Tips And Guides

Nearly every telescope on an equatorial mount comes with setting circles. In theory, they show the right ascension and declination to which the telescope is pointed, making it simple to aim at any object whose coordinates you look up. In practice, experienced observers generally regard setting circles as decorations to help sell telescopes, as a source of false hope for beginners, and possibly useful as makeshift frisbees.

We're talking here about traditional mechanical setting circles: rings engraved with lines and numbers on the telescope's two axes. More recent "digital setting circles," electronic readouts that tell where a telescope is pointed, can be vastly more accurate and useful; they're described at the end of this article.

Conventional setting circles are no substitute for learning to find your way around the sky by looking with your eyes. But having absorbed this lesson, many observers scorn their setting circles forever after, even in situations when they might be quite helpful.

The problem is that many adjustments and alignments have to be done very precisely before the circles will display right ascension and declination accurately enough to find objects "blind." Rarely are all of these adjustments made.

But if you have some knowledge of the sky, you can use the circles for less demanding tasks that have looser accuracy requirements. We will discuss this simpler type of use first, then go on to the more exacting applications.

Offsetting from Stars
Their inherent inaccuracies give less trouble if you use setting circles only to measure your way a few degrees across the sky rather than all around the celestial sphere. This is the offsetting method of finding objects from a known star.

This method works even with the oldest-style setting circles that only read hour angle from the celestial meridian instead of right ascension. (These are identified by their 0 to ±12 hour markings that can't be set to anything but 0 when the scope is pointed at the meridian.)

First check that the telescope is polar-aligned moderately well. The polar axis of the mounting should be aimed at the celestial pole to within a couple of degrees. (Instructions for polar alignment come with most equatorial scopes.)

Look up the coordinates of your target object and any fairly bright star within 10° or so of it. Subtract the right ascension and declination of the star from those of the object. The result tells you how far from the star to swing in declination going north (or south if the value is negative), and how far in right ascension going east (or west if negative).

Most setting circles have rulings every 1° in declination and every 5 minutes of time in right ascension. So express your declination offset in degrees and right ascension in minutes. Try to read the declination dial to a tenth of a degree and the right ascension dial to one minute or better.

Offsetting can be very useful if the normal method of finding objects -- star-hopping with the aid of a good map -- isn't working. Perhaps you don't have a map that shows enough stars for you to home in on the exact point. Perhaps your finderscope is too small or the light pollution too bad, or you've repeatedly gotten lost in a difficult field and want to try a new tack.

Offsetting is especially efficient when you plan to survey many objects in a small area of sky. Work out your offsets indoors beforehand, and write them in your observing notebook.

All-Sky Use
If you want to find objects anywhere in the sky by dialing their coordinates, you should understand the many precise adjustments required to your telescope.

Suppose your lowest-power, widest-field eyepiece gives a 1° true field of view, typical of amateur instruments. If the telescope is pointed ½° wrong, your object will be on the edge of the field where it will go unnoticed. Merely to place it closer to the center than to the edge, you have to aim with ¼° accuracy.

What are the adjustments? The axis of the telescope's optical system should be made truly perpendicular to the mount's declination axis. This in turn should be perpendicular to the mount's polar axis. The polar axis must be accurately aligned on the celestial pole. The circles themselves must be positioned just right. Last, you must read the circles accurately -- usually to a small fraction of their finest gradation.

Some of these adjustments have two degrees of freedom, such as in altitude and azimuth when aligning on the celestial pole. So all told, there are eight variables where error can creep in.

Based on the way simple random errors add up, each of these eight adjustments must be good to 0°.09 accuracy to achieve an average total error of 0°.25 in where the telescope is pointed. Half the time the errors will add up to be better than this, half the time worse. To make them fall consistently on the better side, you should strive for even finer accuracy -- say 0°.05 -- in each adjustment.

First, make sure that the optics of the telescope are collimated (aligned) as best you can. Collimation on a reflector is usually just a matter of turning the adjustment screws behind the primary mirror to make a slightly out-of-focus-star image perfectly round when centered. On a Schmidt-Cassegrain telescope, you make tiny adjustments to the screws on the secondary mirror mount. Refractors rarely need collimation. Instructions for collimating a telescope usually come with it.

If you use a star diagonal, such as on a Schmidt-Cassegrain, be sure it too is collimated if it has adjustment screws on its back. Using high power, center the scope on an object while viewing "straight through" without the diagonal. Then insert the diagonal and see if the object is still centered. If it's not, turn the diagonal's adjustment screws until it is.

The reason for getting collimation all squared away first is that when you collimate a telescope, you change its aim point -- that is, the direction of its optical axis with respect to the tube. After you collimate you will have to realign the finderscope to match the main telescope's new aim.

Now swing the tube to about 90° declination. While looking through your lowest power eyepiece, swing the mount back and forth in right ascension by turning the polar axis. You will see the field slowly turning. Make slight adjustments to the declination so the motion of the field is minimized when you turn the scope.

Ideally, you will find a declination position where the stars rotate around the exact center of the field. This happy state of affairs means you have gotten the optical axis truly parallel to the mount's polar axis.

Don't expect it to happen. Instead, you will only be able to find a place where the field motion is minimized, not reduced to zero. The point of sky around which the field appears to rotate will be off to one side, perhaps out of view entirely.

You want to shim the telescope tube in its cradle, or adjust the fork arms if the scope has a fork mount, to bring this point to the center of view. While turning the scope in right ascension, form a mental image of where the field's center of rotation lies. Nudge the scope that way to judge which side of the cradle needs to be shimmed, or which fork arm raised.

You can use strips of brass or plastic or folded-up aluminum foil for shimming. Adjust a fork arm on a Schmidt-Cassegrain scope by loosening the bolts that hold it to the drive base and sliding the arm slightly up or down. (This may be limited by the size of the bolt holes) The adjustment may take quite a bit of trial and error, but it's a job you'll only have to do once.

If your telescope tube can rotate in its cradle (a convenience on many reflectors), you may find you can get closer to the ideal after rotating the tube by some amount. Try this first, then do the shimming. Just remember that in actual use, you may need to rotate the tube back to the position it's in right now before the setting circles will work well. Mark the tube so you can do this if the circles later give problems.

Once you've done the best you can, loosen the declination circle, turn it to read precisely 90°, and retighten it permanently.

Now a confession: we've skipped a step. In the case of a German equatorial mount we haven't checked that the declination axis is perpendicular to the polar axis, and with a fork mount we aren't sure if the optical axis is perpendicular to the declination axis. That's because there is little or nothing you can do about it. Trust the manufacturer and cross your fingers.

The next step is accurate alignment on the celestial pole. Some telescopes come with pole-finding reticles for their finderscopes. Another method that is especially precise is described in the article "Accurate Polar Alignment."

The declination circle need never be touched again. But the right ascension circle does have to be repositioned at the start of each observing session, because the sky is always moving.

Aim at a bright star whose right ascension you know. (It's handy to keep the right ascensions of a dozen bright stars on the inside cover of your observing notebook.) Slide the right ascension circle to read the correct value for that star. On a German equatorial mount, the star should be on the same side of the mount as the objects you'll be looking for.

Now you can dial in the right ascension and declination of any object in the sky. Look in your lowest-power eyepiece, and there it should be.

If your right ascension circle is driven by your telescope's clock drive, as is the case with all Schmidt-Cassegrains we know about and many reflectors, you can dial in object after object all night without touching it again. If the circle is not driven, reposition it to the right ascension of the current object just before swinging to the next.

Technology to the Rescue
New ways have recently been invented to circumvent the problems that make setting circles so error-prone. These methods revolve around the "digital setting circle." In its simplest form, this is nothing more than a readout in little red numbers of what an ordinary setting circle tells you with a dial and pointer. But once this data is electronically encoded, a computer chip can begin to work miracles with it.

In some versions you can simply "initialize" the circles by setting on two or three bright stars at the beginning of a session, and the chip corrects for misalignments of many kinds -- even failure to polar-align at all.

The next step up in sophistication is automatically correcting for lack of perpendicularity in the mount's axes -- compensating for imperfect mechanics by smart electronics.

Team up good digital setting circles with a computerized data base of celestial objects, and you gain the astounding finding capabilities of a "computer assisted" or "robotic" telescope. These are currently working a revolution in high-end amateur astronomy, finally fulfilling the promise of what many people thought setting circles were supposed to do all along.

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

Comrades of the night. Astronomy can acquire new dimensions for those who find a compatible club. Here invisible enthusiasts, marked only by their red flashlights, move about the grounds of the 70-year-old Stellafane clubhouse in Springfield, Vermont, beneath the stars of Sagittarius. Six-minute time exposure by Kevin B. Jones.

"DEAR SKY & TELESCOPE," the letter began. "I am 20 years old and new to astronomy. I have always been fascinated with the stars and universe. What would you suggest my first step be to get into the hobby, so that I might get the most enjoyment out of it?"

It's a good question, one that deserves better answers than most beginners find. Many newcomers to astronomy call us in exasperation after blundering down some wrong trail that leaves them lost and frustrated. Such experiences, widely shared, create a general public impression that astronomy is a tough hobby to get into. But this impression is altogether wrong and unnecessary.

Many other hobbies that have magazines, conventions, and vigorous club scenes have developed effective ways to welcome and orient beginners. Why can't we? For starters, novice astronomers would have more success if a few simple, well-chosen direction signs were posted for them at the beginning of the trail.

What advice would help beginners the most? Sky & Telescope editors brainstormed this question. Pooling thoughts from nearly 100 years of collective experience answering the phone and mail, we came up with a number of pointers to help newcomers past the pitfalls and onto the straightest route to success.

