Have you ever wondered what the night sky looks like on Mars? Or Pluto? Or Alpha Centauri? How about finding out the relative positions of the planets on March 28, 2073 (as a science fiction writer might want to do)? For simple Earth-based stargazing, you would want some kind of planetarium software. There are a lot of great resources out there for that, some free and some not.
However, if you want to really explore the universe from the comfort of your desk, Celestia is the way to go. Celestia is a free, open-source simulation program that creates a 4D map of planets, stars, galaxies, and more, computed in real time. It even includes spacecraft like the International Space Station. It’s a little harder to get up against Earth’s surface and look out, but it gives you a lot more freedom to fly around the Solar System and even the galaxy, to see what the constellations look like from light-years away, or to speed up or reverse time. That’s why a friend of mine has dubbed it, “Google Earth for the universe”.
Better yet, because Celestia is open-source, anyone can create addons with objects that aren’t in the regular database. These can be other spacecraft, asteroids, stars, or galaxies, better maps of planets, or even entirely fictional objects. Many popular addons are available here, and it’s pretty easy to write your own, too. So whether you want to see how other solar systems work or you want to create your own star charts, Celestia is worth a look.