a planisphere, or star wheel, you turn a disk to set your time against
your date. The edge of the star map then represents the horizon all around
you at that time. Some planispheres come with extra features. The most
important aspect of a planisphere, however, is the clarity and realism of
its star map.
Among the many devices on the market,
The Night Sky takes the prize in this regard, and features star maps on
both sides, one minimizing distortion in the north and the other in the
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.
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.
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.
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, and you're all set!
Once you understand the workings of a planisphere, you can "dial in" any constellation visible from your hemisphere and then look at the edge to see when that constellation is visible. Constellations that are visible all-year are known as Circum-Polar, because they seem to spin around the north star Polaris, and never set completely below the horizon. Most other constellations are prominent during a particular season of the year. For example, Orion the Hunter is very easily seen in Winter, but in Summer, it is hidden in the sun's glare and is up during the day.
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