The phases of the moon, explained

What causes the phases

The moon makes no light of its own. At every moment the sun lights exactly one half of it, the half that faces the sun. As the moon goes round the earth, we see that lit half from a changing angle: straight on at full moon, edge-on at the quarters, and not at all at new moon, when the lit half faces away from us.

A common belief is that the earth’s shadow makes the phases. It does not. The earth’s shadow only touches the moon during a lunar eclipse, which can happen only at full moon and only a few times a year. The everyday crescent is the moon’s own night side, seen from the side.

The eight phases, in order

• New moon — the lit side faces away; the moon is in the sky by day and invisible.
• Waxing crescent — a thin sliver after sunset, growing night by night.
• First quarter — half of the disc lit; it is a quarter of the way through the cycle, hence the name.
• Waxing gibbous — more than half lit and still growing.
• Full moon — the whole face lit; it rises at sunset and sets at sunrise.
• Waning gibbous — more than half lit, now shrinking.
• Last quarter — the other half lit; it rises around midnight.
• Waning crescent — a sliver before dawn, until the next new moon.

Only four of them are moments: new moon, first quarter, full moon and last quarter fall at one exact instant, which is what our full moon calendar lists. The other four are the stretches in between.

How long a cycle takes

From one new moon to the next takes 29 days, 12 hours and 44 minutes on average — the synodic month. The moon actually completes its orbit faster, in about 27⅓ days, but by then the earth has moved along its own orbit and the moon needs two more days to line up with the sun again.

Because the moon moves on about 12° a day, it rises roughly 50 minutes later each day. That is why a full moon rises at sunset, a last-quarter moon around midnight and a waxing crescent is only seen in the evening.

Waxing or waning?

In the northern hemisphere a waxing moon is lit on the right and a waning moon on the left. An old memory aid: a waxing moon is shaped like a capital D — the straight back on the left, the bright curve on the right — and a waning moon like a C. The moon, the saying goes, is a liar: D for "decreasing" is exactly when it is growing. In the southern hemisphere it is the other way round, because there the moon looks upside down.

See tonight’s phase

The moon phase today page shows the current phase, the lit percentage and the moon day by day for a whole month; the next full moon page counts down to the next one.