1,400 years, so everyone used the system of first to sixth magnitudes. It worked just fine.

Galileo forced the first change. On turning his newly made telescopes to the sky, Galileo discovered that stars existed that were fainter than Ptolemy's sixth magnitude. "Indeed, with the glass you will detect below stars of the sixth magnitude such a crowd of others that escape natural sight that it is hardly believable," he exulted in his 1610 tract, Sidereus Nuncius. "The largest of these...we may designate as of the seventh magnitude...." Thus did a new term enter the astronomical language, and the magnitude scale became open-ended. Now there could be no turning back. 

As telescopes got bigger and better, astronomers kept adding more magnitudes to the bottom of the scale. Today a pair of 50-millimeter binoculars will show stars of about 9th magnitude, a 6" amateur telescope will reach to 13th, and the Hubble Space Telescope has seen objects as faint as 30th magnitude. 

By the middle of the 19th century astronomers realized there was a pressing need to define the entire magnitude scale, both telescopic and naked-eye, more precisely than by eyeball judgment. They had already determined that a 1st-magnitude star shines with about 100 times the light of a 6th-magnitude star. Accordingly, in 1856 the Oxford astronomer Norman R. Pogson proposed that a difference of five magnitudes be defined as a brightness ratio of exactly 100 to 1. This convenient rule was quickly adopted. One magnitude thus corresponds to a brightness difference of exactly the fifth root of 100, or very close to 2.512 -- a value known as the Pogson ratio.

The resulting magnitude scale is logarithmic, in neat agreement with the 1850s belief that all human senses are logarithmic in their response to stimuli. (The decibel scale for rating loudness was likewise made logarithmic.) Alas, it's not quite so, not for brightness, sound, or anything else. Our perceptions of the world follow power-law curves, not logarithmic ones. Thus a star of magnitude 3.0 does not in fact look exactly halfway in brightness between 2.0 and 4.0. It looks a little fainter than that. The star that looks halfway between 2.0 and 4.0 will be about magnitude 2.8. The wider the magnitude gap, the greater this discrepancy. Accordingly, Sky & Telescope's computer-drawn sky maps use star dots that are sized according to a power-law relation (see the March 1990 issue, page 311).

But the scientific world in the 1850s was gaga for logarithms, so now they are locked into the magnitude system as firmly as Hipparchus's backward numbering.

Now that star magnitudes were ranked on a precise scale, however ill-fitting a one, another problem became unavoidable. Some "1st-magnitude" stars were a whole lot brighter than others. Astronomers had no choice but to extend the scale out to brighter values as well as faint ones. Thus Rigel, Capella, Arcturus, and Vega are magnitude 0 -- an awkward statement that might sound like they have no brightness at all. But it was too late to start over. The magnitude scale extends farther down into negative numbers: Sirius shines at magnitude -1.5, Venus reaches -4.4, the full Moon is about -12.5, and the Sun blazes at magnitude -26.7.

Other Colors, Other Magnitudes
By the late 19th century astronomers were using photography to record the sky and measure star brightnesses, and a new problem cropped up. Some stars having the same brightness to the eye showed different brightnesses on film, and vice versa. Compared to the eye, photographic emulsions were more sensitive to blue light and less so to red light.

Accordingly, two separate scales were devised. Visual magnitude, or mvis, described how a star looked to the eye. Photographic magnitude, or mpg, referred to star images on blue-sensitive black-and-white film. These are now abbreviated mv and mp.

This complication turned out to be a blessing in disguise. The difference between photographic and visual magnitudes was a convenient measure of a star's color. The difference between the two kinds of magnitude was named the "color index." Its value is increasingly positive for yellow, orange, and red stars, and negative for blue ones.

But different photographic emulsions have different spectral responses! And people's eyes differ too. For one thing, your eye lenses turn yellow with age; old people see the world through yellow filters (S&T: September 1991, page 254). Magnitude systems designed for different wavelength ranges had to be more firmly grounded than this.

Today, precise magnitudes are specified by what a standard photoelectric photometer sees through standard color filters. Several photometric systems have been devised; the most familiar is called UBV after the three filters most commonly used. U encompasses the near-ultraviolet, B is blue, and V corresponds fairly closely to the old visual magnitude; its wide peak is in the yellow-green band, where the eye is most sensitive.

Color index is now defined as the B magnitude minus the V magnitude. A pure white star has a B-V of about 0.2, our yellow Sun is 0.63, orange-red Betelgeuse is 1.85, and the bluest star believed possible is -0.4, pale blue-white (see "The Truth About Star Colors," S&T: September 1992, page 266).

