Create Your Own Planisphere with Uncle Al's Sky Wheels

Uncle Al's Sky Wheels
Here is an astronomical tool that will help you find constellations of stars and other things in the sky. All you need to do is print out the following:
and the two star wheels currently available:
After you print them, cut them out and assemble them following the directions that are on the printout. Once you have that in hand, directions below on how to use it will make sense...
Using a Planisphere
In principle nothing could be simpler. You turn a wheel to put your time next to your date, and presto, there's a custom-made map of the stars that are above your horizon for that moment. The edge of the oval star map represents the horizon all around you, as you would see if you were standing in an open field and turned around in a complete circle. The part of the map at the oval's center represents the sky overhead -- much like the all-sky constellation map in each month's issue of Sky & Telescope.
In practice, several complications can throw beginners off. The worst is that a planisphere's map is necessarily small and distorted. It compresses the entire celestial hemisphere above and around you into a little thing you hold in your hand. So star patterns appear much bigger in real life than on the map.
Moving your eyes just a little way across the map corresponds to swinging your gaze across a huge sweep of sky. The east and west horizons may look close together on a planisphere, but of course when east is in front of you west is behind your back. Glancing from the map's edge to center corresponds to craning your gaze from horizontal to straight up.
There's only one way to get to know a map like this. Hold it out in front of you as you face the horizon. Twist it around so the map edge labeled with the direction you're facing is down. The correct horizon on the map will now appear horizontal and match the horizon in front of you. Now you can compare stars above the horizon on the map with those you're facing in the sky.
Then there's the distortion issue. On a planisphere designed for use in the Northern Hemisphere, constellations in the southern part of the sky are stretched sideways, taffy-like, making it hard to compare them with real star patterns. This problem does not exist on a well-designed map for fixed dates and times, such as the one in the center of each month's Sky & Telescope. Some planisphere designers have come up with a partial solution. David Chandler's planisphere The Night Sky presents two maps, one on each side. One minimizes distortion north of the celestial equator, the other south of it. Just flip it over for the best view.
A further complication is that a planisphere works correctly for only one latitude on Earth. Most today are made in several editions, each for a particular latitude.
Then there's the matter of daylight saving time. When this is in effect (from the first Sunday in April to the last Sunday in October in most parts of the United States), remember to "fall back" to standard time by subtracting an hour from what your clock says before you set the planisphere's dial.
Actually, planispheres don't employ standard time either, but rather local mean time. The difference, which depends on where you live in your time zone, can amount to a half hour or more. Instructions for finding your local mean time correction are included with the Skygazer's Almanac in each January's issue of Sky & Telescope. Fortunately, even a half hour one way or the other doesn't really matter for most star finding.
In fact, if you just want to know which constellations are up and where they are, a planisphere's limitations can largely be overlooked. It's remarkable that such a simple working model of the sky can work so well.

© Copyright VCAS 2006-2011,    ||   comment send to Webmaster    ||   Website design by A Vescio