Planisphere
Appearance
![](http://upload.wikimedia.org/wikipedia/commons/thumb/9/97/Planisphere.jpg/220px-Planisphere.jpg)
In astronomy, a planisphere is a projection of the celestial sphere onto a plane. Because of the projection, certain parts of the celestial sphere cannot be shown. Hipparchus discovered that it was possible to do a projection which keeps the angles intact.