Kepler’s Laws: Patterns of Planetary Motion
Key idea: Describe how orbital speed changes along an ellipse.
Use this instrument to connect orbit shape to speed changes (Kepler 2) and connect orbit size to period scaling (Kepler 3).
Explore
An interactive museum of astronomy reasoning instruments.
Explore observables, models, and inferences—like doing experiments on ideas.
Play with the universe. Learn the physics.
Cosmic Playground is in active beta. A badge on an exhibit says how ready it is to assign; an exhibit with no badge is finished.
Start here
Looking for a guided path?Featured sequence
Begin with the core orbit ideas before branching out.
Retrograde Motion: Apparent Longitude from Relative Motion
Key idea: Define retrograde motion as an apparent reversal caused by viewing geometry and relative motion.
Use a simple Keplerian model to visualize how the direction to a planet can briefly reverse in the sky (retrograde) even though the planet n
Binary Orbits: Dynamical Reasoning Lab
Key idea: Use momentum conservation to explain why the lighter body moves faster around the barycenter.
This instrument turns binary motion into a reasoning workflow: predict first, test conservation constraints, connect orbital dynamics to rad
- Predict
- Play
- Explain
A simple loop for every exhibit.
Earth & Sky
Moon phases, seasons, and the view from Earth.
Angular Size: The Sky’s Ruler
Key idea: Relate an object’s apparent size to its physical size and distance.
This demo explores why objects can look big or small in the sky depending on both their physical size and their distance.
Moon Phases: Light, Not Shadow
Key idea: Explain why the Moon has phases using the Sun–Earth–Moon geometry.
A short, interactive model for relating phase angle to what fraction of the Moon appears illuminated.
Seasons: Why Tilt Matters
Key idea: Explain why axial tilt causes seasons.
This demo lets you change day-of-year, axial tilt, and latitude to explore how sun angle and day length change through the year in each hemi
Retrograde Motion: Apparent Longitude from Relative Motion
Key idea: Define retrograde motion as an apparent reversal caused by viewing geometry and relative motion.
Use a simple Keplerian model to visualize how the direction to a planet can briefly reverse in the sky (retrograde) even though the planet n
Eclipse Geometry: Shadows in Space
Key idea: Explain eclipses using alignment and shadow geometry.
This demo helps explain why eclipses do not occur every month: the Moon must be at New/Full *and* near a node so its ecliptic latitude $|\be
Planetary Conjunctions: Alignments in the Sky
Key idea: Define conjunction and opposition as seen from Earth.
Explore how conjunctions and oppositions are line-of-sight alignments caused by relative motion, and estimate how long it takes for the same
Orbits
Kepler’s laws, retrograde motion, and gravitational dynamics.
Conservation Laws: Energy & Momentum
Key idea: Identify quantities that remain constant under specific assumptions.
Start with a circular case ($v/v_{\rm circ}=1$), press Play on the Elliptical preset to watch $K$ and $U$ trade, then press Escape ($\sqrt{2
Binary Orbits: Dynamical Reasoning Lab
Key idea: Use momentum conservation to explain why the lighter body moves faster around the barycenter.
This instrument turns binary motion into a reasoning workflow: predict first, test conservation constraints, connect orbital dynamics to rad
Kepler’s Laws: Patterns of Planetary Motion
Key idea: Describe how orbital speed changes along an ellipse.
Use this instrument to connect orbit shape to speed changes (Kepler 2) and connect orbit size to period scaling (Kepler 3).
Retrograde Motion: Apparent Longitude from Relative Motion
Key idea: Define retrograde motion as an apparent reversal caused by viewing geometry and relative motion.
Use a simple Keplerian model to visualize how the direction to a planet can briefly reverse in the sky (retrograde) even though the planet n
Planetary Conjunctions: Alignments in the Sky
Key idea: Define conjunction and opposition as seen from Earth.
Explore how conjunctions and oppositions are line-of-sight alignments caused by relative motion, and estimate how long it takes for the same
Light & Spectra
Blackbody radiation, the electromagnetic spectrum, and spectral analysis.
Spectral Lines & the Bohr Atom
Key idea: Explain why atoms emit and absorb light only at specific wavelengths using the Bohr model.
Infer atomic structure from line wavelengths, then invert the task: observed $\lambda$ to inferred transition. The instrument links Bohr str
Electromagnetic Spectrum: Light Beyond Visible
Key idea: Order EM bands by wavelength and frequency.
This demo helps you connect wavelength, frequency, and photon energy across the electromagnetic spectrum.
Doppler Shift of Light
Key idea: Relate radial motion to observed wavelength and frequency shifts in spectral lines.
Explore how relative motion shifts spectral lines and why astronomers can measure radial velocity from light alone.
Blackbody Radiation: Thermal Spectrum and Temperature
Key idea: Relate temperature to Planck-curve shape and Wien peak shift.
Use the temperature slider (or presets) to compare where the spectrum peaks and how the overall power changes.
Telescopes
Resolution, diffraction, and optical design.
Telescope Resolution: Sharper Eyes
Key idea: Distinguish magnification from resolution.
This demo shows how telescope resolution depends on wavelength and aperture, and how atmospheric seeing can blur images from the ground.
Data & Inference
Statistics, measurement, and scientific reasoning.
Parallax Distance: Measuring the Stars
Key idea: Trace causality: Earth moves in orbit, line-of-sight changes, apparent star position shifts.
This demo makes the full causal chain legible: Earth moves, line-of-sight changes, and the target star appears to shift against fixed backgr
Galaxy Rotation Curves
Key idea: Interpret galaxy rotation curves as velocity-versus-radius observables inferred from Doppler measurements.
Explore why spiral-galaxy rotation curves stay nearly flat at large radius and what that implies about dark matter across scales.
Cluster Census: Sampling Stars from the Initial Mass Function
Key idea: Star formation makes a population, not a star, and the heaviest star in a small cluster is mostly luck.
Star formation does not make one star at a time. It makes a population, and the distribution of masses in that population — the initial mass
Doppler Shift of Light
Key idea: Relate radial motion to observed wavelength and frequency shifts in spectral lines.
Explore how relative motion shifts spectral lines and why astronomers can measure radial velocity from light alone.
Stars
Stellar structure, evolution, and the physics of stellar interiors.
Cluster Census: Sampling Stars from the Initial Mass Function
Key idea: Star formation makes a population, not a star, and the heaviest star in a small cluster is mostly luck.
Star formation does not make one star at a time. It makes a population, and the distribution of masses in that population — the initial mass
Hydrostatic Equilibrium Explorer
Key idea: Explain hydrostatic equilibrium as a balance between inward gravity and outward pressure support.
This ASTR 201 explorer turns stellar structure into a balancing act: gravity sets the required pressure gradient, pressure scale height tell
HR Diagram Inference Lab
Key idea: Use observer-space and theorist-space views to interpret stellar populations as measurement-built maps.
This ASTR 201 instrument treats the HR diagram as both a measurement-built map and a hidden-variable inference engine, with staged reveals a
Galaxies
Galaxy types, rotation, and large-scale structure.
Galaxy Rotation Curves
Key idea: Interpret galaxy rotation curves as velocity-versus-radius observables inferred from Doppler measurements.
Explore why spiral-galaxy rotation curves stay nearly flat at large radius and what that implies about dark matter across scales.
Cosmology
Expansion, dark energy, and the cosmic microwave background.
Galaxy Rotation Curves
Key idea: Interpret galaxy rotation curves as velocity-versus-radius observables inferred from Doppler measurements.
Explore why spiral-galaxy rotation curves stay nearly flat at large radius and what that implies about dark matter across scales.
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