So successful was the UBV system that it was extended redward with R and I filters to define standard red and near-infrared magnitudes. Hence it is sometimes called UBVRI. Infrared astronomers have carried it to still longer wavelengths, picking up alphabetically after I to define the J, K, L, M, N, and Q bands (S&T: June 1995, page 23). These were chosen to match the wavelengths of infrared "windows" in the atmosphere where absorption by water vapor does not entirely block the view.

Appearance and Reality
What, then, is an object's real brightness? How much total energy is it sending to us at all wavelengths combined, visible and invisible?

The answer is called the bolometric magnitude, mbol, because total radiation was once measured with a device called a bolometer. The bolometric magnitude has been called the God's-eye view of an object's true luster. Astrophysicists value it as the true measure of energy emission as seen from the location of Earth. The bolometric correction tells how much brighter the bolometric magnitude is than the V magnitude. Its value is always negative, because any star or object emits at least some radiation outside the visual range. 

Up to now we've been dealing only with apparent magnitudes -- how bright things look from Earth. We don't know how intrinsically bright an object is until we also take its distance into account. Thus astronomers created the absolute magnitude scale. An object's absolute magnitude is simply how bright it would appear if placed at a standard distance of 10 parsecs (32.6 light-years).

Seen from this distance, the Sun would shine at an unimpressive visual magnitude 4.85. Rigel would blaze at a dazzling -8, nearly as bright as the quarter Moon. The red dwarf Proxima Centauri, the closest star to the solar system, would appear to be magnitude 15.6, the tiniest little glimmer visible in a 16" telescope! Knowing absolute magnitudes makes plain how vastly diverse are the objects that we casually lump together under the single word "star." 

Absolute magnitudes are always written with a capital M, apparent magnitudes with a lower-case m. Any type of apparent magnitude -- photographic, bolometric, or whatever -- can be converted to absolute.

Lastly, for comets and asteroids a very different "absolute magnitude" is used. It tells how bright they would appear to an observer standing on the Sun if the object were one astronomical unit away.

So, are magnitudes too complicated? Not at all. They're as simple as they can be considering their historical roots and what they have to describe today. Hipparchus would be enchanted.

Alan MacRobert is an Associate Editor of Sky & Telescope magazine and an avid backyard astronomer.

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The Earth is at the center of the celestial sphere, an imaginary surface infinitely far away on which the planets, stars, and galaxies seem to be printed. On the celestial sphere, lines of right ascension and declination correspond to longitude and latitude on Earth. When a telescope's right-ascension axis is lined up with the Earth's axis, as shown here, the telescope can turn on it to follow the apparent rotation of the sky.

NEWCOMERS to astronomy can get thrown for a loop when they first encounter declination and right ascension. Why are the positions of stars that are light-years away in the depths of space stated in a system that's tied to latitude and longitude on Earth?

The celestial coordinate system, which serves modern astronomy so well, is firmly grounded in the faulty world-view of the ancients. They believed the Earth was motionless and at the center of creation. The sky, they thought, was exactly what it looks like: a hollow hemisphere arching over the Earth like a great dome. The stars? "They're fireflies," explains Timón in The Lion King, "stuck to that big, uh, blue-black thing up there." 

The celestial dome with its starry decorations had to be a complete celestial sphere, early skywatchers figured out, because we never see a bottom rim as the dome tilts up and rotates around the Earth once a day. Parts of the celestial sphere are always setting behind the western horizon, while other parts are rising in the east. At any time half of the celestial sphere is above the horizon, half below.

Even today this is how the cosmic setup actually looks. Never mind that we're on a moving dust mote orbiting a star in the fringe of a galaxy. In astronomy, appearances and reality are more different than in any other area of human experience. Perhaps for this reason, astronomers are quite comfortable living with both -- as long as the two are kept in their proper relationship. The celestial sphere, with its infinitely large radius, appears to turn daily around our motionless Earth, from which we use telescopes to examine wonders painted on its inside surface. The illustration here presents the scene. 

From Earth to Sky
Whenever you want to specify a point on the surface of a sphere, you'll probably use what geometers call spherical coordinates. In the case of Earth, these are named latitude and longitude.

Imagine the lines of latitude and longitude ballooning outward from the Earth and printing themselves on the inside of the sky sphere. They are now called, respectively, declination and right ascension.

Directly out from the Earth's equator, 0° latitude, is the celestial equator, 0° declination. If you stand on the Earth's equator, the celestial equator passes overhead.  

Stand on the North Pole, latitude 90° N, and overhead will be the north celestial pole, declination +90°.

At any other latitude -- let's say Kansas City at 39° N -- the corresponding declination line crosses your zenith: in this case declination +39°. (By custom, declinations north and south of the equator are called + and - rather than N and S.) This is the declination of the bright star Vega. So once a day, Vega passes overhead as seen from the latitude of Kansas City.

