Moon Phases Explained: Why the Moon Changes Shape

Ask a room of adults why the moon has phases and many will say "Earth's shadow". It's one of the most common science misconceptions — and it's wrong. Earth's shadow only falls on the moon during a lunar eclipse, which happens a few times a year at most. Phases happen every month.

The real reason

The sun always lights half of the moon, just as it always lights half of Earth (that's day and night). As the moon orbits Earth about once a month, we see that sunlit half from different angles:

• When the moon is between Earth and the sun, the lit half faces away from us — a new moon.
• When Earth is between the sun and the moon (but not exactly in line), the whole lit half faces us — a full moon.
• In between, we see part of the lit half — crescents, quarters and gibbous moons.

The eight phases in order

• New moon — dark (or nearly invisible).
• Waxing crescent — a thin sliver, growing.
• First quarter — half the disk lit (we are a quarter of the way through the cycle, which is why it's called a quarter).
• Waxing gibbous — more than half, still growing.
• Full moon — the whole disk lit.
• Waning gibbous — more than half, shrinking.
• Third (last) quarter — the other half lit.
• Waning crescent — a thin sliver before the next new moon.

The full cycle from one new moon to the next takes about 29.5 days.

Waxing or waning?

Waxing means growing; waning means shrinking. From the Northern Hemisphere, a waxing moon is lit on the right, a waning moon on the left. A memory trick: the lit part makes the shape of a D as it grows ("developing") and a C as it closes. In the Southern Hemisphere it is the other way round, because people there see the moon "upside down" compared with the north.

The lamp-and-ball demo

• In a dark room, put a lamp without a shade at one side to be the sun.
• A student holds a ball (the moon) at arm's length, slightly above their head. Their head is Earth.
• They turn slowly in a circle, keeping the ball in front of them.
• Everyone watches the lit part of the ball change from new, to crescent, to quarter, to full and back.

Ask: "Did the lamp ever stop lighting half the ball?" The answer — no — is the whole idea.

Eclipses

A solar eclipse happens at new moon, when the moon passes directly between the sun and Earth. A lunar eclipse happens at full moon, when Earth's shadow falls on the moon. We don't get eclipses every month because the moon's orbit is tilted about 5° compared with Earth's, so the three bodies usually don't line up exactly.

In NGSS, moon phases, eclipses and seasons are part of middle-school Earth and space science (MS-ESS1-1), built on the Grade 1 study of patterns in the sky.