Phases and eclipses

Half of the Moon is always lit by the Sun, just as half of the Earth is always in daylight. What changes is how much of that lit half is turned towards us.

The Moon goes round the Earth about once a month. When it is on the Sun’s side of us, the lit half faces away and we see nothing: new Moon. A week later we see it side-on, half lit and half dark. A week after that it is on the far side of us from the Sun, with its whole lit face towards us: full Moon. Then the same in reverse.

The dark part is not the Earth’s shadow. It is simply the Moon’s own night side.

The Earth’s shadow does reach the Moon now and then, and that is an eclipse of the Moon. It is rare because the Moon’s path is slightly tilted, so most months the Moon passes just above or below the shadow.

Two kinds of month. Measured against the stars, the Moon goes round the Earth in 27.32 days. The phases take longer to repeat, 29.53 days, because in that time the Earth has moved about a twelfth of the way round the Sun and the Moon has to travel a bit further to get back in line.

Reading the phase. The lit edge of the Moon always faces the Sun. A thin crescent in the evening sky is a young Moon following the Sun down. A half Moon high at sunset is a week old. A full Moon rises as the Sun sets and stays up all night. After that it rises later each night, by about 50 minutes, and the lit part shrinks from the other side.

Why there is not an eclipse every month. The Moon’s orbit is tilted by 5.1° to the Earth’s. That sounds small, but the Earth’s shadow at the Moon’s distance is only about 1.4° wide, so the Moon usually misses it. An eclipse needs the full or new Moon to fall near one of the two points where the tilted orbits cross. That line-up comes round about every six months.

An eclipse of the Moon happens when the full Moon passes through the Earth’s shadow. Everyone on the night side of the Earth can watch, and the whole thing can last hours. In the deepest part of the shadow the Moon does not vanish. It turns a dull red, lit by sunlight that has been bent through the Earth’s air.

An eclipse of the Sun happens when the new Moon passes in front of the Sun. The dark core of the Moon’s shadow is narrow by the time it reaches us, never more than about 270 km across, and it sweeps over the ground at more than 1,700 km/h. Only people on that narrow track see the Sun fully covered, and never for longer than about seven and a half minutes.

A total eclipse works because of a coincidence. The Sun is about 400 times wider than the Moon and, at present, about 400 times further away, so the two look the same size. When the Moon is at the far end of its orbit it looks slightly too small, and a ring of Sun is left showing.