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

Ask people what causes the Moon’s phases and a common answer comes back quickly: Earth’s shadow. It sounds right. The Moon has a dark part, Earth is nearby, Earth casts a shadow, done.

It’s wrong, and it’s wrong in a way that’s easy to check from your back garden. The real explanation is simpler, and once you’ve seen it you can predict when the Moon will rise, which way the crescent will face and why you can sometimes see the Moon in a bright blue sky.

Half the Moon is always lit

The Sun lights the Moon the same way it lights the Earth: one half is in daylight, the other in night. That never changes. At any moment, exactly half the Moon is lit.

The phases are about how much of that lit half faces us.

As the Moon orbits the Earth, about once every 29.5 days, we see the 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 few days 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, so the Moon looks like a half circle. (Confusingly, a “quarter” Moon looks half lit, because it’s a quarter of the way round its orbit.)
• Gibbous: we see most of the lit half.
• Full moon: the Moon is on the far side of Earth from the Sun. The whole lit half faces us.

Then the cycle runs backwards: waning gibbous, last quarter, waning crescent and back to new. Move the Moon around in the toy at the top and you can see each phase appear as the angle changes.

Why it can’t be Earth’s shadow

There’s a clean way to rule out the shadow. For Earth’s shadow to fall on the Moon, Earth has to be between the Sun and the Moon. That puts the Sun and the Moon on opposite sides of the sky.

But you can often see a half Moon in the afternoon while the Sun is still up. Both are on the same side of us. Earth isn’t in between, so there can’t be any shadow. The dark part of a half Moon is simply the Moon’s night side, turned partly towards us.

A second clue is the shape. Earth’s shadow is round, and if it fell on the Moon its edge would be a curve bending the same way at every phase, like a bite taken out. The boundary of a crescent bends the other way: it’s the curved line where day turns to night on a sphere, seen from the side.

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. Your head plays the part of the Earth, the lamp the Sun, the ball the Moon.

You’ll also see why the full moon is opposite the Sun: when you face away from the lamp, the ball in front of you is fully lit. And you’ll see why the new moon is invisible: when the ball is between you and the lamp, its lit side faces the lamp, not you. (Hold it slightly above your head’s shadow so that you don’t accidentally make an eclipse.)

The Moon’s timetable

Because the Moon goes round the Earth once a month while the Earth spins once a day, the Moon falls about 50 minutes further behind the Sun each day. That’s why its phase is tied to when it rises.

• A new moon rises and sets with the Sun, so it’s lost in daylight.
• A first-quarter Moon rises around midday and sets around midnight. It’s the one you see in the afternoon.
• A full moon rises at sunset and sets at sunrise, up all night.
• A last-quarter Moon rises around midnight and is still up in the morning, which is why early risers see it in the blue sky on the way to work.

Once you know this, you can do a small trick: look at the Moon and, from its shape, say roughly what time it is.

Which way is it lit?

From the northern hemisphere, a waxing (growing) Moon is lit on the right, and a waning (shrinking) Moon on the left. A handy rhyme is that a Moon shaped like a backwards C is coming (waxing), and like a C is going. From the southern hemisphere it’s the other way round, because people there look at the Moon “upside down” compared with the north.

Near the equator, a crescent lies on its back like a smile or a boat, because the Sun sets almost straight down below the horizon and the lit edge points down at it. The farther from the equator you live, the more the crescent tips up on its side.

The ghost on the dark side

On a clear evening a few days after new moon, look carefully at a thin crescent. You can often see the rest of the Moon faintly glowing, a ghostly full disc with a bright edge. That glow is earthshine: sunlight that has bounced off the Earth, travelled to the Moon and bounced back to you.

Leonardo da Vinci worked this out in the early 1500s and described it in one of his notebooks. Seen from the Moon at that moment, Earth would be nearly full and very bright, so there’s plenty of light to bounce.

The far side, not the dark side

The Moon spins exactly once for every orbit, so the same side always faces Earth. Over billions of years, Earth’s gravity slowed its spin until it matched, a process called tidal locking.

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 we see nothing, the far side is in full daylight. Nobody saw it at all until 1959, when the Soviet probe Luna 3 photographed it, and the first landing on it was China’s Chang’e 4 in 2019.

Supermoons and blue moons

Two terms crop up in the news. A supermoon is a full moon that happens near the closest point of the Moon’s slightly oval orbit. It looks up to about 14% wider and 30% brighter than a full moon at the farthest point, a real difference that’s hard to notice without a side-by-side photo. A blue moon, in the modern sense, is a second full moon in a single calendar month, which happens every two or three years, because a lunar month is a little shorter than most calendar months. It isn’t blue.

Neither has anything to do with shadows. Both are just the same lit half, seen from a slightly different distance or on a slightly awkward date.

So when does Earth’s shadow matter?

Earth’s shadow does fall on the Moon sometimes: during a lunar eclipse. That can only happen at full moon, when Earth is between the Sun and the Moon. During a total lunar eclipse the Moon doesn’t vanish but turns a deep coppery red, because Earth’s atmosphere bends a little red sunlight round into the shadow. It’s every sunrise and sunset on Earth, projected onto the Moon.

But if eclipses happen at full moon, why isn’t every full moon an eclipse? 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 one of the two points where the orbits cross do we get an eclipse, which is why they come a few times a year rather than every month.

Test yourself

The Moon’s phases are the first question in the free space test, alongside other things about the sky that most of us picture wrongly. If you liked this, what causes the seasons takes apart another sky myth with a lamp and an orange.