Hours and Degrees
Of course Vega doesn't move; it's the Earth that's turning. But we're talking appearances here. The celestial sphere seems to rotate around our motionless world once in about 24 hours.

This daily motion is the basis of the numbering system used in right ascension. Instead of counting in degrees, as with longitude around the Earth, right ascension is usually counted in hours, from 0 to 24 around the sky. This is just a different way of putting dividing marks on a circle. One hour in this scheme is 1/24 of a circle, or 15°.

The benefit of this numbering system is that as the Earth rotates, you see the sky turn by about 1 hour of right ascension for each hour of time. This makes it easy to figure out when celestial objects will come in and out of view. The stars become a giant 24-hour clock. 

Since ancient Babylonia, people have divided both degrees and hours into finer units by means of base-60 arithmetic. In 1° there are 60 arcminutes, written 60'. One arcminute contains 60 arcseconds, written 60". A good telescope in good sky conditions can resolve details about as fine as 1" on the surface of the celestial sphere. By comparison, 1" of latitude on Earth is about 101 feet. So if you had a telescope at the center of a transparent Earth, you could resolve details about the size of a house lot up on the surface.

Because declination is given in degrees, fine gradations of it are usually expressed in the Babylonian system of arcminutes and arcseconds. For instance, Vega's exact declination (2000.0 coordinates) is +38° 47' 01".

Hours of right ascension are divided into minutes and seconds of time, not of arc. In one hour (1h) are, naturally enough, 60 minutes, written 60m. In one minute of right ascension are 60 seconds, written 60s. Vega's right ascension is 18h 36m 56.3s.

Notice the different notation for the different kinds of minutes and seconds. They're truly different. Just as 1h contains 15°, so does 1m contain 15' and 1s contain 15".

Starting Points
Any spherical coordinate system comes with a natural, built-in zero value for its "latitude" coordinate, whether it is called latitude, declination, or something else. This reference marker is the equator. No other latitude line is like it.

But there's no such natural zero point for counting longitude -- in the sky's case right ascension. All lines of longitude or right ascension are alike. So a zero point has to be picked arbitrarily. On Earth, 0° longitude has long been defined as a line engraved on a brass plate set in the floor under a position-measuring telescope at the Old Royal Observatory in Greenwich, England. In the sky, 0h right ascension is defined as where the plane of the Earth's orbit around the Sun (the ecliptic) crosses the celestial equator in Pisces. This point is called, for historical reasons, the First Point of Aries.

Precession
The First Point of Aries really was in Aries when it was named roughly 2,000 years ago. It has crept into the stars of Pisces because of precession, a slow shift in the orientation of the Earth's axis with respect to the rest of the universe.

Put a spinning top at an angle on a table and it too will precess. Its spin axis will slowly circle around the upward direction of the force that the table applies to the point of the top. In exactly the same manner, the spinning Earth slowly precesses because of the force that the tidal gravitational tugs of the Moon and Sun apply to the Earth's slight equatorial bulge.

Hence we see the north celestial pole, which is currently located close to Polaris, swing across the stars in a wide loop around the north ecliptic pole every 26,000 years. The moving celestial pole drags the whole celestial-coordinate system -- the whole grid of declination and right ascension -- along with it.

Contrary to popular belief, precession does not shift the Earth's axis with respect to the Earth's own geography. The terrestrial North Pole doesn't move to a new location (at least not much on the time scale we're talking about). Precession won't give walruses a tropical suntan. The only noticeable changes are those that result from the grid of celestial coordinates moving against the stars. In 12,000 years, for instance, Vega will be the north star, and Orion will be a constellation of summer, not winter.

Because the coordinate grid insists on sliding around this way, a star's right ascension and declination are continually changing. To fix a star's position you need to specify the date for which a right ascension and declination apply. The current standard is "equinox 2000.0," shorthand for "right ascension and declination at the moment the year 2000 begins." The previous standard, still encountered on some star charts, was 1950.0.

For moving objects such as the Sun, Moon, and planets, right ascension and declination are often given for the "equinox of date": that is, correct for the actual date listed. In Sky & Telescope's monthly table of Sun and planet positions near the center of each issue, positions are given in the coordinate system for each date listed.

Rarely, however, do backyard astronomers need to worry about precession. From 1950 to 2000 the coordinate grid creeps along the ecliptic by only 0.7°, less than the width of the lowest-power view in many telescopes. And that amount applies only at the ecliptic itself. The total shift is less elsewhere, declining to essentially zero at the ecliptic poles.

Which way does precession go? It makes a star's right ascension increase each year. That is, an old right-ascension value precedes the newer value in amount as well as date.

