Eclipse Geometry: Shadows in Space
draft readiness: experimental EarthSky Both 12 min
Core demo behavior is implemented, but parity and launch-gate signoff are still pending.
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Predict
Predict
If the Moon orbits Earth once per month, why don’t we have eclipses every month?
Play
Play
- Explore alignments and note when an eclipse is possible.
- Change the tilt/offset and see how eclipse likelihood changes.
- Compare solar vs lunar eclipse conditions.
Explain
Explain
Use alignment and orbital tilt to explain why eclipses are rare.
Learning goals
- Explain eclipses using alignment and shadow geometry.
- Distinguish umbra vs penumbra in a qualitative way.
- Explain why eclipses do not occur every month.
Misconceptions targeted
- Eclipses happen every time there’s a new or full Moon.
Model notes
- This is a simplified geometric model: eclipses require (1) syzygy (New/Full) and (2) the Moon near a node (small $|\beta|$).
- Ecliptic latitude is computed from orbital tilt $i$ and distance from the ascending node $\Omega$: $$\beta = \arcsin\!\big(\sin i\ \sin(\lambda_M - \Omega)\big).$$
- Eclipse “how close is close enough?” thresholds come from a physically motivated shadow-cone model (similar triangles).
- Earth–Moon distance is selectable (km) and affects eclipse type (e.g., central solar eclipses can be total at perigee-like distances and annular at apogee-like distances).
- Interactive outcomes use a pedagogical tolerance ($\Delta$ within $5^\circ$ of New/Full); the long-run simulation uses constant-rate angle evolution to show eclipse seasons.
About this demo
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 $|\beta|$ is small.
Use the distance presets to connect eclipse geometry to angular size: for central solar alignment, the same geometry can yield total or annular eclipses depending on Earth–Moon distance.