stellarium

Stellarium turns your screen into a planetarium. Give it a date and a location and it renders the sky as you would actually see it, naked eye, binoculars, or through a specific telescope and eyepiece you configure. Windows, macOS, Linux, and Haiku, shipped as a normal desktop download under GPL-2.0. No account, no tier, no upsell.

Depth is what separates it from a phone star app. It carries multiple sky cultures rather than only the Greek constellations, renders 3D landscapes so the horizon matches your actual site, and its plugin system includes real telescope control. Amateurs use it to plan a session before going outside. Schools use it because putting it on thirty machines costs nothing.

Install is a download and a double click. Nothing to host, nothing to maintain, no ops burden whether you are one person or a whole department.

The catch is the mobile story. The desktop app is free and open source, but the phone app is a separate product from Noctua Software, not from this repository. Stellarium Mobile is free to download and the Plus upgrade is an in-app purchase at $18.99, with a cheaper recurring option. Free on your laptop does not mean free in your pocket.