11.2 Why the Moon Changes Shape

The Moon’s shape never really changes — it’s always a sphere. What changes is how much of its sunlit half we can see from Earth.

The Moon Borrows Its Light The Moon gives off no light of its own — it shines by reflecting sunlight. The half of the Moon facing the Sun is always lit; the half facing away is dark. As the Moon revolves around the Earth, we see different amounts of its lit half — and that is what makes the phases.

Activity 11.2 — A ball, a torch, and your head In a dark room, your head is the Earth, a torch is the Sun, and a ball on a stick is the Moon. Hold the ball opposite the torch (behind your head): you see its whole lit face — a full Moon. Hold it toward the torch: you see only its dark side — a new Moon. In between, the line dividing light and dark looks curved — exactly like the crescent and gibbous Moon!

Slide through a lunar month below and watch the phase change.

Moon phase explorer
Slide through ~29.5 days and watch the Moon change.
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Drawing it all together

What Causes the Phases As the Moon orbits Earth (about once a month), the angle between the Sun, Moon and Earth keeps changing, so the fraction of the lit half we see keeps changing — giving the cycle of phases. On new Moon day the Moon is closest to the Sun in the sky; on full Moon day, farthest — just as Activity 11.1 showed.

And the 50-minute daily delay? The Moon moves a little further along its orbit each day, so the Earth must rotate a bit extra (about 50 minutes’ worth) before the Moon returns to the same spot in the sky.

A Very Common Mistake Phases are NOT caused by the Earth’s shadow falling on the Moon! That’s a popular but wrong idea. Phases come from the changing Sun–Moon–Earth orientation. The Earth’s shadow on the Moon is something different — a lunar eclipse (which only happens at full Moon; a solar eclipse only at new Moon). Eclipses don’t happen every month because the Moon’s orbit is slightly tilted.