1. Ransack your public library.
Astronomy is a learning hobby. Its joys come from intellectual discovery and knowledge of the cryptic night sky. But unless you live near an especially large and active astronomy club, you have to make these discoveries, and gain this knowledge, by yourself. In other words, you need to become self-taught.

The public library is the beginner's most important astronomical tool. Maybe you found Sky & Telescope there. Comb through the astronomy shelf for beginner's guides. Look for aids to learning the stars you see in the evening sky. One of the best is the big two-page sky map that appears near the center of every month's Sky & Telescope, which the library should have. When a topic interests you, follow it up in further books.

Many people's first impulse, judging from the phone calls, is to look for someone else to handle their education -- an evening course offering, a planetarium, or some other third party. These can be stimulating and helpful. But almost never do they present what you need to know right now, and you waste an enormous amount of time commuting when you should be observing. Self-education is something you do yourself, with books, using the library.

2. Learn the sky with the naked eye.
Astronomy is an outdoor nature hobby. Go into the night and learn the starry names and patterns overhead. Sky & Telescope will always have its big, round all-sky map for evening star-finding. Other books and materials will fill in the lore and mythology of the constellations the map shows, and how the stars change through the night and the seasons. Even if you go no further, the ability to look up and say "There's Arcturus!" will provide pleasure, and perhaps a sense of place in the cosmos, for the rest of your life.

3. Don't rush to buy a telescope.
Many hobbies require a big cash outlay up front. But astronomy, being a learning hobby, has no such entrance fee. Conversely, paying a fee will not buy your way in.

Thinking otherwise is the most common beginner's mistake. Half the people who call for help ask, "How do I see anything with this %@&*# telescope?!" They assumed that making a big purchase was the essential first step.

It doesn't work that way. To put a telescope to rewarding use, you first need to know the constellations as seen with the naked eye, be able to find things among them with sky charts, know something of what a telescope will and will not do, and know enough about the objects you're seeking to recognize and appreciate them.

The most successful, lifelong amateur astronomers are often the ones who began with the least equipment. What they lacked in gear they had to make up for in study, sky knowledge, map use, and fine-tuning their observing eyes. These skills stood them in good stead when the gear came later.

Is there a shortcut? In recent years computerized, robotic scopes have come on the market that point at astronomical objects automatically. They represent an enormous change. No longer do you need to know the sky.

Once fully set up, a computerized scope is a lot faster than the old way of learning the sky and using a map -- assuming you know what's worth telling the computer to point at. But they're expensive, and opinions about them are divided. For beginners, at least, there's some consensus that a computerized scope can be a crutch that prevents you from learning to get around by yourself and will leave you helpless if anything goes wrong. Moreover, you miss out on the pleasures of making your own journeys through the heavens.

At star parties beneath gorgeous black, star-sprinkled skies, we have seen beginners struggling for hours with electronics when they should have been sweeping the heavens overhead. Is this just the carping of old fogeys? The jury is still out.

4. Start with binoculars.
A pair of binoculars is the ideal "first telescope," for several reasons. Binoculars show you a wide field, making it easy to find your way around; a higher-power telescope magnifies only a tiny, hard-to-locate spot of sky. Binoculars give you a view that's right-side up and straight in front of you, making it easy to see where you're pointing. An astronomical telescope's view is upside down, sometimes mirror-imaged, and usually presented at right angles to the line of sight. Binoculars are also fairly inexpensive, widely available, and a breeze to carry and store.

And their performance is surprisingly respectable. Ordinary 7- to 10-power binoculars improve on the naked-eye view about as much as a good amateur telescope improves on the binoculars. In other words they get you halfway there for something like a tenth to a quarter of the price -- an excellent cost-benefit ratio.

For astronomy, the larger the front lenses are the better. High optical quality is important too. But any binocular that's already knocking around the back of your closet is enough to launch an amateur-astronomy career.

5. Get serious about maps and guidebooks.
Once you have the binoculars, what do you do with them? You can have fun looking at the Moon and sweeping the star fields of the Milky Way, but that will wear thin after a while. However, if you've learned the constellations and obtained detailed sky maps, binoculars can keep you busy for a lifetime.

They'll reveal most of the 110 "M objects," the star clusters, galaxies, and nebulae cataloged by Charles Messier in the late 18th century. Binoculars will show the ever-changing positions of Jupiter's satellites and the crescent phase of Venus. On the Moon you can learn dozens of craters, plains, and mountain ranges by name. You can split scores of colorful double stars and spend years following the fadings and brightenings of variable stars. If you know what to look for.

A sailor of the seas needs top-notch charts, and so does a sailor of the stars. Fine maps bring the fascination of hunting out faint secrets in hidden sky realms. Many reference books describe what's to be hunted and the nature of the objects you find. Moreover, the skills you'll develop using maps and reference books with binoculars are exactly the skills you'll need to put a telescope to good use.

6. Find other amateurs.
Self-education is fine as far as it goes, but there's nothing like sharing an interest with others. There are more than 400 astronomy clubs in North America alone -- see Sky & Telescope's Astronomical Directory right here on the Sky & Telescope Web site. Call the clubs near you. Maybe you'll get invited to monthly meetings or nighttime star parties and make a lot of new friends. Clubs range from tiny to huge, from moribund to vital. But none would have published a phone number unless they hoped you would call.

Computer networks offer another way to contact other amateurs. CompuServe, GEnie, America Online, and the Internet all have active astronomy areas. These present a constant flow of interesting news and chatter by amateurs who are quick to offer help, opinions, and advice.

7. When it's time for a telescope, plunge in deep.
Eventually you'll know you're ready. You'll have spent hours poring over books and ad brochures. You'll know the different kinds of telescopes, what you can expect of them, and what you'll do with the one you pick.

This is no time to scrimp on quality; shun the flimsy, semi-toy "department store" scopes that may have caught your eye. The telescope you want has two essentials. One is a solid, steady, smoothly working mount. The other is high-quality optics -- "diffraction-limited" or better. You may also want large aperture (size), but don't forget portability and convenience. The telescope shouldn't be so heavy that you can't tote it outdoors, set it up, and take it down reasonably easily. The old saying is true: "The best telescope for you is the one you'll use the most.

Can't afford it? Save up until you can. Another year of using binoculars while building a savings account will be time you'll never regret. It's foolish to blow half-accumulated telescope money on something second rate that will disappoint. Or consider building the scope yourself, an activity that many clubs support.

8. Lose your ego.
Astronomy teaches patience and humility -- and you'd better be prepared to learn them. There's nothing you can do about the clouds blocking your view, the extreme distance and faintness of the objects you desire most, or the timing of the long-anticipated event for which you got all set up one minute late. The universe will not bend to your wishes; you must take it on its own terms.

Most of the objects within reach of any telescope, no matter how large or small it is, are barely within its reach. Most of the time you'll be hunting for things that appear very dim, small, or both. If flashy visuals are what you're after, go watch TV.

"Worthiness" is the term entering the amateur language for the humble perseverance that brings the rewards in this hobby. The term was coined by Ken Fulton, author of The Light-Hearted Astronomer (1984) -- a book describing the hobby as a jungle full of snares, quicksand, and wild beasts that only those with the spiritual skills of a martial artist can traverse unmauled. It's really not that bad -- but there are definitely times when a Zen calmness will help you through.

9. Relax and have fun.
Part of losing your ego is not getting upset at your telescope because it's less than perfect. Perfection doesn't exist, no matter what you paid. Don't be compulsive about things like cleaning lenses and mirrors or the organization of your observing notebook.

And don't feel compelled to do "useful work" right away. Ultimately, the most rewarding branches of amateur astronomy involve scientific data collecting -- venturing into the nightly wilderness to bring home a few bits of data that will advance humanity's knowledge of the universe in some tiny but real way. Such a project often marks the transformation from "beginner" to "advanced amateur," from casual sightseer to cosmic fanatic. But it only works for some people, and only when they're good and ready.

Amateur astronomy should be calming and fun. If you find yourself getting wound up over your eyepiece's aberrations or Pluto's invisibility, take a deep breath and remember that you're doing this because you enjoy it. Take it only as fast or as slow, as intense or as easy, as is right for you.

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

For simple visual observing without setting circles, you don't need to align a telescope's equatorial mount very well on the north celestial pole. Just plunk it down so that the polar axis (the right ascension axis) is aimed at Polaris as best you can judge by eyeballing it. The mount will then do its job.

For long-exposure astrophotography, however, the polar alignment must be a lot better.

The "declination drift method" is the most accurate way to accomplish this. The method is straightforward, but it does require some time and patience.

First aim the mount's polar axis roughly at Polaris. Now point the telescope at a star that's somewhat above the celestial equator and as close to south as you can judge by looking opposite Polaris. Put in a high-power eyepiece. If the eyepiece has cross hairs, center the star on them. Otherwise put the star on the north or south edge of the field and defocus it a little. Turn on the clock drive, and ignore any east-west drift.

If the star drifts south in the eyepiece, the polar axis is pointing too far east.

Shift the polar axis left or right accordingly, until there is no more drift.

Now go back and repeat from the beginning, because each adjustment throws the previous one slightly off. When all visible drift is eliminated the telescope is very accurately aligned, and you can take long deep-sky exposures.

If your eastern sky is blocked, you can use a star low in the west and reverse the words "too high" and "too low" in the above instructions. If you're in the Earth's Southern Hemisphere, reverse the words "north" and "south."

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

By the time you set out into the night with a telescope, you should know the constellations well enough to find your way around the sky.

