Watch the Moon night after night and something odd happens: its face keeps changing. One evening it is a thin silver curve, like a fingernail clipping. A week later it is a neat half-circle. A week after that it is a big round full Moon, bright enough to throw shadows in the garden. Then, slowly, it shrinks away to nothing again.
Here is the surprise: the Moon isn't really changing shape at all. It is the same round ball of rock the whole time. What changes is how much of it we can see lit up — and once you know why, you will be able to look at tonight's Moon and say exactly where it is on its journey around us.
The most important fact about the Moon is this: it makes no light of its own. It is a dark, dusty rock, like a giant grey pebble in the sky. Every scrap of light we see coming from it is sunlight bouncing off it — the Moon is really a huge mirror-ish ball, shining only because the Sun shines on it.
And the Sun can only ever light up one half of the Moon at a time — the half facing the Sun — exactly as it lights only the daytime half of the Earth while the other half has night. The lit half is always there, always the same size. The changing face we see happens because, from where we stand, we look at that lit half from a different angle each night.
On the left is a bird's-eye view: the Moon circling the Earth while sunlight streams in from the right, so the Moon's right side is always the bright, sunny side. On the right is the same Moon as it looks from Earth. Slide slowly through the month and watch the bright shape grow from nothing, swell to a full circle, and shrink back again — while the lit half never actually changes.
Notice the trick: when the Moon sits between us and the Sun, its sunny side is turned away from us and we see a dark disc. When it is on the far side of Earth from the Sun, the whole sunny side faces us and it looks full. In between, we catch the lit half side-on and see a crescent or a half.
The month starts at the New Moon, when the sunny side is turned away and the Moon almost vanishes. From there the bright part grows a little more each night. We say the Moon is waxing — an old word that simply means growing.
After the full Moon the show runs in reverse. The bright part shrinks a little every night, and we say the Moon is waning — an old word meaning shrinking. It slides back through gibbous, to a half, to a crescent, until it disappears into the New Moon again — and then the whole thing starts over.
The full loop — New Moon, all the way round, back to New Moon — takes about 29 days, which is just under a month. In fact that is where our word "month" comes from: a moon-th, one journey of the Moon. A handy way to tell waxing from waning: a waxing Moon is lit on its right side, and a waning Moon is lit on its left.
Look at the Moon whenever you like — tonight, next month, or in a photo from a hundred years ago — and you will always see the same face, with the same dark patches and craters. The Moon never turns round to show us its back. Why not?
Because the Moon spins exactly once for every trip it makes around the Earth. The turning and the orbiting are perfectly in step — scientists call this being tidally locked — so the same side is forever pointed our way. The other side, which we never see from home, is often called the "dark side of the Moon." That name is a myth: the far side gets just as much sunlight as the side we know. It isn't dark at all — it's just hidden. Nobody set eyes on it until spacecraft flew around the back and beamed pictures home, and it turned out to be rugged and packed with craters.
People mix these two up all the time, but they are completely different. A phase is the everyday thing this whole page is about: which slice of the Moon's sunny half happens to be turned towards us. It changes gently, night by night, all month long, and it needs no shadow at all — only sunlight and a point of view.
An eclipse is a rare event where the Sun, Earth and Moon line up almost perfectly and one really does throw a shadow on another. In a lunar eclipse the Earth slips directly between the Sun and a full Moon, and its shadow slides across the Moon, turning it a dusky red for an hour or two. That happens only a couple of times a year. Every other Moon shape you have ever seen was a phase, not an eclipse.