The Night Sky

by David Chandler
plastic or cardboard planisphere chart
order from: David Chandler ( ) or Sky and Telescope ( )

All right, it's not really a book and you can read it cover to cover (i.e. front side and back side) in about 25 seconds. It's still one of the most useful accessories for any amateur astronomer. Just as you'd want a Thomas Bros. Street Atlas or Auto Club map to find your way around Los Angeles, you need a star chart of some sort to find your way around the night sky. And just as you wouldn't need the New York Times World Atlas if you merely wanted to find you way to the local Trader Joe's, similarly keep your goal in mind when you shop for a sky chart. There are many varieties of astronomical chart, from single page maps of the monthly sky to excruciatingly detailed, multi-volume hardcover or gigabyte scale CCD digitized maps.

The challenge in designing a simple, friendly star chart is that you can only see half of the full sky at any one time, the other half being hidden by the Earth that we're standing on. Furthermore, the half that we see is constantly changing both as the earth rotates on its axis and as we travel through our annual orbit round the Sun. And to complicate matters, the percentage of the full sky potentially visible from any location depends on your latitude on the curved surface of the Earth. All this makes knowing where and when to look for the Sun, or Moon, or a given planet or star one of the most difficult problems for the beginning student or amateur astronomer to deal with. Most students find black hole physics easy by comparison. But there's a simple, inexpensive device (going back to the days of mediaeval astronomical manuscripts) that can help -- the planisphere.

The planisphere is simply a turnable, circular star chart fitted into a flat sleeve or frame of paper or plastic which covers all of the star chart except for what's visible through an oval "window" cut into the frame. A planisphere is designed to show, through its window, the stars visible from a given locale, the window's shape being determined by the observer's latitude. You can "dial" the planisphere to show the stars visible any hour of the day or night for any day of the year simply by lining up the observing date printed on the outer edge of the circular star chart with the observing hour printed on the frame. Then hold the planisphere over your head, line up chart's north with true north, and you're all set to start identifying the constellations in your sky from your "personalized" chart. What could be simpler?

It's a great way to learn the constellations from your backyard or to look up (months in advance) what will be visible when you take that camping trip you're planning. There are only two considerations to bear in mind. First, a planisphere does not show the positons of the planets. Because they constantly move against the background of stars, any planisphere with the planets printed in place would be out of date in a few weeks, hence, they're not shown at all. Secondly, planispheres are designed for an ideal "home" latitude. Los Angeles is at 34 degrees North latitude. Try using a planisphere designed for L.A. in Seattle and you'll find that the North Star is much higher in your sky than the chart shows and that many of the stars displayed to the south on your L.A. chart are simply not visible. Fortunately planispheres are made for a variety of latitude ranges in both northern and southern hemispheres.

There are many "makes" of planisphere, and most large bookstores with a decent astronomy section will sell at least one type. Many planetariums sell their own planisphere, such as the local Griffith Observatory's Astrorama, optimized for L.A.'s latitude of 34° North (now out of print). Noted astronomical author and "professional" amateur astronomer David Levy markets a gigantic 16 inch diameter planisphere which is beautiful, but to my mind a litle too "supersized." My own favorite is David Chandler's The Night Sky planisphere, available in a budget cardboard version or a durable and elegant plastic version. The Night Sky is a high precision instrument available for latitude ranges 20°-30° North, 30°-40° North, 40°-50° North, 50°-60° North, and a Southern Hemisphere version. Since any planisphere will induce some distortion into the map of the sky farthest from the pole (for us, the southern sky), David Chandler invented a two-sided planisphere where the back/flip side shows the southern sky with the distortion corrected to give a more accurate view.

A planisphere will help you to either satisfy your curiosity and identify that group of stars you've been eyeing the past few nights or learn the sky well enough to become a real naked eye astronomy whiz. It's all up to you. You can go as far as you want. And since the starry sky changes so slowly, you can use a good planisphere, like Chandler's The Night Sky, for your whole life and then pass it on to your great grandchildren to use. A thing of beauty is a joy forever.