Why the Moon Seems to Change Shape

The Moon makes no light of its own

The Moon is a ball of rock. It does not glow. Everything we see of it is sunlight bouncing off its surface, which is why it looks bright at night and rather pale in the afternoon sky.

Because the Sun is a long way off in one particular direction, it lights exactly one half of the Moon at any moment — the half facing it. The other half is in darkness. That is true all the time, tonight and every night, and it never changes.

What changes is where we are standing in relation to that lit half. The Moon goes round the Earth roughly once a month, so our view of it swings round too. Sometimes we look straight at the lit side and see a full circle. Sometimes we look at it from the side and see half of it. Sometimes we are almost behind it, looking at the dark side, and see nothing at all.

The eight phases, in order

The full cycle takes about 29.5 days, which is where the word month comes from. Astronomers give eight of the views names, though the Moon passes smoothly through every stage in between.

Two words are worth learning because they save a great deal of confusion. Waxing means growing — more of the lit half is coming into view each night. Waning means shrinking. And gibbous simply means more than half but not yet full. From the northern hemisphere the lit part grows from the right-hand side; from Australia or South Africa it grows from the left, because the observer is effectively upside down relative to a northern one.

• New Moon — the lit half faces entirely away from us, so we see nothing.
• Waxing crescent — a thin curve of light, growing night by night.
• First quarter — exactly half the disc is lit, with a straight edge down the middle.
• Waxing gibbous — more than half lit and still growing.
• Full Moon — the whole lit half faces us, a complete bright circle.
• Waning gibbous — more than half lit, now shrinking.
• Third quarter — half lit again, but the other half from first quarter.
• Waning crescent — a thin curve, shrinking away to nothing.

The dark part is not a shadow

This is the one thing most people have wrong, and it is worth stating on its own. When you look at a crescent Moon, the dark portion of the disc is not the Earth's shadow falling across it. The Earth's shadow is nowhere near it.

The dark portion is simply the part of the Moon that the Sun is not shining on. It is night-time on that part of the Moon, in exactly the way it is night-time on the half of the Earth facing away from the Sun. Nothing is blocking the light — the light was never pointed there in the first place.

If phases were caused by the Earth's shadow, we would see the Moon dimmed a little every single night, and we would have to explain how the Earth manages to cast a curved shadow that grows and shrinks on a monthly schedule. It does not, and it cannot.

Eclipses are the shadow ones, and they are rare

Shadows do get involved, but only occasionally, and then something visibly unusual happens. In a lunar eclipse the Earth passes directly between the Sun and a full Moon, and the Earth's shadow falls across the Moon. The whole disc dims over an hour or two and often turns a deep copper colour rather than going black, because a little sunlight bends through the Earth's atmosphere on its way past.

In a solar eclipse the Moon passes directly between the Sun and the Earth, and it is the Moon's shadow that lands on us. That shadow is small, so a total solar eclipse is visible only from a narrow strip of the Earth's surface and lasts only a few minutes at any one place.

The obvious question is why this does not happen every month, since the Moon passes between us and the Sun every month at new Moon. The answer is that the Moon's orbit is tilted by about five degrees compared with the Earth's path around the Sun, so most months it passes a little above or a little below the exact line and no shadow lands anywhere. Only when the three line up closely enough does an eclipse occur.

Two things to try this week

The phases make much more sense when they have been produced rather than described. Take a lamp with the shade off into a dark room, hold a ball at arm's length, and turn slowly on the spot with the ball held out in front of you. Your head is the Earth, the ball is the Moon, the lamp is the Sun. Watch the lit part of the ball narrow to a crescent, vanish, and grow back as you turn. Every phase appears, in order, and nothing casts a shadow on the ball at any point.

Then look for the Moon in the afternoon. Most children are surprised that it is often up in broad daylight, and it is: a first quarter Moon is easy to spot in the eastern sky before sunset. Seeing it in a blue sky settles the question of whether the Moon makes its own light better than any explanation can.