Stellarium along with Celestia is one of the most interesting piece of software for those interested in Astronomy. Those who like to visit the planetarium will possibly love this program, especially given the fact that it is free (licensed under GPL). It brings the power of a panetarium (minus the viewing area) to your computer. Given the kind of light pollution at night in urban areas, Stellarium serves as an exploration tool for Astronomy enthusiasts.
Stellarium renders realistic skies in real time, enabling you to explore the sky sitting right in front of your computer. It gives you a number of options to customise what you see. You can choose your geographic location, & it renders the sky accordingly. It also lets you adjust the rate at which time elapses, so that you can rewind or fast forward the time.
The view is basically from the earth, & it allows you to zoom in on objects (like planets & satellites) to get a closer look, just like when using a telescope. You can also toggle the rendering of the atmosphere, to get a better view of heavenly bodies when the sun is shining. In fact, you can even watch the sun rise & set, & all other bodies move too. The rendering of the atmosphere gives you the different lighting effects due to the sun, like the sky turning red & then dark as the sun sets.
You can also watch the constellations, & it also gives you the option to superimpose pictures (artistic impressions) on some of the common ones. There are numerous other features available in the software, & I haven’t managed to try out all of them yet.
The program can be run full screen or windowed. The system requirements are not very high (should run on machines about 5-6 yrs old), though the faster the better, as the program is quite processor intensive.
It might take you a while to get the hang of the controls, but once you do then you can really start exploring. The program is not very large, & at around 16 MB is a worthwhile download, especially if you are interested in Astronomy.
There is also a user guide available on the site which can help you a lot in setting up & using the program.