What phase is the Moon tonight?

The Moon makes no light of its own. What we see is sunlight bouncing off its grey rock and dust. At every moment the Sun lights exactly half of the Moon, the half that faces it, just as it lights half of the Earth. The other half is in night.

As the Moon circles us, we see that lit half from a changing angle. When the Moon is on the far side of the Earth from the Sun, its whole day side faces us: a full Moon. When it passes between us and the Sun, its day side faces away and we see almost nothing: a new Moon. In between, we see part of the lit half. That changing shape is the phase.

There are eight named steps. After new Moon comes a waxing crescent, a thin sliver that grows night by night. At first quarter we see half the face lit; it is called a quarter because the Moon is a quarter of the way round. Then comes a waxing gibbous Moon, more than half lit, and the full Moon. After that the same shapes return in reverse, waning gibbous, last quarter and waning crescent, until the Moon slips into the Sun's glare again.

From one new Moon to the next takes 30 days on average. Many cultures built their calendars on this rhythm, and the Chinese, Hebrew and Hindu calendars still follow it, adding an extra month now and then to stay in step with the seasons.

Measured against the stars, the Moon goes once round the Earth in only 27 days. The cycle of phases takes longer because the Earth moves on along its path round the Sun in the meantime. To line up with the Sun again and show the same phase, the Moon has to travel a little further.

Not every cycle is the same length. One can run several hours longer or shorter than the average, because the Moon's orbit is an oval and the Sun's pull keeps stretching and twisting it.

The next full Moon is 20 days away. Around new Moon, 4.7 days from now, come the darkest nights of the month, the best for faint stars and meteors.

Whatever its phase, the Moon always shows us the same face: the same dark patches, the same bright craters. That is not because it doesn't spin. It turns once on its axis in exactly the time it takes to go round the Earth, 27 days. So as it circles us, it keeps the same side turned our way.

This is called tidal locking, and it is common: all the large moons in the solar system are locked to their planets. When the Moon was young, hot and closer to us, the Earth's pull stretched it into a slight bulge. As the Moon spun, that bulge was always a little out of line, and the tug of war slowed the spin until one turn matched one orbit.

It does not mean the far side is dark. It gets just as much sunlight as the side we see. At new Moon, when we see no lit face at all, the far side is in full daylight.

The match isn't perfect, either. Because the orbit is an oval and tilted, the Moon seems to nod and rock a little over each month, and over time we can peek a little way round its edges.

Look at a thin crescent Moon in a darkening sky, a few days before or after new Moon. Often the rest of the disc shows too, a faint grey ball cradled in the bright curve. That soft glow is earthshine.

It is sunlight that has bounced off the Earth, crossed to the Moon and bounced back to your eyes. When the Moon is a thin crescent to us, the Earth seen from the Moon is nearly full: a big, bright disc in the lunar sky. Its light is enough to dimly light the Moon's night side.

So when you see earthshine, part of what you are looking at is our own planet's light, sent back to us from the Moon. Right now the Moon is 382,139 kilometres away, and light takes 1.24 seconds to cross that gap. The glow you see has made the trip twice, out and back, before it reaches you.

Everyone on Earth sees the same phase on the same night. A full Moon over Copenhagen is a full Moon over Sydney. What changes is which way up it is.

From the Southern Hemisphere, the Moon looks upside down compared with the view from the north. A waning crescent that is lit on the left for someone in Europe is lit on the right for someone in Australia, and a waxing crescent swaps the other way. The familiar dark patches are turned round too.

The reason is the way we face to see it. From the middle of the Northern Hemisphere, the Moon mostly crosses the southern half of the sky, so people face south to look at it. From South Africa, Chile or Australia it mostly crosses the northern half, and people face north. Turn around, and the picture turns with you.

So tonight the same lit half of the same Moon shines down on everyone who looks up, wherever they stand, only tilted to suit each place. Two people on opposite sides of the world, both watching it, are looking at one and the same thing.