What are some alternatives?
When comparing Celestia and Space Engine, you can also consider the following products
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
Gaia Sky - 3D visualization software for ESA's (European Space Agency) Gaia mission to chart about 1...
KStars - KStars is a Desktop Planetarium for KDE.
Universe Sandbox - Universe Sandbox ² is a physics-based space simulator.
Cosmonium - Cosmonium is a 3D astronomy and space exploration program.