Why the Moon changes shape

Why does the Moon look different almost every night?

Here is the Moon through the month: a thin curve, a half, a full circle, then shrinking again.

Here is the surprise: the Moon never really changes shape. It is always a round ball.

• Always round!

The Moon makes no light of its own. It only reflects light from the Sun, like a mirror.

Sunlight always lights up half of the Moon, the half that faces the Sun. The Moon travels around the Earth, taking about one month to go all the way around.

As it moves, we see different amounts of its lit half from Earth. That is the changing shape.

When the Moon is between us and the Sun, its lit side faces away. We see almost nothing. That is the new moon. When the Moon is on the far side of Earth, we see its whole lit face. That is the full moon. In between, we see a slice of the lit half. It grows, then shrinks, in a steady pattern.

Then it shrinks again, and the whole cycle repeats about every twenty nine days.

Some people think Earth's shadow causes the phases. It does not. That only happens in a rare eclipse.

• Phases are not shadow

Quick check to see if it clicked. The Moon is always a ball, and its phases come from how much sunlit half we see as it orbits.