Move the Moon around Earth and compare two viewpoints at once. Make a crescent, then tilt the orbit to discover why an eclipse is a special alignment.
Enable JavaScript to change the conditions and run the interactive experiment.
The Sun lights roughly half the Moon. As the Moon goes around Earth, we see different amounts of that lit half. Ordinary phases do not need Earth’s shadow.
Can you make a crescent without Earth’s shadow covering the Moon?
The Moon’s own day/night boundary makes ordinary phases. Earth’s shadow is involved in a lunar eclipse, a separate alignment.
In the space model the Sun is far to the right, and its light direction stays fixed. Move the Moon and watch which part of its sphere faces the Sun. The adjacent disk always shows the view from Earth, regardless of your orbit camera.
At new Moon the Moon is roughly sunward and its near side is mostly dark. At full Moon it is roughly opposite the Sun. Quarter phases mean half the visible disk is illuminated, not that only a quarter of the whole Moon receives sunlight.
The lunar orbit is tilted by about 5° relative to Earth’s orbital plane. New and full positions often pass above or below a shadow alignment. Move the line of nodes to compare an ordinary conjunction with an eclipse opportunity.
For a distant Sun, the illuminated disk fraction is (1+cos α)/2, where α is the Sun–Moon–observer angle at the Moon. The implementation derives this from the Moon direction. It is a geometric area fraction, not calibrated photographic brightness.
The displayed Earth and Moon are enlarged for readability. The separate alignment check uses Earth radius 1, Moon radius 0.2727 and separation 60.3, based on representative NASA/JPL sizes and distance. Camera changes do not change those numbers.
The alignment check uses parallel sunlight and projected disk overlap. A finite Sun makes real umbra and penumbra geometry more complex. This model does not classify total, partial or annular eclipses, predict dates, local visibility or durations. The phase cycle is about 29.5 days; the playback has no calendar.
Place the lamp securely. Use the ball as the Moon and your viewing position as Earth. Never use the Sun or look into a bright lamp.
Hold the ball at a comfortable distance. Turn to observe different angles of its lit half. Keep it slightly above your head’s shadow for ordinary phases.
Separately let your head’s shadow fall on the ball. Name the difference between a phase and this shadow event. Sketch both.
Can one light and one ball explain the changing shape?
Not to scale. Reflected room light and nearby lamplight soften the boundary. No Sun or eclipse viewing is required.
NASA explanation of illumination, phase order and the roughly 29.5-day cycle.
NASA · Moon phasesNASA alignment and shadow explanation.
NASA · EclipsesJPL astronomical parameters; representative model ratios are rounded.
JPL · Planetary parametersJPL satellite physical parameters; volume-equivalent mean radius.
JPL · Moon radiusMean distance about 384400 km and mean radius about 1737.4 km; these are not a date-specific ephemeris.
NASA · Moon factsIndependent subject review is pending.
Read the sources and model assumptionsSlide an object toward a lens. Follow the rays, find a sharp image, and discover the moment a camera becomes a magnifier.
Interactive explorationPull a ridge apart over millions of years. Read the age and magnetic pattern left behind, then compare spreading, sliding and subduction.
Release a few virtual balls, then a thousand. Change the chance of a right turn and compare the results with a mathematical prediction.