http://www.stellarium.org
Stellarium is a free open source planetarium for your computer. Stellarium is an open source desktop planetarium for Linux/Unix,
Windows and MacOSX. It renders the skies in realtime using OpenGL,
which means the skies will look exactly like what you see with your
eyes, binoculars, or a small telescope. Stellarium is very simple to
use, which is one of its biggest advantages: it can easily be used by
beginners. It includes realistic atmosphere, sunrise, sunsets,
and over 150,000 stars with info. It also lists constellations from
many different cultures, and you can even add your own custom
constellations. Check out their
amazing screenshot
gallery.
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