An all-sky constellation map (such as the one in or near the center of Sky & Telescope magazine every month) will get you started. Think of this as like a map of the world; if you don't know where Japan or England are, you need to learn.

But once you've found England on a world map, it's not much good for getting you to a particular street address in Tunbridge Wells. In addition to a wide-scale constellation guide, an astronomer needs a more detailed, magnified sky atlas in order to locate specific points of interest.

One of the most widely used atlases is Wil Tirion's Sky Atlas 2000.0. It covers the celestial sphere in 26 big charts that plot a total of 43,000 stars and 2,500 other objects.

Such maps may look terribly complex at first. But step back for a minute and look at only the brighter stars; they form the same constellation patterns familiar from your naked-eye all-sky map.

Suppose, for instance, you've learned Gemini as it's drawn on the monthly Sky & Telescope charts, where the stars are connected to form two stick figures holding hands. These same stick figures appear on Chart 5 of the Tirion atlas -- but at a larger scale and almost lost in a wealth of detail.

Directions and Distances
First get familiar with directions on the map. East, you'll notice, is left of north on sky maps, not to the right like on maps of the ground. The reason is simple: You look down at the ground but up at the sky. (If you looked up through the bottom of a land map of, say, the United States -- as if you were at the center of
a transparent Earth -- it too would have east left when north was up.)

One trick for keeping east and west straight on a celestial map is remembering that right ascension increases to the east. If hours of right ascension are printed on the map, they'll set you straight.

The next step is to learn the map's scale. You have to know how much of what's printed on paper appears in the eyepiece of your finderscope before you can compare the map to what you see!

First determine the size of your finderscope's field. Locate two stars that just fit into its edges (try pairs in the Big Dipper or Cassiopeia). Then see how many degrees apart these stars are on the map, by referring to the declination scale along the sides. That's the diameter of your finder's field.

Now do the same to find the field diameter of the main telescope's lowest-power eyepiece. It will probably be only about 1° or so -- the area of sky your little fingernail covers at arm's length.

This is so small that it may be hard to identify a good star pair on your map to measure the field size. Here's another way. Aim at any star within about 10° of the celestial equator -- in Orion's Belt, for example, or the Circlet of Pisces. Center the star. Then turn off the telescope's clock drive (if any) and time how long the star takes to drift from the center to the field's edge. The time in seconds, divided by 120, equals the diameter of the field in degrees.

Now, using the scale on the margin of the charts, make little rings out of wire -- or draw circles on clear plastic -- corresponding to your field sizes. An example is shown in the photograph here. By sliding these circles across the charts, you can see exactly what star patterns will pass through your field of view when you sweep across the sky.

Beginners are always surprised at how tiny the view really is. Keep these little tools with the charts; you'll need them whenever you observe.

Now we're ready to go on our first deep-sky hunt.

A Practice Star-Hop
Any observing session should begin with some indoor planning. Let's use Gemini for a dry run. If you know the constellations, you can find its two bright stars, Castor and Pollux, in the sky. These will be our starting point.

A couple inches south of Castor on the appropriate Sky Atlas 2000.0 chart is the planetary nebula NGC 2392, indicated by a little open circle with four spikes. This looks like it might be a nice item to check out. We look it up in Burnham's Celestial Handbook. It's described as a small, round glow of 8th magnitude, which is bright enough to show in most telescopes. So far so good.

The next step is to plan how to get there by star-hopping. This just means following a trail of stars to move the telescope from a place we know, such as Pollux, to some place we don't, such as the location of the nebula. The trick is not to get lost on the way.

Take the wire ring that corresponds to your finder's field and center it on Pollux. Several fainter stars are in the circle, just as they would be if you were looking through the finderscope at the sky. The bright star closest to Pollux is 75 or Sigma Geminorum, to the north of Pollux (in the direction of Castor, which is out of the field of view). Near the southwest edge of the field is the star 69 or Upsilon Geminorum. It forms a long right triangle with Pollux and 75; Pollux is at the right angle.
This triangle confirms 69's identity in the sky, where there's no convenient label next to it.

Shift the wire ring to center on 69; this corresponds to moving the telescope. Two new pairs of stars have now entered the west edge of the field a little north of center: 64, 65, 60, and 59, the four of them forming a distinctive shape. Shift the ring to center on 60. The fainter 59 just to its southwest will confirm
you've got the right one.

Star 57 is now just on the south edge of the field. Shift south by half the width of the field so 57 is centered; bright Delta is now waiting just outside to the south. Shift south again an equal amount; Delta quickly appears and can be centered just after 57 leaves to the north.

See how Delta forms an equilateral triangle with 56 and 63, to its south and east? With 63 now identified -- aided by two fainter stars on either side of it -- we're less than 1° from our prey. Note the flat triangle that 63 and 61 form with the nebula. The shape of the triangle allows us to target the correct position even if the nebula is invisible, as it may be in the finder. The two faint stars just southeast of the nebula will help confirm the exact spot.

From Map to Sky: Know Your Directions
If we do this outdoors at night and move the telescope to match each step on the map, NGC 2392 should now be visible in the main eyepiece: a small, dim, eerie round glow quite unlike the pointlike stars, grayish-green in color and with a very faint star at its exact center -- a prize worthy of the rather complicated chase.

The star-hopping route may seem like a lot of trouble to the beginner, whose impulse is just to sweep from Pollux "about the right distance that way." But most deep-sky objects are many times dimmer than the faintest stars on the chart and won't catch your attention even if, by luck, your tiny telescopic
field happens to sweep right across them. The only way to succeed is to know exactly where you are at all times. If you suspect you're lost, go back and start over. Have patience. You'll speed up later when practice increases your skill.

The biggest pitfall in going from map to sky is keeping directions straight.

Remember that in the sky, celestial north is not up but toward Polaris, no matter how cockeyed this direction may be in the eyepiece. To find north as seen in the eyepiece, just nudge the telescope a bit toward Polaris. New stars will enter from the field's north side, showing you where this is. Turn the map
around accordingly, so north on the map is oriented in this direction. This north-nudging trick will become such a habit at the telescope that you'll forget you're even doing it.

If you have an equatorial mount, turn the eyepiece of the finderscope so the crosshairs line up with the telescope's motion as you sweep north-south or east-west. The crosshairs will now mark the four cardinal directions no matter where you point the scope.

Okay -- you've found north in the eyepiece. East and west can be a bit trickier, depending on your telescope.

East is 90° counterclockwise from north if you're looking at a "correct" or right-reading image, just like on a map. A correct image is given by an optical system that has an even number of mirrors. Examples are a Newtonian reflector, which has two mirrors, or a straight-through refractor, which has zero.

But east is 90° clockwise from north in a mirror image, which is what you see in any optical system that reflects light an odd number of times. A mirror image view is very hard to compare with a correct-image map.

Note that this is not the same as the image merely being turned upside down. In that case you could just turn the map upside down too. A mirror image cannot be made correct no matter how you turn it.

The usual culprit is a star diagonal on a refractor or Schmidt-Cassegrain telescope. To get a correct image you can simply take out the diagonal and reinsert the eyepiece to view straight through. This is especially important to do to your finderscope, if it came with a diagonal.

Alternatively, you can photocopy your map, turn the photocopy over, and shine a flashlight up through the paper from beneath to view a mirror image of the printing through the paper. Better yet, photocopy maps onto clear acetate Viewgraph sheets, turn the Viewgraphs over, and tape them to a red background.

Some amateurs who insist on using their star diagonals while star-hopping have resorted to propping up a small mirror on their chart table and viewing their maps in the mirror. This way you see what you get. Or you can buy a diagonal that is made with an Amici prism, which employs two reflections instead of an ordinary diagonal's one.

When star-hopping, always think in terms of north, south, east, and west -- never up, down, left, or right, or you'll quickly get lost in trackless wastes of space. Once you get the hang of it you'll always be mumbling as you turn from map to scope: "Starting from that bright one in the north of the kite shape...half a finder field east to the pair in the skinny triangle...then a quarter finder field south to the one at the west end of the flat triangle..." Triangles are the most basic units of star-hop patterns, and you'll be seeing a lot of them.

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

Part of learning astronomy is learning the language.

Astronomy, like any other endeavor, has its own jargon. Newcomers can get thrown by such arcane-sounding terms as "arcsecond," "4th magnitude,"and "right ascension." But they're not as tough as they sound. Here's a quick rundown of the most important astronomy terms for you to know.

Sky Measures
Beginners often have trouble describing distances in the sky. You may get into a conversation that sounds like this:

"Do you see those two stars? The ones that look about eight inches apart?"

"Yeah, but they look more like six feet apart. . . ."

The problem here is that distances on the sky can't be expressed in linear units like feet or inches. The way to do it is by angular measure.

Astronomers might describe the two stars as 10 degrees apart. That means if lines were drawn from your eye to each star, the two lines would form an angle of 10 degrees at your eye.

Hold your fist at arm's length and sight past it with one eye. Your fist covers about 10 degrees of sky from side to side. A fingertip held at arm's length covers about 1 degrees. The Sun and Moon are each 1/2 degrees wide. The Big Dipper is 25 degrees long. From the horizon to the point overhead (the zenith) is 90 degrees.

There are finer divisions of angular measure. A degree is made up of 60 arcminutes, and each arcminute is divided into 60 arcseconds.

If two objects lie a quarter degree apart, astronomers might note that as 15 arcminutes, abbreviated 15'. The brightest planets usually appear just a few dozen arcseconds across as seen from Earth. A 5" telescope can resolve details 1 arcsecond (1") across. This is the width of a penny seen at a distance of 2.5 miles.