As for right ascension itself, just remember that it increases to the east. If you get confused about which way is east on a star map that shows right ascension, this little mnemonic will get you squared away.

Alan MacRobert is an Associate Editor of Sky & Telescope magazine and an avid backyard astronomer.

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The Names of Deep-Sky Objects By Brian Skiff

Look through any issue of Sky & Telescope and you'll find a wide variety of seemingly incomprehensible names for celestial objects: Arp 220, MWC 560, QSO 0957+561, 1E 1740.7-2942, 3C 273, PSR 1257+12. Some apply only to galaxies, others to nebulae, pulsars, quasars, star clusters, radio emitters, X-ray sources -- everything in the astrophysical zoo.

Luckily, backyard astronomers with ordinary telescopes needn't memorize the names of hundreds of authors, acronyms for space-borne experiments, or codes for sources at invisible wavelengths. Celestial objects that are easily seen by eye were, naturally enough, the first ones cataloged. These early lists were fairly simple, so the names of the brightest and most interesting objects are fairly easy to keep track of. These designations are also steeped in history, and this serves as a way of remembering them, just as constellation patterns served for thousands of years as mnemonic devices for storytellers. 

Deep-sky objects such as star clusters and nebulae began to draw attention as soon as telescopes were pointed to the night sky. Among Galileo's early accomplishments was resolving the Praesepe cluster for the first time, showing that what had previously been regarded as a little cloud was a cluster of dozens of stars. Small lists of other deep-sky objects were compiled by several observers in the 17th and early 18th centuries. But it wasn't until the late 18th century that the French astronomer Charles Messier began tallying them in substantial numbers, publishing several lists that were to become the now-familiar catalog bearing his name.

Messier (pronounced "MESS-yay") was a comet hunter, and the main purpose of his list was to provide himself and others with a roster of cometlike objects to ignore. Although Messier discovered a large portion of those in his list, many had already been found by others (sometimes unbeknown to him), especially by his colleague Pierre Méchain, whom he duly credited.

Many people have wondered why Messier numbered such big, obvious objects as the Pleiades (dubbed Messier 45, or M45) and the Orion Nebula (M42). These were already well known -- surely they could not have been mistaken for comets! But as Arizona deep-sky observer Steve Coe reminded me once, people back then sometimes comet-hunted by naked eye. When the Pleiades rise low in the east at dawn in May, one can easily imagine mistaking them for a bright comet nearly hidden in the growing daylight.

But Messier certainly included some of his new "nebulae" (as practically everything nonstellar was called back then) just because they were new. So one wonders why the Double Cluster in Perseus and a few other very bright objects were left out of his collection.

Although the Messier catalog is 200 years old it is still the most commonly used list, simply because it contains most bright nonstellar objects in the northern two-thirds of the sky. It also contains a sampling of each major class of visual object. Practically every amateur who becomes interested in viewing clusters and galaxies gets started by working from the Messier list. All 103 of the M objects (or 107, 109, or 110, depending on which later additions you accept) can be seen in a 6" telescope even under suburban skies. Some observers have bagged all the Messier objects with a 2.4" refractor, and from a very dark site they're all visible in 8 x 50 binoculars.

The NGC and Beyond
The next big catalog to appear that remains in wide use today was the NGC . The New General Catalogue of Nebulae and Clusters of Stars by John L. E. Dreyer (pronounced "Dryer") appeared in 1888. It is a compendium of all the lists of nonstellar objects compiled by the many 19th-century observers who had been ransacking the sky. Chief among these lists was John Herschel's monumental "General Catalogue of Nebulae" published in 1864, which is why Dreyer called his own general catalog "new." The NGC contains 7,840 objects of many types, numbered in order of equinox-1860 right ascension.

As discoveries kept pouring in, Dreyer published two supplements to the NGC in 1895 and 1908 titled the Index Catalogues, abbreviated IC. They brought the total to 13,226. These three lists should really be considered a single work. They include nearly every extended (non-pointlike) telescopic object beyond the solar system that is visible with, say, an 8- to 12" telescope from a backyard observing site with slight to moderate light pollution. Almost any NGC object can be detected with a sharp 12" telescope working from a first-class observing site. The NGC includes hundreds of objects that are as bright as the fainter half of the Messier catalog, so for small apertures it fills in virtually all the blanks. 

About half the IC objects were discovered visually, but much of the second IC catalog (objects numbered IC 1530 and up) consists of photographic discoveries. These are quite faint or have very low surface brightnesses, making them difficult or impossible to see visually. There are notable exceptions, such as the open cluster Messier 25 in Sagittarius, which despite being a naked-eye object was not assigned a designation by Dreyer until the second Index Catalogue; it's IC 4725. A lovely big open cluster for binoculars in Ophiuchus is missing from both the M and NGC lists; Dreyer finally tabulated it as IC 4665. Inexplicably, the Pleiades never received an NGC or IC number.

