Celestia

Celestia is a free, three-dimensional astronomy program that allows the user to view objects ranging in size from artificial satellites to full galaxies in three dimensions, using OpenGL technology. Unlike most other virtual planetariums, the user is free to travel through the universe. Celestia displays over 120,000 stars based on the HIPPARCOS catalog. The program uses the very accurate VSOP87 trajectory calculation system, making it easy to see certain solar and lunar eclipses, and to display the orbits of planets (including exoplanets), their satellites, asteroids, comets and spacecraft.

Users can change the number of visible stars and display them in three different styles: - as dots (most realistic for LCD monitors) - as fuzzy dots (good for CRT monitors) - as disks (not realistic, but much easier to pick the right star or determine what color it is). Celestia lets you fly through the virtual universe with simple controls. You can move at different speeds, from 0.001 meters per second to several million light years per second. The current speed is displayed in the lower left corner. Users can view any object from any angle, from spaceships to galaxies. They can simply watch them move, follow, or rotate in sync with them.

Celestia can simulate any time in the past, present and future for many millions of years in different directions, but the accuracy of the orbits is maintained between a few thousand years and the present day. Time can also be reversed or stopped altogether. Hovering the cursor over any object displays basic information about its size, distance from the central object of the system (if any), temperature, and the current distance from it. Names of important surface objects can be displayed on planets and satellites. Celestia displays planet and satellite atmospheres, moving clouds, eclipse shadows, sunsets and dawns, detailed object surface structures, planetary rings, night light of cities on Earth, constellation lines, their boundaries and illustrations.