Sky Co-ordinates
From the Earth's surface, the night sky looks like a huge dome with stars stuck on its inside surface. If the Earth were swept out from under us, we'd see stars all around us, and we'd have the breathtaking sensation of hanging at the center of an immense, star-speckled sphere.

Astronomers designate the positions of stars by where they lie on this celestial sphere.

Picture the Earth hanging at the center of it. Imagine the Earth's lines of latitude and longitude expanding outward and printing themselves on the inside of the celestial sphere. They now provide a coordinate grid on the sky that tells the position of any star, just as latitude and longitude tell the position of any point on Earth. In the sky, "latitude" is called declination and "longitude" is called right ascension.

There's a slight complication. These coordinates change slightly over the years, due to a slow shift of the Earth's orientation in space known as precession. When right ascension and declination are given in books and atlases, you might see them accompanied with a year date such as 1950 or 2000. This is the year for which the position is strictly correct.

Magnitudes
The brightness of a star (or anything else in the sky) is called its magnitude. You'll encounter this term often.

The magnitude system began about 2,100 years ago when the Greek astronomer Hipparchus divided stars into brightness classes. He called the brightest ones "1st magnitude." Those a little fainter he termed "2nd magnitude," and so on down to the faintest ones he could see, "6th magnitude."

With the invention of the telescope in the 17th century, observers could see even fainter stars. Thus 7th, 8th, and 9th magnitudes were added. Today binoculars will show stars as faint as 9th magnitude, and an amateur's 6" telescope will go to about 13th. The largest and most sensitive telescopes used by professional astronomers can reach to about 29th magnitude.

It turns out that some of Hipparchus's "1st-magnitude" stars are brighter than others. To accommodate them, the scale now extends into negative numbers. Vega is zero (0) magnitude, and Sirius, the brightest star in the sky, is magnitude –1.4. Venus is even brighter, usually magnitude –4. The full Moon is –13, and the Sun shines at magnitude –27.

Distances
The Earth orbits (circles around) the Sun once a year at a distance averaging 93 million miles. That distance is called one astronomical unit (a.u.).

The distance light travels in a year, 6 trillion miles or 63,000 a.u., is called a light-year. Note that the light-year is a measure of distance, not time.

Most of the brightest stars in the sky lie a few dozen to a couple thousand light-years away. The nearest large galaxy beyond our own Milky Way is 2.5 million light-years distant.

What's the most important thing to know about a star? Its apparent magnitude might top the list, but right behind would be its spectral type. Without it the star is a meaningless dot of light. Add a few letters and numbers, such as G2V or B5IV-Vshnne, and the star suddenly gains personality and character. To those who can read its meaning, the spectral code tells at a glance just what kind of object the star is -- its color, size, and luminosity, its history and future, its peculiarities, and how it compares with the Sun and stars of all other types.

The modern spectral classification system is so successful that it has hardly been changed since 1943. It is based on just two physical properties that imprint themselves on the spectrum of a star's light: the star's temperature and atmospheric pressure. These reveal an abundance of information that paints the star's portrait and tells its life story.

The temperature sets the star's color and tells its surface brightness, how much light it emits from each square meter of its surface. The pressure depends on the star's surface gravity and therefore, roughly, on its size -- telling whether it is a giant, dwarf, or something in between. The size and surface brightness in turn yield the star's luminosity (its total light output, or absolute magnitude) and often its evolutionary status as young, middle-aged, or nearing death. The luminosity also gives a good idea of the star's distance. Appended to the basic spectral type may be letters for chemical peculiarities, an extended atmosphere, unusual surface activity, fast rotation, or other special characteristics.

Every starwatcher needs to have a feel for spectral types. Here are the most important things to know.

Dissecting Starlight
The tale begins in 1802, when the English experimenter William Wollaston passed a beam of sunlight through a thin slit and then through a prism. The slit provided a sharp, high-resolution view of the familiar rainbow spectrum, with no colors overlapping each other. When seen this way, Wollaston noticed, the Sun's spectrum was marked by many narrow, black lines of various intensities. These dark lines stayed at exactly the same places in the colorful band from day to day and year to year. They were later measured and cataloged by Josef von Fraunhofer, for whom they are still called "Fraunhofer lines."

Similar spectral lines showed up in laboratory experiments. Using a slit and prism, physicists discovered that when a solid, a liquid, or a dense gas is heated to glow, it emits a smooth spectrum of light with no lines: a continuum. A rarefied hot gas, on the other hand, glows only in certain colors, or wavelengths: bright, narrow emission lines instead of a rainbow band. If a cooler sample of the same gas is placed in front of a glowing object, dark absorption lines appear at the wavelengths where the emission lines would be if the gas were hot.

By 1859 the situation was clear: we see the Sun's hot surface through a cooler solar atmosphere that imposes the dark lines.

Every element, every chemical compound, shows its own set of spectral lines. They are as unique as fingerprints. They reveal not only which atoms and molecules are present but also many other physical conditions, starting with temperature. Here, scientists realized, was a way to bring the Sun down into the laboratory. When they put slit-and-prism devices (spectroscopes) on telescopes, they could even see spectral lines in the light of stars.

It was the 19th century's greatest astronomical breakthrough. Philosophers had cited the makeup of stars as something beyond all possible human knowing. Now finding the composition of the Sun and stars was just a matter of comparing spectral lines seen in a telescope to those in a laboratory. This wasn't always simple, but it gave birth to modern astrophysics -- the treatment of stars as physical objects to be studied and understood, rather than as mere points of light on the sky to be measured.

Spectral Classes
The first great classifier of stellar spectra was Angelo Secchi in Rome. In the 1860s he examined the spectra of hundreds of stars visually in a telescope and classed them into five main types, mostly named for bright examples. Sirian stars, for instance, showed spectra like Sirius's: dominated by absorption lines of hydrogen atoms.

Today's classification scheme was born at Harvard College Observatory. Starting in 1886 under Edward C. Pickering, the observatory staff photographed and classified thousands of stellar spectra. They assigned them letters from A through Q, generally in alphabetical order from the simplest-looking to the most complex. But soon a more natural system became clear. By rearranging and merging classifications, Antonia C. Maury and Annie J. Cannon found they could fit nearly all stars' spectra into one smooth, continuous sequence. The sequence matched the stars' color temperatures, from the hottest, blue-white stars to cool, orange-red ones.

But it was too late to reassign the letters. When the dust cleared, the rearranged sequence ran O B A F G K M from hot to cool. Spectral types on the blue end were called "early" and those on the red end "late." These terms are still used today, though the incorrect idea of stellar evolution they embody -- that stars simply cool with age -- has been obsolete for generations.

The sequence could be cut even more finely. Cannon subdivided each letter with numbers from 0 to 9, so that a spectrum whose appearance placed it halfway between standard G0 and K0 stars was called G5. Using this scheme, Cannon led the classification at Harvard of 325,300 spectra recorded on wide-field photographs. The resulting Henry Draper Catalogue (HD) and Henry Draper Extension (HDE), published beginning in 1918, remain standard references today.

The time-honored mnemonic for remembering the spectral sequence, invented by Henry Norris Russell when astronomy's leadership was all male, is "Oh Be A Fine Girl Kiss Me." In 1995 Mercury magazine published a student's rejoinder: "Only Boys Accepting Feminism Get Kissed Meaningfully." Take your pick.

A few other spectral types don't fit the sequence but instead parallel it. Type W or Wolf-Rayet stars are as hot and blue as the hottest O stars but show strong emission lines, either of nitrogen (WN), carbon and oxygen (WC), or neither (WR). Emission lines indicate an especially large, thick shroud of hot gas surrounding a star.

Some giant stars at the cool end of the spectrum have an excess of carbon. These were originally called R and N but have been merged to form type C. "Carbon stars" can often be spotted at a glance in a telescope by their deep red color. A bright example in the autumn sky is 19 Piscium (TX Piscium) in the Circlet of Pisces, spectral type C5. Their distinctive absorption bands (masses of overlapping spectral lines) due to the carbon compounds C2, CN, and CH darken or "blanket" the blue end of the spectrum. In other words, a carbon star's atmosphere is a red filter. When seen in emission instead of absorption, these same spectral bands glow blue; the same compounds that redden a carbon star give comets their blue-green tint.

The rare type-S stars are also red giants. They parallel type M but show strong bands of zirconium oxide and lanthanum oxide instead of an M star's titanium oxide. We can imagine that planets of S stars, bathed in chemically peculiar stellar winds, might be encrusted with gems of cubic zirconia.

Luminosity Classes
Even in stars of the same spectral type, the absorption lines don't always look alike. In some stars the lines are narrow and sharp; in others they are broadened by various effects. Chief among these is atmospheric pressure, which also changes the intensity ratios of certain pressure-sensitive lines.

Astronomers quickly realized that atmospheric pressure tells a star's surface gravity and therefore suggests its size. Narrow lines indicate an immense, bloated star with a weakly compressed atmosphere far from its center of gravity. In the Henry Draper Catalogue, spectral types were prefixed with d for dwarf, sg for subgiant, g for giant, and c for supergiant.

You'll still run across these letters from time to time, but beginning in 1941 they were replaced by a more detailed scheme first published by William W. Morgan and Philip C. Keenan. With only minor changes, this "MK" system of spectral classification remains the standard today. Stars are assigned to luminosity classes by Roman numerals: I for supergiants (often subdivided into classes Ia-0, Ia, Iab, and Ib in order of decreasing luminosity), II for bright giants, III for normal giants, IV for subgiants, V for dwarfs on the main sequence, and occasionally VI for subdwarfs.

