What are some alternatives?
When comparing Cosmonium and Celestia, you can also consider the following products
Space Engine - Space Engine is a realistic virtual Universe you can explore on your computer.
Gaia Sky - 3D visualization software for ESA's (European Space Agency) Gaia mission to chart about 1...
Stellarium - Stellarium is a free open source planetarium for your computer.
Solar System Scope - 3D model of the Solar System and night sky in real time
Universe Sandbox - Universe Sandbox ² is a physics-based space simulator.
KStars - KStars is a Desktop Planetarium for KDE.