Moon phase

How it works

The synodic month, from one new moon to the next, averages 29.53059 days. Take the number of days between your date and a known new moon, divide by that figure, and the fractional remainder tells you how far through the cycle you are. Zero is new, half is full.

The average is doing a lot of work here. The real interval varies by up to thirteen hours either side because the Moon's orbit is elliptical and the Sun perturbs it, so this method can be a few hours out. For picking a date to stand in a field, it is entirely sufficient.

The eight phases

• New — the Moon is between Earth and Sun; the lit face is turned away.
• Waxing crescent — a sliver in the western sky after sunset.
• First quarter — half lit, and a quarter of the way round the orbit, which is why the name confuses everyone.
• Waxing gibbous — more than half, still filling.
• Full — Earth between Moon and Sun; rises about as the Sun sets.
• Waning gibbous — past full, emptying.
• Last quarter — half lit again, the other half.
• Waning crescent — the last sliver, visible before dawn.