Stellarium

Stellarium draws the sky as it appears from a chosen point on Earth at a chosen moment. It renders stars from bundled catalogues, the planets and their moons, comets and asteroids, and deep-sky objects, and it simulates the atmosphere: light scattering, twilight colours, and the refraction that lifts objects sitting near the horizon. A ground panorama is drawn beneath the sky, so the view resembles what an observer would face.

Time is a control rather than a constant. The clock can be paused, wound backwards or accelerated, which is how eclipses, conjunctions, transits and the drift of planets over months are examined. Constellation lines, figures and star names are supplied for a number of sky cultures, not only the Greek-derived Western set.

Plugins push it towards practical observing: an eyepiece simulator that frames the field as a given telescope with a given eyepiece or camera sensor would, satellite tracking computed from orbital elements, and telescope control that slews a connected mount to the selected object. A red night-vision mode keeps the screen usable in the dark.