Why Does the Moon Have Phases? (It's Not Earth's Shadow)

Ask people what causes the Moon’s phases, and a common answer is Earth’s shadow falling across it. It’s a natural guess, and it’s wrong. Earth’s shadow only reaches the Moon during a lunar eclipse, a few times a year at most.

The real reason

The Sun always lights up half of the Moon, the half facing the Sun, just as it always lights up half of the Earth (that’s where it’s daytime).

As the Moon orbits the Earth, about once every 29.5 days, we see that sunlit half from different angles:

• New moon: the Moon is roughly between the Earth and the Sun. Its lit half faces away from us, so we see almost nothing.
• Crescent: a week or so either side of new moon, we see a thin edge of the lit half.
• First and last quarter: the Moon is at right angles to the Sun, as seen from Earth. We see half of the lit half: the Moon looks like a half circle.
• Gibbous: we see most of the lit half.
• Full moon: the Moon is on the far side of the Earth from the Sun. The whole lit half faces us.

Then the cycle runs backwards: waning gibbous, last quarter, waning crescent, and new moon again.

Try it at home

Stand in a dark room with a single lamp on one side. Hold a ball at arm’s length and turn slowly on the spot, watching the ball. You’ll see it go through every phase, crescent to full and back, with no shadow involved. You’re the Earth, the ball is the Moon, and the lamp is the Sun.

Why isn’t every full moon an eclipse?

If a full moon happens when the Moon is on the far side of the Earth from the Sun, why doesn’t Earth’s shadow cover it every month?

Because the Moon’s orbit is tilted by about 5 degrees compared with Earth’s orbit around the Sun. Most months, the full moon passes a little above or below Earth’s shadow. Only when a full moon happens near where the two orbits cross do we get a lunar eclipse.

The far side, not the dark side

The Moon spins exactly once for every orbit, so the same side always faces Earth. The side we never see is often called the “dark side”, but it gets just as much sunlight as the near side. At new moon, when the side facing us is dark, the far side is fully lit. “Far side” is the better name.

Which side is lit?

From the northern hemisphere, a waxing (growing) Moon is lit on the right, and a waning (shrinking) Moon on the left. From the southern hemisphere, it’s the other way round, because people there see the sky upside down compared with the north.

Try it

Move the Moon around its orbit above and watch the phase change.

The free space test covers the Moon’s phases, the dark side and eight more things we picture wrongly about the sky.