As sky photography improved, Maximilian Wolf and others produced long lists of additional photographic discoveries. Some included hundreds of very faint objects found on a single plate -- some of them now known to be emulsion flaws rather than anything celestial. By the time of the Index Catalogues it was clear that the number of "nebulae" (galaxies) was skyrocketing, which is probably why Dreyer quit cataloging and spent his later years concentrating on his other love, the history of astronomy.

He picked a good stopping place. It's a tribute to the usefulness and quality of Dreyer's lists that the abbreviations NGC and IC remain ubiquitous in professional literature a century later, after almost every other aspect of the science has changed beyond recognition. Long out of print, the NGC and IC were reissued with modern coordinates in a newly edited edition by Roger W. Sinnott titled NGC 2000.0 (Sky Publishing Corp., 1988).

Huge numbers of objects, galaxies in particular, continued to be recorded and cataloged in works such as the southern photographic surveys by Harvard Observatory. For amateurs who push beyond the limits of the NGC and IC, the next designation usually encountered is UGC -- for the Uppsala General Catalogue of Galaxies by Peter Nilson (Uppsala Astronomical Observatory, 1973). It includes the 12,940 brightest galaxies north of declination -2½° (1950.0). Delving still deeper, one soon meets the Master List of Nonstellar Optical Astronomical Objects (MOL) by Robert S. Dixon and George Sonneborn (Ohio State University Press, 1980). This huge, diverse compendium packs 185,000 brief listings from 270 catalogs into a single volume.

Babels of Designations
Everything grows more complex with time. As astronomy has expanded, many objects have acquired an abundance of different names. 

One way a familiar entity acquires a new name is to be included in a list of special objects. Thus the galaxy Arp 220, mentioned at the start of this article, is IC 4553 and IC 4554. Because of its bizarre shape (it's probably two spirals colliding and merging), Halton Arp included it in his Atlas of Peculiar Galaxies published in 1966. I prefer the older designation, however, since the pair were first reported by S. Javelle in 1903.

More designations get added when an object is detected at wavelengths outside the visible. One of the brightest galaxies in the Virgo cluster is Messier 87 (NGC 4486), but it is also known as Virgo A, 3C 274, 1ES 1228+126, 87GB 122819.0+124029, and IRAS F12282+1240. These are among the 20 names of this one galaxy listed in the NASA Extragalactic Database (NED), a bibliographic computer catalog maintained by the Jet Propulsion Laboratory.   

"Virgo A" dates from the early days of radio astronomy when the resolution of radio telescopes was so poor that a source's location couldn't be determined much better than to within a whole constellation. The name 3C 274 comes from the "Third Cambridge" catalog of radio sources compiled using the great dish at Jodrell Bank, England, and published in 1959. Like Messier's optical list, these early radio surveys corralled most of the best objects. So even though better radio catalogs are now available, the brightest quasar in the sky (optically a 13th-magnitude "star" in Virgo) will forever be known as 3C 273.

The last three names mentioned above are built from celestial coordinates, in this case equinox-1950 right ascension and declination. Look again at 1ES 1228+126. The two blocks of numbers mean right ascension 12h 28m, declination +12.6°. Positional names of this sort are useful and nearly inexhaustible but quite cumbersome.

"1ES" is the name of a 1992 catalog of sources detected at X-ray wavelengths by the Einstein satellite. The "87GB" catalog resulted from a radio-continuum survey of the northern sky done at Green Bank, the U. S. national radio astronomy facility in West Virginia, and published in 1987. Finally, IRAS was the Infrared Astronomical Satellite, launched in 1983, which made the first far-infrared survey of the sky. M87 showed up as a "faint" source in the IRAS data catalog published in 1990, earning it one more designation. M87, an object Messier called a "nebula without a star," has now been observed across the electromagnetic spectrum from the X-ray (a few angstroms wavelength) to the radio (centimeters and meters wavelength). All kinds of photons are coming out of that galaxy!

Laying Down the Law
This naming business clearly threatens to get out of hand. But the complication is utterly necessary if really large numbers of objects are to be specified unambiguously. The 87GB and IRAS catalogs have around 55,000 and 65,000 entries, respectively. The Hubble Guide Star Catalog has 18,819,291. The Sloan Digital Sky Survey now being developed will measure and tabulate an estimated 50 million galaxies and 200 million stars (Sky & Telescope: October 1992, page 370), each one demanding its own individual identity.

In an effort to manage the mammoth bookkeeping tasks of the future, the International Astronomical Union has urged astronomers to assign names within a single well-defined but fairly flexible system. New names are supposed to have two elements, an "origin" and a "sequence." Optionally a "specifier" may be added too. 