Thus a designation such as G2V, the Sun's spectral type, tells temperature and luminosity. When these are plotted against each other on a graph, the result is called a Hertzsprung-Russell or H-R diagram. This has been a fundamental astrophysical tool ever since it was invented around 1911. Most stars gather in certain narrow regions of the H-R diagram according to their masses and ages.

Stars arrive on the main sequence soon after they are born, and this is where they spend most of their lives. Massive stars blaze brightly on the hot, blue end of the main sequence. They burn up their nuclear fuel in only millions or tens of millions of years. Stars with lower masses comprise the yellow, orange, and red dwarfs on the lower-right part of the main sequence, where they remain for billions of years.

As a star begins to exhaust the hydrogen fuel in its core, it evolves away from the main sequence toward the upper right and becomes a red giant or supergiant. Stars that began with more than six times the Sun's mass then evolve left and right through complicated loops on the H-R diagram as if in a frenzy to keep up their energy production, then finally explode as supernovae. Less massive giants evolve to the left and then down to become white dwarfs; this is the track the Sun will trace through the H-R diagram 8 billion years from now (S&T: May 1994, page 12).

Odds and Ends
Spectra can reveal many other things about stars. Accordingly, lowercase letters are sometimes added to the end of a spectral type to indicate peculiarities. A list appears in the following table.

Some Spectral Peculiarity Codes
Code	Meaning
comp	Composite spectrum; two spectral types areblended, indicating that the star is an unresolved binary
e	Emission lines are present (usually hydrogen)
m	Abnormally strong "metals" (elements other than hydrogen and helium) for a star of a given spectral type; usually applied to A stars
n	Broad ("nebulous") absorption lines due to fast rotation
nn	Very broad lines due to very fast rotation
neb	A nebula's spectrum is mixed with the star's
p	Unspecified peculiarity, except when used with type A, where it denotes abnormally strong lines of "metals" (related to Am stars)
s	Very narrow ("sharp") lines
sh	Shell star (B to F main-sequence star with emission lines from a shell of gas)
var	Varying spectral type
wl	Weak lines (suggesting an ancient, "metal"-poor star)

Symbols can be added for elements showing abnormally Strong lines. For example, Epsilon Ursae Majoris in the Big Dipper is type A0p IV:(CrEu), indicating strong chromium and europium lines. The colon means uncertainty in the IV luminosity class.

Certain spectral subtleties are not widely known among amateurs. Some visual observers pride themselves on being able to nail a star's type to the nearest letter by its color in the eyepiece. Color is indeed a close indicator of spectral type for stars earlier (hotter) than about K5, assuming no interstellar reddening is present. But the relationship often breaks down among the later K and M stars. Compare the tint of Betelgeuse, type M2 Iab, to that of Aldebaran, K5 III. Most people can't see a difference. At the same time, two red giants of the identical type may show different tints; compare Mu and Eta Geminorum, both cataloged as type M3 III.

In addition, dwarf G, K, and M stars are not as red as giants and supergiants of the same types. The color difference is equivalent to about one-half to one letter class.

Lastly, differences between spectra are far greater than differences in the actual compositions of stars. An A star might seem to be almost pure hydrogen, while a K star shows only trace evidence of hydrogen in a spectrum packed with lines of "metals" (the astronomer's term for all elements other than hydrogen and helium). But A and K stars are made of the same stuff. Different atoms and ions merely display their spectral lines at different temperatures. Even carbon stars are made mostly of hydrogen and helium. The true "abundances" of elements can indeed be measured in a star. But it's a tough job of comparing precise line strengths in a high-quality spectrum with those predicted by atomic theory or measured in the lab.

For much of the 20th century, the study of visible-light spectra practically was astronomy. In recent decades the opening of nonvisible wavelengths and other exciting advances have distracted attention from this field. Nevertheless it remains the bedrock on which modern astronomy rests.

Further Reading: Kaler, James B. Stars and Their Spectra. Cambridge University Press, 1989. Sections of this book were serialized in Sky & Telescope in 10 parts from February 1986 to May 1988.

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

If a rose by any other name would smell as sweet, would BD +38°3238 by any other name sing more sweetly to lovers down from the summer sky? Everyone who starts out in astronomy faces a bewildering variety of numbers and letters denoting the great works of creation. Sometimes the nomenclature almost seems designed to confuse. Anyone can look up and recognize a star as Vega -- so why does it also need the names BD +38°3238, Alpha Lyrae, 3 Lyrae, HR 7001, GC 25466, HD 172167, SAO 67174, ADS 11510, and dozens of others?

At least beginners aren't alone in their confusion. The First Dictionary of the Nomenclature of Celestial Objects, 1983, describes well over 1,000 different naming systems currently in use, mostly for faint objects studied by professionals. Its editors despair of the list ever being made orderly, reasonable, or complete. Celestial nomenclature is too freakish for that, too full of schemes from times long past that just grew.

A well-rounded amateur needs to know only a tiny fraction of these naming systems. In this article we'll cover those most often encountered for stars, with their meanings and histories. Another article covers nomenclature of deep-sky objects.

Where the Heck is Zujj Al Nushshabah?
Since ancient times stars, like people, have had their own proper names, such as Vega or Deneb. But today proper names are widely used only for the brightest few dozen stars -- and it's a good thing. Star names are poetic and embody old constellation lore (usually in garbled Arabic), but confusion runs wild. "Deneb" to most people interested in astronomy means the brightest star in Cygnus. But the same name has also been bestowed, at some time, on at least five other stars. It simply means "tail," a body part that a lot of constellations possess. Which one do you really mean?

Moreover, the list quickly becomes too big. The Yale Bright Star Catalogue, 4th edition, 1982, gives some 845 star names, more than most people would ever want to memorize. Every astronomer knows what you mean by Sirius or Polaris, but not one in a hundred could identify Pishpai (Mu Geminorum), Alsciaukat (31 Lyncis), Dhur (Delta Leonis), or Zujj al Nushshabah (Gamma Sagittarii).

More tractable is the Greek-letter system introduced by the German astronomer Johann Bayer in 1603. In his beautiful star atlas Uranometria published that year, Bayer identified many stars in each constellation with lower-case Greek letters. He often named a constellation's brightest star Alpha, then sorted the rest into brightness classes and assigned letters within each class in order from the head to the feet of the traditional constellation figure.

Bayer's letters caught on immediately. They are used with the Latin genitive of the constellation name, so the leading star in Centaurus is Alpha Centauri. This simply means "Alpha of Centaurus." Back when most educated people knew Latin and Greek this phrasing flowed off the tongue naturally, but today it's many skywatchers' first exposure to the Greek alphabet and Latin declensions. Sooner or later everyone who deals with stars has to sit down and learn the Greek letters (in the box here) and the genitives of the 88 constellation names (listed in the back of most astronomy handbooks).

There are swarms of stars per constellation but only 24 Greek letters. Sometimes one letter is used repeatedly with superscripts to cover several adjacent stars, such as Pi¹ through Pi^6 Orionis, the ragged row forming Orion's Shield. But as more and more stars needed names because of better sky surveys, astronomers adopted numbers.

Around 1712 England's Astronomer Royal, John Flamsteed, began numbering stars in each constellation from west to east in order of right ascension -- a big help when looking for a star on a map. For instance, 80 Virginis is east of 79 Virginis and west of 81 Virginis (at least in the coordinate system Flamsteed used, equinox 1725, which still matches today's celestial east and west pretty well).

All bright stars were numbered whether they had a Greek letter or not, which is why Alpha Lyrae is also 3 Lyrae. In all, 2,682 stars received Flamsteed numbers. The highest number within any constellation is 140 Tauri.

All nice and logical -- but when it comes to celestial nomenclature, there's a fly in every ointment. When the constellation borders were formalized in 1930, many Flamsteed stars found themselves stranded in exile. Thus 30 Monocerotis is today considered to be in Hydra, and 49 Serpentis is in Hercules. Such names are best discreetly swept under the rug, never to be used.

Nobody got around to numbering stars farther south than could be seen from England. So in far-southern constellations one often encounters upper- and lower-case Roman letters, such as g Carinae and L² Puppis. These were applied all over the sky by various star mappers from Bayer on, but in the northern sky they have largely passed out of use.

Herculean Lists
By the 19th century all these naming efforts were falling far short of the mushrooming need. Telescopes were revealing stars by the hundreds of thousands, every one of them an individual crying out for its own identity.

In 1859 the German astronomer F. W. A. Argelander at Bonn Observatory began measuring star positions with a 3" refractor to compile a gigantic list, the Bonner Durchmusterung (Bonn Survey). The BD eventually included 324,188 stars to about magnitude 9.5. Argelander and his successors divided the sky into thin bands 1° of declination wide wrapping around 24 hours of right ascension. Stars within each band were numbered in order of right ascension; constellations were ignored. Thus Vega's designation BD +38°3238 means it was the 3,238th star counting from 0h right ascension in the zone between declination +38° and +39°.

The original BD covered just over half the sky, from the north pole to declination -2°. A later southward extension, the SBD, continued the system down to declination -23° to garner another 133,659 stars. The Cordoba Durchmusterung (CD or CoD) completed the job to the south celestial pole with 613,953 more, so that visual durchmusterung, or "DM," names were bestowed on a grand total of 1,071,800 stars.

The BD, with its detailed star charts to 9th or 10th magnitude and its reliable, well-checked list of positions, remained an essential everyday tool of working astronomers for nearly a century. Durchmusterung designations are still often encountered. The star magnitudes in these catalogs, however, are notoriously unreliable by modern standards. Most were merely quick eyeball estimates.

