A Planisphere

The planisphere is an analog computer for calculating the positions of the stars. The name refers to the representation of the celestial sphere on the flat plane of the page. Because of the motion of the earth, the appearance of the sky changes both with the hour of the day and the time of the year, as well as your location on the earth. A single printed star chart cannot capture all these combinations; for completeness, you need a book of them. On the other hand, at the twist of a dial a planisphere can show the positions of the stars at any time, on any day, for a particular latitude, so it is as good as a whole book of star charts.

This Web document includes planispheres for use at 40° N, 45° N or 50° N latitude, that you can download, print, and assemble yourself.

DOWNLOAD AND PRINT

Download and print the bottom plate, and choose the top plate for the latitude closest to your own. (You might find it instructive to download more than one top plate and see how they differ.)

NOTE: If the edges get cut off when you print, try turning on the "auto-rotate and center pages" option when you print (in Acrobat Reader).

INSTRUCTIONS FOR ASSEMBLY

Photocopy the top plate onto a transparency (or print directly on a transparency, if you can). The photocopy may not be exactly the same size as the original; if not, make a plain paper copy of the bottom plate so that it is reduced or enlarged by the same amount. Glue the bottom plate onto a sheet of cardboard. Cut off everything outside the outermost circle on both plates. Attach the plates with a pin through the center (marked by an asterisk) so they are centered and can rotate freely. (The mark at the center of the bottom plate may be hard to spot: it is almost on top of the North Star, Polaris.) Bend the pin so the point lies flat against the back of the bottom plate, and tape it there. Your assembled planisphere should resemble the figure at the top of this page.

INSTRUCTIONS FOR USE

1. To see what the stars look like at a particular time on a particular day, dial the planisphere so that the time on the inner ring lines up with the date on the outer ring. (The times marked are Standard Time; during Daylight Savings Time, the clock reads one hour later than the planisphere.) The entire portion of sky visible at that time on that day will appear through the window.
2. Bigger dots represent brighter stars. (Don't look for big dots in the sky. All stars appear as points of light in all but the very most advanced telescopes.)
3. If you hold the planisphere over your head, it will show you basically what you see in the sky. The oval represents the horizon. Stars further away from the edge appear higher in the sky. The zenith, the point directly overhead, is marked with a "Z". Thus, those stars between the part of the oval marked "East" and the point marked "Z" will appear in the eastern sky between the horizon and the zenith.
4. The planisphere only shows stars brighter than magnitude 4. If you are far from city lights, you will be able to see many, many more stars than are shown on the planisphere! To orient yourself, try to concentrate on the brighter stars and the constellations that are easiest to identify, such as the Summer Triangle (the stars Vega, Deneb and Altair), the Square of Pegasus in fall, Orion in winter, the Big Dipper in spring, or any other constellations you know. Then you can try to identify dimmer stars and constellations by their relation to the stars you already know.
5. Due to the limitations inherent in making a flat map of the celestial sphere, constellations near the southern horizon are stretched along the horizon in the representation of the planisphere.
6. The Sun, Moon and planets are not shown on the planisphere, because they do not always appear in the same part of the sky. However, they always stay near the dashed line indicating the circle of the ecliptic. If you see a relatively bright object near the ecliptic that isn't moving, doesn't twinkle and is not marked on the planisphere, it is probably a planet.

This site has been selected as a valuable educational Internet resource for Discovery Channel School.

To Washington and Jefferson College web page.

Copyright © 1997 M. S. Pettersen
Permission is granted to make copies for individual use, not for redistribution.
This document was last updated March 13, 2003.