For example, while deep-sky observing a few years ago I came upon what appears to be an unreported open cluster in Auriga near the asterism of the Kids. If I want to name it, I could do so the old-fashioned way: start a list named for myself and call the cluster Skiff 1. This would be appropriate if I were publishing a lengthy table of newly identified clusters. As long as I never publish a different kind of list and nobody else named Skiff does either, there would be no problem calling new clusters Skiff 1, 2, 3, and so on.

Whether these names are justified would ultimately depend on whether they get used. Will other cluster specialists adopt them as the most useful for their needs? If not, I've just added to the confusion.

A better name for Skiff 1 might be BAS J0458.2+4301. "BAS" is the "origin"; it's my initials. One or two letters wouldn't be acceptable; S is already commonly assigned to emission nebulae cataloged by Stewart Sharpless in 1959, and BS is widely used for stars in the Yale Bright Star Catalogue. BAS isn't taken, however, so I could claim it. The "sequence" gives the position instead of just a serial number. The J indicates that the numbers following are right ascension and declination in the J2000 system, the precisely defined coordinate grid for epoch 2000.0. The numbers are given here to a precision of 0.1 minute of right ascension and one arcminute of declination; other degrees of precision are often used. In a similar way I could use B1950 coordinates, or maybe galactic latitude and longitude. When older designations don't specify a coordinate epoch, it's assumed to be for 1950.0.

I can add objects to my collection more or less endlessly as long as no more than one falls in the same square-arc-minute "bin" on the sky defined by the coordinates. (There are about 150 million such bins on the celestial sphere.) In a pinch one could add a "specifier." An interacting pair of galaxies, for instance, might be called BAS J1234.5-3456 (SW) and BAS J1234.5-3456 (NE), indicating the southwest and northeast components of this fictitious object. 

Although a scheme like this is necessary for the multitude of objects inhabiting the sky, I find it soulless and clinical. At a 1984 meeting of star cluster specialists, Berkeley astronomer Ivan King reacted to one presentation by mocking the prospect of identifying even well-known objects with long strings of digits. At one point he told the speaker, "I'm glad to see that you identified M49 as NGC 4472, because, although you are a man of the 18th century, I live in the 19th century myself and prefer NGC numbers." Out alone on a clear night with a telescope, the romance dwelling in those simpler names is far more attractive.

Brian Skiff is a researcher at Lowell Observatory and an avid amateur astronomer.

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Dozing at the telescope, comes a vision: a bare dirt yard both familiar and strange, small now, hemmed in, closely fenced. Beside it a house of intimate memory, lit on all sides by night glare from strange constructions in the middle distance. An old man, his face like time in a mirror, is carrying a load of fabric and poles over his shoulder. A young girl dances around him.

"Gee, Grampa, I was always wanting for you to show me the stars. What's that one up there?"

"That's not a star, Kimu, that's the security balloon. Smile at it so it'll see who you are. Now give me a hand with this, would you?"

"Okay, what is this stuff?"

"My observatory. We'll just put it down here in the middle of the yard ... unfold this part here...stand back.... Okay, you want to yank the rope?"

"Sure, here goes. Wow! Triple cherries!" 

Up leaps a wide, black cylinder. It snaps taut -- a giant top hat swaying and settling on its brim.

"Now then, we just open the flap and step in. Pretty fine, isn't it? Every real amateur has a cocoon -- the old-fashioned outdoor guys like me, I mean. Ant-proof floor. Adjustable any way you want. It's everything but bulletproof. I know the inventor, Steve Kufeld the Third. He's made a million bucks off these things; bought himself a car."

"It's dark in here."

"That's the idea. Can't see any of those big old lights any more, can you?"

"That's what I mean. Is this safe?"

"Darkness never hurt a soul, Kimu. Darkness is the astronomer's friend."

"Can we see real stars from in here?"

"Not yet, but we might. You have to give your eyes time to adapt to the dark. Let's close the top some more too. Now wait in here while I get the scope."

The man emerges from the flap, then disappears into the house with a screen-door slam that hasn't changed in 36 years. In a minute he returns carrying a nest of bars, fabric, and struts on a sturdy-looking equilateral triangle. Resting it on his hip, he ducks through the flap.

"Here's the telescope, Kimu, and it's a sweetie. A 15"er with a mirror you wouldn't believe. Everything built right in. You can't buy 'em this good, you gotta put 'em together yourself."

He sets the device in the middle of the floor. It stands hardly three feet high; a disk of bright gray sky shows reflected in its center. Tiny red lights flare up momentarily on one side of the triangular base; they reveal the old man and child kneeling side by side. The lights dim to pinpoints; motors quietly whir.

