Moon Phases: Light, Not Shadow
Key idea: A short, interactive model for relating phase angle to what fraction of the Moon appears illuminated.
A short, interactive model for relating phase angle to what fraction of the Moon appears illuminated.
Earth & Sky
Moon phases, seasons, eclipses, and the geometry of what we see from Earth. These demos build intuition for how angles, distances, and illumination combine to produce the sky we observe.
6
exhibits
Suggested sequence
Not mandatory, but this order builds concepts incrementally.
Exhibits
Filter these in ExploreSeasons: Why Tilt Matters
Key idea: 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
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
Angular Size: The Sky’s Ruler
Key idea: This demo explores why objects can look big or small in the sky depending on both their physical size and their distance.
This demo explores why objects can look big or small in the sky depending on both their physical size and their distance.
Eclipse Geometry: Shadows in Space
Key idea: 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
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: Explore how conjunctions and oppositions are line-of-sight alignments caused by relative motion, and estimate how long it takes for the same
Explore how conjunctions and oppositions are line-of-sight alignments caused by relative motion, and estimate how long it takes for the same
Retrograde Motion: Apparent Longitude from Relative Motion
Key idea: 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
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
Key concepts
Learning goals aggregated from all exhibits in this topic.
- Relate an object’s apparent size to its physical size and distance.
- Interpret small angles as a practical measurement tool in astronomy.
- Explain why nearby objects can look larger than distant ones.
- Explain eclipses using alignment and shadow geometry.
- Distinguish umbra vs penumbra in a qualitative way.
- Explain why eclipses do not occur every month.
- Explain why the Moon has phases using the Sun–Earth–Moon geometry.
- Connect phase angle to the fraction of the Moon that appears lit.
- Distinguish illuminated fraction from how much of the Moon you can see.
- Define conjunction and opposition as seen from Earth.
- Explain why conjunctions repeat using Earth’s motion relative to another planet.
- Use the model to estimate the time between successive conjunctions (synodic period).
- Define retrograde motion as an apparent reversal caused by viewing geometry and relative motion.
- Interpret apparent (sky) longitude $\lambda_{\mathrm{app}}$ as the direction from an observer planet to a target planet in an inertial frame.
- Identify stationary points as times when $d\tilde{\lambda}/dt = 0$ and connect them to the start/end of retrograde.
- Explain why axial tilt causes seasons.
- Relate sun angle/day length to heating.
- Distinguish tilt-driven seasons from distance myths.