Variable stars have a naming system all their own. This too was instigated by the energetic Argelander. He denoted the first variable star found in a constellation by the capital letter R with the genitive of the constellation name, since the previous letter, Q, was the highest Bayer had gone in Roman star lettering. The next variable would be named S, and so on to Z. After Z came RR, RS, and so on to RZ, then SS to SZ, on up to ZZ. If a variable already had a Greek letter, Argelander left it alone.

But new variable stars kept getting discovered! After ZZ, astronomers decided to go to AA, AB, and on to AZ (omitting J since in some languages it could be confused with I), then BB to BZ, on up to QZ.

Even these 334 designations proved insufficient for the variables in some crowded constellations. Rather than start an even more awkward three-letter system, astronomers ruled that further variables in a constellation would simply be designated V335, V336, and so on forever. It was a wise move. By 1990 the highest numbered variable was V4153 Sagittarii.

Multiplying Catalogs
The next great, widely used star list to appear after the BD was the Henry Draper Catalogue of stellar spectra, compiled by Annie J. Cannon at Harvard from 1911 to 1915 and published from 1918 to 1924. It includes 225,300 stars numbered in simple order of right ascension. More were added later in the Henry Draper Extension; these bear HDE numbers. Any star with an HD or HDE designation is guaranteed to have had its spectrum measured.

Meanwhile another catalog had been issued at Harvard: the Revised Harvard Photometry of 1908, which sought to provide accurate magnitudes for the brightest 9,110 stars to about magnitude 6.5. Stars in this catalog bear HR numbers. Even now the HR list remains the basis of the modern Yale Bright Star Catalogue, widely used for its detailed information about naked-eye stars.

Perhaps the most common star-numbering system today is the SAO designation. It refers to the Smithsonian Astrophysical Observatory Star Catalog (1966), which also was produced (with companion star charts) on Harvard's campus. This catalog gives very accurate positions for 258,997 stars to about 9th magnitude, though coverage is spotty for the fainter ones. The SAO stars are numbered by right ascension within 10° bands of declination from the north to the south pole.

SAO numbers have almost completely supplanted the once widely used GC designations, taken from the General Catalogue of 33,342 Stars by Benjamin Boss (1937).

The latest and greatest star list is the Hubble Space Telescope Guide Star Catalog. It is too big ever to print; instead it's distributed on two CD-ROMs. The GSC lists positions generally good to nearly 1 arcsecond and magnitudes accurate to a few tenths for 18,819,291 objects from 9th to usually about 13th or 14th magnitude, sometimes as faint as 15th. Of this total, 15,169,873 are listed as being stars; most of the remaining 3.6 million objects are small, faint galaxies. Most have never been examined by human eyes; they were measured automatically from photographic plates.

A typical individual in this list is GSC 1234 1132, a 13.3-magnitude luminary in Taurus. The first four digits specify one of 9,537 small regions of the sky; the last four give the object's serial number within this region.

Many more lists have been compiled for special purposes. A star with an ADS number is in the Aitken double star catalog (1932); IDS refers to the Index Catalogue of Visual Double Stars (Lick Observatory, 1963). These comprehensive lists are more rational than the 150-odd types of older double-star designations you are likely to encounter, generally named for astronomers who published lists, however short, of their own discoveries. Nevertheless the older names are so much a part of double-star usage that there will be no getting rid of them.

Lunar occultation observers often refer to stars by ZC number, referring to the Zodiacal Catalogue of stars that the Moon can cover. And so on, and so on.

P.S.: Name-It-Yourself Stars Are a Hoax!

Often Sky & Telescope gets inquiries about a company that
advertises on the radio that it will name a star for you or a
loved one for $45 or so. You get a pretty certificate and some
papers. Is this for real, we are asked?

No. The certificate is a "novelty item" only.

With just as much validity, you can step outside on a clear night, choose any star you like, and name it for anyone you want. For free.

We know a number of amateur astronomers who have done this for their spouses or children. To one of the Sky & Telescope editors, Iota Ursae Majoris is "Lucy's Star" and Zeta Hydrae is "Andrew's Star." Why not?

Why pay some commercial outfit to mediate your personal life? Even a fancy certificate, if it appeals to you, can be made with shareware for a lot less than $45. The company advertises that it keeps the names in a Swiss bank vault, as if that means something. If that appeals to you, you can put a piece of paper with a star name in your own bank's safe-deposit box. But why bother?

Sometimes planetariums "sell" stars on their domes to help raise needed funds. They are careful to tell donors that the certificate they get denotes a contribution to a worthy institution, not the purchase of a real star name. If you insist on paying someone else to pretend to name a star, this is a more worthwhile way to do it.

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

Timekeeping the old-fashioned way. The first sundial was probably just a stick in the ground; this fine porcelain dial, made in revolutionary France in 1793, is no great improvement. It still keeps apparent solar time, which is out of step with modern clock time on several counts. (The larger of the two hour rings is marked for a 10-hour day and 100-minute hours, innovations that lived and died with the French Revolution.) Courtesy Museum of Fine Arts, Boston.

KNOWING TIME is simple in everyday life. You look at a clock. You assume that everyone else's clock in your time zone reads the same. And that's that.

For astronomers, however, time can become quite complex. The reason is that our units of time measurement -- the day and its subdivisions of hour, minute, and second -- are based on astronomical phenomena that are themselves more complex than you might think.

Most of these complications have been smoothed out of our everyday civil time system by official edict. The result is a simple, easy-to-use timekeeping arrangement that serves society well -- as long as nobody looks too closely at the sky. Do so, and all the carefully hidden fudge factors erupt back into view.

Here, then, is a summary of the time systems that a well-informed amateur should know.

Local Apparent Time (LAT)
Also called apparent solar time or sundial time, is what everyone used long ago when they told time by the Sun. Noon was what most people still think is noon: when the Sun crosses the meridian -- that is, when the Sun is due south (for people at north temperate latitudes), at its highest point of the day, and halfway between sunrise and sunset. The very word "meridian" is from the Latin for "mid-day."

But when reasonably accurate clocks were invented, careful timekeepers noticed that something was wrong with solar time. The Sun sometimes runs up to 16 minutes fast in its daily travels across the sky, and sometimes as much as 14 minutes slow, depending on the season.

This effect arises from the tilt of the Earth's axis and the ellipticity of the Earth's orbit around the Sun. To escape the problem, our next time system was invented.

Local Mean Time (LMT).
Astronomers created an imaginary, well-behaved mean Sun that travels along the celestial equator at a uniform rate to make its annual circuit around the constellations. The mean Sun has the average or mean right ascension of the real Sun. Noon became the moment when the mean Sun crossed the meridian.

The number of minutes the real Sun lags behind or runs ahead of the mean Sun was named the equation of time. Its value for any date can be looked up in an almanac or can be read from the graphic Sky-Gazer's Almanac in the center of Sky & Telescope's January issues.

But this adjustment wasn't enough. An even worse problem results from the fact that the Earth is round.

Standard Time. Because the Earth's surface curves, "overhead" at your location is a different direction than "overhead" just a few miles away. Similarly, when the Sun or a star is on your meridian it has not yet reached the meridian of someone to your west, and it has already crossed the meridian of someone to your east.

The time depends on where you stand — if you’re using Local Mean
Time, the system governing star charts, planispheres, and the meridian
crossings of the Sun and stars. At 40° latitude, people living just 13
miles east or west of each other had clocks differing by 1 minute until
standard time zones were imposed

At 40° latitude the difference amounts to one minute of time for every 13 miles east or west. To a person watching the sky 13 miles west of you, the time seems to be 11:59 when you swear it's 12:00 and someone 13 miles east insists it's 12:01. This is why Local Mean Time is local. It depends on your location.

This didn't matter when travel and communication were slow. The problem grew more acute in the 19th century. The widespread use of telegraphs and railroads finally forced a change. How could you catch a train when every town and every railroad company kept a slightly different time?

In 1883 the United States was divided into standard time zones; the rest of the world soon followed. In each zone, all clocks are set to the Local Mean Time of a standard longitude: 75° west for Eastern Standard Time, 90° for Central, 105° for Mountain, and 120° for Pacific. Each time zone differs from its neighbors by one hour because these longitudes are 15° apart -- 1/24 of the way around the Earth.

Standard time was a great advance for society. But not for skywatchers. Planispheres (star wheels) still work in Local Mean Time (LMT). So does every all-sky map that shows horizons, such as the one in or near the center of Sky & Telescope every month. So does the Sky-Gazer's Almanac in our January issues, the "Local Time of Transit" scale on our Sun, Moon, and Planets This Month chart, and every other map, device, or calculation that shows astronomical objects with respect to your horizon, zenith, or meridian without taking your local longitude explicitly into account.

Luckily, correcting for LMT is simple. For every degree you are west of your time zone's standard longitude, add four minutes to LMT to get standard time. For each degree you are east, subtract four minutes.

To make sure you don't do it backward, use this formula: Standard time = LMT + Correction, where the correction is positive west of your time zone meridian, negative east of it. Find and learn your correction; you'll use it forever.

To get daylight saving time, of course, add an hour to standard time. Daylight saving time is currently used in the United States (except Arizona, Hawaii, and a few Midwestern counties) from 2:00 a.m. on the first Sunday in April to 2:00 a.m. on the last Sunday in October.

Universal Time (UT).
Standard time (and its daylight-saving variant) serves fine within a given time zone. But when a time applies worldwide, such as in an astronomical almanac, which time zone should be favored?