"First thing, it's got to figure out where it is. That means it's got to look at a few stars to get oriented. I'll give it some more sky." He pulls cords on the fabric wall; the top of the cylinder irises wide. The machinery clicks, whines, and swings around, then beeps with satisfaction.

"Now, what would you like to see?"

"I want to see real stars to show my class."

"You bet. We'll see lots of them. Ready? M11."

More clicking and whirring. The girl unrolls her computer. Its screen glows dimly gray -- then resolves into a bright spangle of blue, white, and orange dots that lights up her face.

"Oh, it's just like the pictures! What is it?"

"That's Messier 11 in Scutum. They used to call it the Wild Duck Cluster, back when you could hunt ducks and eat them."

"Ewww, gross, how would you know where they'd been. Are you sure this isn't just a picture?"

"Well, Kimu, it's funny you asked. I've got another little surprise here. This gizmo" -- he detaches a long black tube stowed on the triangle's side -- "is something I made myself. It's got a relay-lens doohickey on this end, a stack of special filters, and up here is a real, old-fashioned eyepiece. The 13-millimeter Nagler my dad gave me. Just a minute now." He reaches into the struts and bars, grunting. The computer goes blank. He snaps the tube into the machinery; its end barely emerges at floor level.

"Now get down and have a look in there. Here's the focus button."

Silence from the girl. "They're...they're pretty neat. They're tiny. I've never seen that hi-res a screen."

"That's no screen, that's the real thing. You are looking directly at stars in space."

Long silence. "Wait till I tell everyone. Real stars. I thought they'd be bigger."

"They're supposed to look small. Stars are so far away they appear to be mathematical points, no matter how big they really are. If they look big on the screen, the screen's telling you a damn lie."

"This is what you do in here? Just look at stuff?"

"Some of the time, yeah. I put in my old eyepiece, stretch out, and get comfortable."

"What about those projects you told the school about?"

"Oh, those too. I'm doing one tonight. I've got an astronomy buddy in Poona; we're working on this star together. A 17th-magnitude eclipsing binary; sometimes fades right down to 19th. No one's worked up a proper study of it yet. It must be morning where he is; let's call him. But, uh, let's not say anything about using an eyepiece, okay? That'll be our secret."

The man's weathered face shows for a moment in the light of the telephone's menu. "Petrov, you there? I'm just setting up here."

"Ya, I been running all night," comes a tiny voice. "Urrrgh. I just woke up. And been running the scope on Mindanao since yesterday afternoon, and Jerry let me turn on his rig in Cornwall. You go all night, we'll get 20 hours continuous. You in that stupid tent thing of yours?"

"You bet, and this is my granddaughter Kimu. Say hi, Kimu."

"How you expect to follow a star all night if you gotta sit there tilting a tent hole around? You'll fall asleep."

"Some of us are just weird, Pete. You'll never know." 

"I gotta go. The wife's telling me to put my head under the pillow if I'm gonna talk on the phone."

The girl shifts on the fabric floor. "Does that mean we have to look at the same star all night? Can't we look at some other things too?"

"Sure we can, between frames."

"I think I like the eyepiece best. You're looking right at real stars. I've never seen that."

"Kimu, I remember when it was so dark in this yard you could see the Milky Way from one side of the sky to the other. There were woods, and peepers, and fireflies....

"The Milky Way, that's the galaxy we live in, right? What did it look like?"

"It looked a lot like other galaxies, only you saw it real big and edge-on because we're right inside it. I can show you one that looks the same way. NGC 891."

The telescope whirrs and rearranges itself; the man fiddles with it, then moves to the wall and pulls ropes. The fabric cylinder bends to the northeast. The computer screen fills with starry cloudiness that resolves, more slowly than before, into a pale, mottled band of light split down the middle by a dark, knotted dust lane. The picture gradually continues to sharpen. The two figures press close to the screen. 

"That's what the Milky Way looked like? Right up in the sky?"

"Yes, Margy and I could walk out the back door and see it. And it wasn't a little thing like this. It went all the way from behind the pine trees that used to be there on the hill, way up overhead, and all the way down behind where the towers are. It was immense. And you didn't need anything to see it. You just stepped out and looked. And the stars were everywhere."

The girl gazes at the screen in silence. "The people back then...," she begins. "It must have been like they lived right in outer space, to see that! I want to go into space someday. I've always wanted to go into space. I want to look right out my port and see stars, real stars everywhere." 

Alan MacRobert is an Associate Editor of Sky & Telescope magazine and an avid backyard astronomer.

