Big questions.
Hands-on answers.
Ten free astronomy simulations and calculators to make the invisible visible. Pick an experiment and start changing the universe — no sign-up, no download.
10 experiments to explore
Orbit simulator
Change the shape of an orbit and discover why planets move faster near their star.
Open experiment Solar systemMoon phase explorer
Move through a lunar month and see how sunlight creates the Moon’s familiar phases.
Open experiment Solar systemGravity & escape velocity
Build a world and calculate its gravitational pull, your weight, and its escape speed.
Open experiment Deep spaceLight travel time
Discover how long light takes to cross the solar system and the distances between stars.
Open experiment StarsStar color & luminosity
Explore how a star’s temperature and radius change its color, peak wavelength, and luminosity.
Open experiment ObservingTelescope calculator
Match telescope and eyepiece to calculate magnification, exit pupil, and approximate field of view.
Open experiment Deep spaceExoplanet transit
Watch a planet cross a star and reveal the dip in brightness astronomers look for.
Open experiment StarsStellar parallax
Turn a star’s annual parallax into a distance in parsecs and light-years.
Open experiment Deep spaceBlack hole radius
Calculate the Schwarzschild radius for a black hole of a chosen mass.
Open experiment ObservingStellar brightness
Compare the apparent brightness of two objects using the astronomical magnitude scale.
Open experimentNo experiments match that search. Try “Moon” or choose another topic.