Logically enough, the "universal" time zone that was agreed upon is that of 0° longitude. This longitude is, by definition, that of a line engraved in a brass plate in the floor of the Old Royal Observatory at Greenwich, England. Hence UT was long known as Greenwich Mean Time (GMT).

By tradition UT is stated in the 24-hour system, whereby noon is called 12:00, 1 p.m. is 13:00, 2 p.m. is 14:00, and so on. Midnight is called 0:00.

One of the first things a beginner must learn is how to turn UT into standard time. It's easy. To get Eastern Standard Time, just subtract 5 hours from UT. For CST subtract 6 hours, for MST 7, for PST 8. Other time zones have their relations to UT listed in many places. (To get daylight saving time, remember to subtract one hour less than these values.)

Of course the date must be given in the same system as the time! If you get a negative time by subtracting from UT, add 24 hours. In this case the result is on the date before the UT date. For instance, 2:00 April 15th UT is 10:00 p.m. Eastern Daylight Time April 14th. These instructions and an example are in the Calendar Notes section of Sky & Telescope every month.

Many amateurs find it easiest just to remember when 0:00 UT (often written 0h) happens in their time zone. For example, 0h UT is 7 p.m. EST (8 p.m. EDT) on the previous date.

Ephemeris Time; Dynamical Time.
Once the worldwide system of time zones was in place, with UT proudly heading up the list, all should have been well forever after. But such was not to be.

Astronomers working with solar system dynamics noticed something very disturbing. The day itself varies in length.

The Earth's rotation slows down and speeds up by small amounts unpredictably, while undergoing a very long-term slowing trend. The gradual slowing is caused by the friction of tides raised by the Moon and Sun. Slow, irregular changes are thought to involve motions of material in the Earth's fluid interior. Changes in winds, air masses, snow packs, and other factors cause shorter-term variations.

Faced with this problem, astronomers in 1960 instituted Ephemeris Time (ET). This time system runs perfectly steadily regardless of the Earth's rotation, almost as if the Earth didn't exist. It is used for most celestial calculations and almanac (ephemeris) predictions, especially those having to do with the motions of the Moon, planets, and other solar system bodies in space.

Ephemeris Time matched UT around 1902. Since then UT has gradually drifted away from it, so that now (as of 1996) UT lags behind by about 62 seconds.

In 1984 ET was renamed Terrestrial Dynamical Time (TDT or TT); also created was Barycentric Dynamical Time (TDB), which is referred to the solar system's center of mass. For amateur purposes they can be considered identical, since they differ by only milliseconds.

If you encounter a time given in ET or Dynamical Time, and if one-minute accuracy matters, you need to know the difference from UT. Almanacs list this difference, which is known as Delta T. Use the formula UT = Dynamical Time - Delta T. It is impossible to forecast Delta T precisely because the Earth's fitful rotation rate is too unpredictable.

Coordinated Universal Time (UTC).
Civilization at large, not just astronomers, needs a smoothly running time system like Dynamical Time. But most of humanity is also tied to the natural cycle of the day, variable though it may be. What to do?

Part of the solution has been to redefine the basic time unit, the second. No longer is a second exactly 1/86,400 of a mean solar day. Since 1967 the second has been defined as how long cesium-133 atoms take to emit 9,192,631,770 cycles of a certain microwave radiation in an atomic clock.

With the second no longer defined astronomically, the Earth can spin as it pleases without upsetting the world's clocks. But there is a price to pay. A day no longer has 24 hours. In 1983 there were an average of 24.00000063 hours in a day, and in 1986 there were 24.00000034.

To keep our clocks in close step with the turning of the Earth, a leap second is inserted into Universal Time every year or so when required. A leap second may be added at the end of June 30th or December 31st UT, giving the last minute of the chosen day 61 seconds.

The result is Coordinated Universal Time or UTC (its acronym in French), the system by which all the world's clocks are set. UTC is the basis for all time-signal radio broadcasts and other time services. In non-astronomical circles it is sometimes called World time, Z time, or Zulu.

But the occasional leap-second jerks in UTC go unfelt, of course, by the Earth, planets, and stars. Almanac predictions given in "UT" are actually in a system known as UT1, which is always within 0.9 second of UTC. Therefore, when specifying "UT" to better than 1-second accuracy, you should state whether you mean UTC or UT1 unless this is obvious from the context -- such as if the time came from a radio time-station signal.

There is also a UT0, which is nearly the same as UT1 but includes the tiny effect of the Earth's crust moving with respect to its axis (polar motion), and a UT2, which is obsolete.

Sidereal time.
This is simply the right ascension of stars on your local meridian at any moment. Sidereal time runs about 4 minutes a day faster than all the time systems described above. An old amateur astronomer's trick is to adjust a wind-up clock to run 4 minutes a day fast, set it to local sidereal time, and use it to tell what constellations are on the meridian and what star charts to use. For instance if the clock reads 5:30 a.m., right ascension 5h 30m is on your meridian, and there you'll find Orion.

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

M51, better known as the Whirlpool Galaxy, lies in the northern
constellation Canes Venatici, along with its diminutive companion NGC
5195. This is how the pair looked to deep-sky aficionado Roger N. Clark,
who observed with an 8" f/11.5 Cassegrain telescope at magnifications
between 117x and 334x. From Visual Astronomy of the Deep Sky, © 1990
Roger N. Clark

OKAY, YOU'RE PRETTY SURE you've finally got your telescope aimed at the position of the object of your desire. The crosshairs of your finder are on its exact location according to the map in front of you. Now what can you hope to see?

If it's a bright star it will be obvious and beautiful but contain no detail. A star as seen in a telescope is a tiny blaze of brilliant light looking about the same as a star does to the naked eye, only brighter.

More interesting but generally more difficult are "deep-sky objects." This term covers the vast variety of nebulae, star clusters, galaxies, and anything else beyond the solar system that appears extended: having a visible size, rather than just being a starlike point. Many hundreds of these ghostly glows and subtle spatterings are within reach of a modest telescope.

Once you're precisely aimed you may see, with luck, a very dim, shapeless, glowing smudge floating among the stars. While finding it may bring a thrill of accomplishment, many novices are let down by the sight. "Is that all there is...to galaxies? It's nothing like the pictures in the books!"

You've just come up against the fact that the human eye cannot perform as well as a camera does at very low light levels. We are daytime animals that evolved in the skirts of a blazing sun; our eyes are not well designed for the dark of night and space. Your real-life view of a galaxy will never match the spectacular photos so common in books and magazines. But here lies the challenge. Many deep-sky objects do show a surprising wealth of detail when studied long and well even with the eyes nature gave you.

A telescope serves a different function on deep-sky objects than on the Moon, planets, or scenes on Earth. In those cases, its main purpose is to magnify distant detail. With deep-sky objects, on the other hand, a telescope's main purpose is to collect a lot of light for your less-than-sensitive eye. The issue is not that the objects are too small to see without optical aid. It's that they're too dim.

Accordingly, deep-sky observing involves its own techniques. All are aimed at helping the eye to see in near-total darkness. Here are some pointers.

Sky brightness.
The single most important factor in deep-sky observing is light pollution. Its worst effect is on dim, extended objects of just the sort we're considering. A dark sky matters even more than telescope size; a small instrument in the country will show faint nebulae and galaxies better than a large telescope in a city. If you live in a badly light-polluted area, take pleasure in what you can see through the skyglow--but don't blame yourself or your telescope for mediocre results. Plan to bring the telescope on getaways to the country.

Dark adaptation.
The eye takes time to adjust to the dark. Your eyes' pupils expand to nearly their full nighttime size within seconds of when you step out into the dark, but the most important part of dark adaptation involves chemical changes in the retina that require many minutes.

After the first 15 minutes in total darkness you might think you're night vision is fully developed, but no. Tests show that your eyes gain about another two magnitudes of sensitivity -- in other words, a factor of six in how faint you can see -- during the next 15 minutes. Thereafter, dark adaptation improves very slightly for 90 minutes more. So don't expect to see faint objects at their best until a half hour or more into an observing session.

In practice, complete darkness is unattainable. Light pollution aside, you need some light to see what you're doing. Astronomers have long used a dim red flashlight because red light has less effect on night vision. The reason is that in near-darkness you see with the "rod" cells in your retina, and these are blind to the far red end of the spectrum. When you see red light your "cone" cells are at work; these are the receptors responsible for normal daytime color vision. (You have three types of cones -- red, green, and blue -- but only one type of rod, which is insensitive to red.) The idea is to use the red cones for reading charts and swapping eyepieces, while protecting the rods for the most delicate work at the eyepiece.

Red paper rubber-banded over the front of a flashlight provides a dim, diffuse glow. In a two-battery flashlight, install a bulb rated for three or four batteries. Its light will be dim and somewhat reddened, and the batteries will last longer.

Much better than the traditional flashlight and red filter, however, is a red LED (light-emitting diode) flashlight. Its red is purer and deeper, so the division between rod and cone vision is more sharply drawn. LEDs also use much less current, so the batteries last for years. Many LED flashlights for astronomers are now available. Or see the article "Make Your Own Red LED Light." FRED

Another trick for preserving dark adaptation is to observe with one eye and read charts with the other. Keep the observing eye closed or covered with an eye patch when not in use.

Averted vision. When you look directly at something, its image falls on the fovea centralis of your retina. This spot is packed with bright-light receptors, the cone cells, and gives sharp resolution under strong illumination. But the fovea is fairly blind in dim light. So to see something faint, you have to look slightly away from it. Doing so moves the image off the fovea and onto parts of the retina that have more rod cells.