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EVERY ASTRONOMER is familiar with the artificial skyglow that hangs over populated areas, washing out almost everyone's view of the universe to a greater or lesser degree. In the last two generations, light pollution has spread from a problem in cities to a major astronomical disruption almost everywhere. 

But some aspects of light pollution are not widely appreciated by amateur astronomers. Knowledge is power; here are facts that may help you avoid some of the problem and combat the rest more effectively.

Glare versus skyglow. The most annoying and destructive problem is light that beams directly into your eye from a bright bulb. This is called glare; it comes from fixtures that are poorly designed or improperly aimed, perhaps most of those currently in use. When glare crosses property lines and creates a nuisance, it's called "light trespass." Glare is often the easiest problem to avoid -- by setting up your telescope in a shadowy corner, erecting a tarpaulin to shade the telescope, or negotiating with your neighbors or local government to have the offending light turned off or replaced with a modern one of better design.

Skyglow is what the term "light pollution" properly denotes. The sky has a certain minimum surface brightness even in the most pristine, unspoiled environment. This natural component of skyglow has four sources: faint airglow in the upper atmosphere (a permanent, low-grade aurora), sunlight reflected off interplanetary dust (zodiacal light), starlight scattered in the atmosphere, and background light from faint, unresolved stars and nebulosity. Airglow peaks around the maximum of the 11-year sunspot cycle; the other sources vary with the hour of night and the seasons. But their combined average is well known.

A typical suburban sky today is about 5 to 10 times brighter at the zenith than the natural sky. In city centers the zenith may be 25 or 50 times brighter than the natural background.

Full-cutoff shielding inside light fixtures is the essential remedy for both glare and skyglow. A lamp should send all its light down where the light is intended to be used, not upward or sideways. "Full cutoff" is usually taken to mean that no light rays from the fixture go above the horizon, and that at least 90 percent of the light is blocked in the near-sideways range from 0° to 20° below the horizontal plane.

Light that shines in this near-sideways range contributes nothing to most lighting needs. It is merely a dazzling annoyance in the eyes of people nearby and dissipates uselessly into the distance. Of course, light spilling upward is wasted totally. Tremendous above-the-horizon waste is tolerated because it goes unseen. People who install lights don't normally check them at night from high in the air! The electricity cost of this wasted light has been put at $1 billion to $2 billion annually in the United States.

Near-horizontal light is especially destructive. A light beam aimed straight up is usually not the worst kind. It escapes into space quickly, passing through what astronomers call one "air mass." A ray aimed 10° above the horizon, on the other hand, passes through 5.6 times as much atmosphere -- 5.6 air masses -- polluting all the way. A ray that skims the horizon pollutes up to 10 air masses, though not much of the light is left by the time it goes through the last few of them.

The situation is comparable to atmospheric extinction of starlight arriving in the opposite direction. When a light ray travels straight up through clear air from sea level, only 20 to 30 percent of it is absorbed or scattered by the atmosphere. The rest escapes harmlessly into space. When the same ray is aimed 5° above the horizon, about 90 percent of it is absorbed or scattered. Thus it causes three or four times as much pollution, when all the damage is summed up over an area many miles across. (That, anyway, is the situation in clear air. Aerosols can complicate the picture.)

Add the fact that most lights provide some blockage at high angles, and it becomes clear that most of the light-pollution war will be won or lost in the narrow battleground just a little way above the horizon. At least this is true at sites fairly far from the offending lights -- the semirural areas that seem to have suffered the worst degradation in the last 20 years. Right inside a city, rays at higher angles (and reflected from the ground) are the primary problem. 

Some skyglow is surprisingly local. You can often see more stars 15 miles from a city than a quarter mile from a bad rural shopping center.

I've made extensive sky-brightness measurements of the zenith at two sites in Middletown, Connecticut: at the Van Vleck Observatory on the Wesleyan University campus, and at my home two miles away in wooded, semirural suburbia. The campus had, until recently, a night sky more than 20 times brighter than the natural sky background. The sky over my house is only four to five times brighter than the natural level. The change in two miles was remarkable -- from a nearly invisible Milky Way to views of the Sagittarius and Scutum starclouds on good nights.

In 1994 the university agreed to replace most of its walkway lights within a block of the observatory with properly shielded fixtures. The sky brightness at the zenith dropped by almost half -- a dramatic improvement of 0.6 or 0.7 magnitude. 

Such observations point up the importance of dealing with local lights. You don't have to convert an entire city to see results. Hartford, Connecticut, a metropolitan area of a million people, is only 15 miles north of the Wesleyan campus. Its lights obtrude only marginally. Those of New York City about 90 miles southwest interfere not at all.

Another example appears on the light-pollution map of the Washington, D.C., region made by the Northern Virginia Astronomy Club and published in the June 1996 Sky & Telescope