لماذا يبدو القمر وكأنه يغيّر شكله

Ever seen the Moon look like a sliver one night and a circle another? Picture a flashlight shining on a ball. As the ball moves around you, the part that looks bright changes, but the ball stays round. The Moon’s changing looks are called phases.

The Sun is the flashlight, and sunlight reflects off the Moon. It lights half the Moon at any moment. As the Moon travels around Earth, we see different amounts of that lit half. The Moon isn’t changing shape; only our view of its sunlit side changes.

The view goes from a dark new moon to a crescent, a half moon, then full, before shrinking again. From one new moon to the next takes about 29.5 days. What makes the Moon’s visible shape change? We see different amounts of its sunlit half.