1 of 3 Orbits
Kepler’s Laws: Patterns of Planetary Motion
Describe how orbital speed changes along an ellipse.
Predict Where in an elliptical orbit do you think a planet moves fastest: near perihelion or aphelion?
Cosmic Playground
Play with the universe. Learn the physics.
Cosmic Playground is in active beta. Expect frequent improvements as demos are hardened.
New here? Answer two questions and we will point you at the right starting place.
How it works
Every demo follows the same three-phase cadence. Commit to a prediction, explore the model, then reconcile what happened.
Predict
Before touching a slider, commit to a prediction. What do you think will happen?
Play
Explore the interactive model. Drag, tweak, break it. See what the physics does.
Explain
Reconcile what happened with what you expected. That gap is where learning lives.
Start here
These three demos introduce the core orbital mechanics concepts. Work through them in order for the best experience.
2 of 3 Earth & Sky
Retrograde Motion: Apparent Longitude from Relative Motion
Define retrograde motion as an apparent reversal caused by viewing geometry and relative motion.
Predict If Earth is moving faster than Mars, what do you predict happens to Mars’s apparent direction in the sky near opposition?
3 of 3 Orbits
Binary Orbits: Dynamical Reasoning Lab
Use momentum conservation to explain why the lighter body moves faster around the barycenter.
Predict When decreases at fixed separation, predict how , , and change before revealing readouts.
Recently updated
Orbits Conservation Laws: Energy & Momentum Identify quantities that remain constant under specific assumptions.
Stars Cluster Census: Sampling Stars from the Initial Mass Function Star formation makes a population, not a star, and the heaviest star in a small cluster is mostly luck.
Stars Hydrostatic Equilibrium Explorer Explain hydrostatic equilibrium as a balance between inward gravity and outward pressure support.
Orbits Binary Orbits: Dynamical Reasoning Lab Use momentum conservation to explain why the lighter body moves faster around the barycenter.
Light & Spectra Doppler Shift of Light Relate radial motion to observed wavelength and frequency shifts in spectral lines.
Galaxies Galaxy Rotation Curves Interpret galaxy rotation curves as velocity-versus-radius observables inferred from Doppler measurements.
Light & Spectra Spectral Lines & the Bohr Atom Explain why atoms emit and absorb light only at specific wavelengths using the Bohr model.
Stars HR Diagram Inference Lab Use observer-space and theorist-space views to interpret stellar populations as measurement-built maps.
For instructors
Station cards, instructor notes, and curated playlists for classroom use.