To see dramatically how this works, stare right at a star. It will disappear. Look away a little; there it is again.

Practice concentrating your attention on something a little off to one side of where your eye is aimed. This technique is called averted vision. You'll be doing it almost all the time when deep-sky observing.

Your eye is most sensitive to a faint object when it lies 8° to 16° from the center of vision in the direction of your nose. Almost as good a position is 6° to 12° above your center of view. Avoid placing the object very far on the ear side of your center of vision. There it may to fall on the retina's blind spot and vanish altogether.

In practice, finding how far to avert your vision is a matter of trial and error. Not enough and you don't get the full benefit; too much and you lose resolving power, the ability to see details.

Wiggling the scope.
Your peripheral vision is highly sensitive to motion. Under certain conditions, wiggling the telescope makes a big, dim ghost of a galaxy or nebula pop into view by averted vision. When the wiggling stops it disappears again into the vague uncertainty of the sky background.

But under other conditions, especially involving faint objects that appear tiny, just the opposite technique may work. According to Colorado astronomer Roger N. Clark in his 1990 book Visual Astronomy of the Deep Sky, some studies indicate that the eye can actually build up an image over time almost like photographic film -- if the image is held perfectly still. In bright light the eye's integration time, or "exposure time," is only about 0.1 second. But in the dark, claims Clark, it's a different story. A faint image may build up toward visibility for as long as six seconds if you can keep it at the same spot on your retina for that long. Doing so is quite contrary to instinct, because in bright light fixating on something tends to make it less visible with time.

Long exposure times might possibly be one reason why an experienced observer sees deep-sky objects that a beginner misses; the veteran has learned, unconsciously, when to keep the eye still. It also may help to explain why bodily comfort is so essential for seeing faint objects. Fatigue and muscle strain increase eye motion.

Using high powers. Conventional wisdom holds that low power works best for deep-sky viewing. After all, low power concentrates an extended object's light into a small area and thus increases its apparent surface brightness (the illumination of a given area on the retina). But as Clark proved after digging through laboratory vision studies, this assumption is usually false. High powers should do better on many faint deep-sky objects. The reason is subtle but key to understanding how low-light vision works, so we'll go into some detail.

The essential point is that the eye, unlike a camera or other purely mechanical lens system, loses resolution in dim light. This is why you can't read a newspaper at night -- even through you can see the newspaper and your eye lens theoretically resolves all the letters just as sharply as in daylight.

Studies show that the eye can resolve detail as fine as 1 arc minute in bright light but can't make out features smaller than about 20 or 30 minutes wide when the illumination is about as dim as the dark-sky background in a telescope. This is almost the size of the Moon as seen with the naked eye. So details in a very faint object can be resolved only if they are magnified to this large an apparent size--which can require using extremely high power!

The explanation lies in how nature has adapted the visual system to cope with night. Photographic film records light passively, but the nerve system in the retina contains a great deal of computing power. In dim light, the retina compares signals from adjacent areas. A faint source covering only a small area -- such as a small galaxy in the eyepiece -- may be completely invisible at the conscious level. But it is being recorded in the retina, as evidenced by the fact that a larger galaxy with the same low surface brightness is visible easily. In effect, when rod cells see a doubtful trace of light they ask other rods nearby if they're seeing it too. If the answer is yes, the signal is passed on up the optic nerve to the brain. If it's no, the signal is disregarded.

When an image is magnified by high power, its surface brightness does grow weaker. But the total number of photons of light entering the eye remains the same. (A photon is the fundamental particle of light. Most people can detect as few as 50 to 150 photons per second entering the eye.) It doesn't really matter that these photons are spread over a wider area; the retinal image-processing system will cope with them. At least within certain limits. A trade-off is needed to reach the optimum power for low-light perception: enough angular size but not too drastic a reduction in surface brightness.

What does all this mean for deep-sky observers? Simply that it's wise to try a wide range of powers on any object. You may be surprised by how much more you'll see with one than another.

One more point: There is a folk belief among observers that a telescope of long focal length (high f/ratio) gives a cleaner, higher-contrast view of dim objects than a short focal-length scope. But f/ratio is not the issue. A long-focus telescope is simply more likely to be used at high power! (It's also more likely to have high-quality optics, because they're easier to manufacture.)

Color.
Deep-sky objects sometimes disappoint beginners not only by their frequent lack of obvious detail, but also by the absence of the brilliant colors recorded in photographs.

In order to see color, we must view something with a surface brightness great enough to stimulate the retina's cone cells, and the list of deep-sky objects this bright is short. The great Orion Nebula M42 qualifies (some people can make out the pastel yellow or orange in parts of its brightest region), as do some small but high-surface-brightness planetary nebulae. The ability to see color in dim objects varies greatly from person to person, and surprises may occur.

Averted vision is not the way to look for color. The cones are thickest in the fovea, so stare right at your object. In this case, the lowest useful power should work best.

Heavy breathing.
When you pour all your concentration into examining a deep-sky object at the very limit of vision, does it get even harder to see after 10 or 15 seconds while the sky background brightens a little into a murky gray? Diagnosis: you're holding your breath without realizing it.

Low oxygen kills night vision fast. An old variable-star observer's trick is to breathe heavily for 15 seconds or so before trying for the very dimmest targets. And keep breathing steadily while you're looking.

Other tips.
Night vision is impaired by alcohol, nicotine, and low blood sugar, so don't drink, smoke, or go hungry while deep-sky observing. Bring a snack. A shortage of vitamin A impairs night vision, but if you've already got enough of it, taking more won't do any good. Virtually no one in the developed world manages to get vitamin-A deficiency any more. So don't expect eating carrots to improve your eyesight.

Prolonged exposure to bright sunlight reduces your ability to dark-adapt for a couple of days, so wear dark glasses at the beach. Make sure the label on the dark glasses says they block ultraviolet light (UVA and UVB); some cheap ones don't. Over the years ultraviolet daylight ages both your eye lens and retina, reducing sensitivity and increasing the likelihood of degenerative diseases. So if you wear eyeglasses outdoors, ask your optometrist to have an ultraviolet-filter coating applied to your glasses. This option is so cheap and easy that everyone buying glasses ought to get it regardless of any immediate medical need.

A rich complex of nebulae in Orion, including the Flame (upper left) and Horsehead (lower right), viewed through an 8" f/11.5 Cassegrain at 82x to 117x. From Visual Astronomy of the Deep Sky, © 1990 Roger N. Clark.

Taking time. Most of all, be patient. If at first you don't see anything at the correct spot, keep looking. Then look some more. You'll be surprised at how much more glimmers into view with prolonged scrutiny -- another faint little star here and there, and just possibly the object of your desire. After you glimpse your quarry once or twice, you'll glimpse it more and more often. After a few minutes you may be able to see it nearly continuously -- what astronomers call "steadily holding" an object. This where you thought at first there was nothing but blank sky.

You can be sure your observing skills will improve with practice. Pushing your vision to its limit is a talent that can only be learned with time. "You must not expect to see at sight," wrote the 18th-century observer William Herschel, often considered the founder of modern astronomy. "Seeing is in some respects an art which must be learned. Many a night have I been practicing to see, and it would be strange if one did not acquire a certain dexterity by such constant practice."

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

Lots of people buy a telescope only to discover that they can't find much of anything with it in the sky.

Their problem? They haven't learned their way around the stars as seen with the naked eye, and they try to use inadequate maps.

Here are some simple tricks for finding your way that should save you a lot of grief.

First things first. You need maps. To start with, you need a simple all-sky map, for use with the naked eye, that shows where to find the brightest stars and constellations as seen at your particular time, date, and latitude on Earth.

A simple planisphere or "star wheel" can do the trick. You turn a plastic or cardboard dial to set your time and date and get a rough map of your whole sky.

The map's edges represent the horizon all around you, as if you were standing in an open field and turning around in a complete circle. Compass directions should be printed around the horizon/edge.

The center of the map represents the part of the sky directly overhead.

A star that's plotted on the map halfway from the edge to the center, therefore, can be found about halfway up the sky -- halfway from horizontal to overhead.

That's really all there is to it!

Many planispheres are offered for sale, all too many of them poorly designed. Look for one with small, fine, carefully drafted star dots and patterns. These will be easier to match to real stars in the sky. Avoid glow-in-the-dark star maps; the glow paint can't be printed very accurately, so the result is usually a map that looks confusingly different from what it's supposed to represent.

An excellent all-sky map appears in or near the center of every month's issue of Sky & Telescope magazine. It works the same way: the big round edge is the horizon all around you (with compass directions labeled), and the center is the point overhead.

This map is drafted for specific times and dates (printed in its upper right corner). This way it avoids the distortion of the southern sky that a planisphere has to be drawn with in order to work for all times and dates.

Many planetarium programs for computers can display and print a customized all-sky map for whatever time, date, latitude, and longitude you specify.

Into the Night
To read the map outdoors, bring along a dim flashlight. The best flashlight for astronomy is red, not white; red light affects your night vision less. You can rubber-band a piece of red paper or plastic over the front of the flashlight. This both dims and reddens the light.

Outdoors with your map, start by looking for only the brightest stars plotted on it. The difference between bright and faint stars in the sky is much greater than is represented on paper. In fact, if you live in a populated area where there is much light pollution (artificial skyglow), the faint stars will be completely invisible.

Also, be aware than the constellations on an all-sky maps appear much smaller than they do in real life. The star patterns you're hunting in the night are mostly big!

Go out often with your map, and use it to learn all the constellations you can. You are establishing the familiar, major landmarks that you'll need when you start using a more detailed map with binoculars or a telescope.

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