Astronomy, considered as one of the oldest sciences, is the scientific study of celestial objects (such as stars, planets, comets, and galaxies) and phenomena that originate outside the Earth's atmosphere (such as the cosmic background radiation). With the aid of powerful telescopes, Astronomers were able to study our vast skies. Since accurate and reliable software related to Astronomy are plenty these days, anyone can now have an opportunity to learn more about Astronomy without the need for telescopes. I’ve collected here a list of well-recognized free/open-source Astronomy software. I hope this helps:
*Celestia is a 3D astronomy program created by Chris Laurel. The program, based on the Hipparcos Catalogue, allows the user to display objects ranging in scale from artificial satellites to entire galaxies in three dimensions using OpenGL, from perspectives which would not be possible from a classic planetarium or other ground based display.