Why does the Moon change shape?

The Moon isn't changing shape: discover how sunlight and our viewing angle create lunar phases.

The short answer

The Moon stays roughly spherical. As it orbits Earth, we see different portions of its sunlit half. Those changing views create the familiar phases: crescent, quarter, gibbous, and full.

Is Earth's shadow causing the phases?

Usually, no. Earth's shadow falls on the Moon during a lunar eclipse, which is different from the ordinary monthly cycle of phases.

The surprising connection: geometry in the sky

Moon phases are a simple demonstration of geometry. The positions of the Sun, Earth, and Moon determine what portion of the illuminated lunar surface we can see.

Try it

In a darkened room, shine a lamp on a ball. Walk around it and notice how the bright portion appears to change from your point of view.