Using a Planisphere

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

In practice, several complications can throw beginners off.

First, sadly, many planispheres on the market are so poorly designed that they would frustrate an expert. The worst problem is carelessly drawn star maps that make all stars, whether very bright or very faint, look too much alike. Planispheres with gimmicky glow-in-the-dark stars are the worst offenders in this regard; the glow paint can't be printed very accurately, so all stars come out looking more or less alike. The result is star patterns that look nothing like what you'll see in the sky.

Other issues are inherent to even well-designed planispheres. One is that the map is necessarily small and distorted. It compresses the entire celestial hemisphere above and around you into a little thing you hold in your hand. So be aware that star patterns will appear much bigger in real life than on the map.

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

There's only one way to get to know a map like this. Hold it out in front of you as you face the horizon. Twist it around so the map edge labeled with the direction you're facing is at the bottom. The correct horizon on the map will now be horizontal to match the horizon in front of you. Now you can compare stars above the horizon on the map with those you're facing in the sky. Ignore all the parts of the map above horizons you're not facing.

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