Moon phases explained: why the Moon changes shape, and when it rises
Why we see a crescent, a half and a full Moon, what the Moon's age means, why it rises about 50 minutes later each night, why we always see the same face, and the best phase for looking through binoculars.
Half lit, always
The Moon makes no light of its own. The Sun always lights exactly half of it, just as it lights half of Earth (day) and leaves the other half dark (night). What changes over the month is how much of that lit half faces us.
At new Moon the Moon lies roughly between Earth and the Sun, so its lit side faces away and we see nothing. As it moves along its orbit we see a growing sliver (waxing crescent), then half (first quarter), then more than half (waxing gibbous), then the whole lit face (full Moon) when it is on the far side of Earth from the Sun. Then the sequence runs backward through waning gibbous, last quarter and waning crescent.
29.5 days and the Moon's age
One full cycle of phases (a synodic month) takes about 29.53 days. The Moon's 'age' is the number of days since the last new Moon: around 7 days at first quarter, about 15 at full and about 22 at last quarter. Traditional lunar calendars, including the Korean and Chinese ones, start each month at new Moon, so the 15th of a lunar month falls near full Moon, which is why festivals such as Chuseok and the Mid-Autumn Festival come with a bright round Moon.
Why it rises 50 minutes later each day
The Moon moves eastward around its orbit by about 13° a day, so Earth has to turn a little further each day to bring it back into view. On average moonrise comes about 50 minutes later each night, though it varies from about 30 minutes to more than an hour through the year.
That leads to a handy pattern: a new Moon rises and sets with the Sun; a first quarter rises around noon and is high at sunset; a full Moon rises at sunset and stays up all night; a last quarter rises around midnight and is up in the morning sky. So if you see the Moon in the daytime, it is near a quarter.
Always the same face
The Moon spins once on its axis in exactly the time it takes to orbit Earth, so the same side always faces us. This tidal locking happened long ago, as Earth's gravity slowed the Moon's spin. Small wobbles called librations let us see about 59% of its surface over time. The 'far side' is not dark: it gets as much sunlight as the near side; at new Moon it is fully lit.
Earthshine, the Moon illusion and supermoons
- Earthshine: a few days either side of new Moon, the dark part of the Moon glows faintly. That is sunlight reflected from Earth onto the Moon and back.
- The Moon illusion: a rising Moon looks huge near the horizon. It is not larger; photos show the same size. The effect is in our brains.
- Supermoon: the Moon's orbit is slightly oval, so a full Moon near the closest point looks up to about 14% wider and 30% brighter than one at the far point. Side by side the difference is clear, alone it is hard to notice.
The best phase for looking
Full Moon is the brightest but the flattest: the Sun shines straight down on the face we see, so there are no shadows. The best views are a few days either side of first or last quarter. Along the terminator, the line between light and dark, the Sun is low on the lunar landscape and long shadows bring craters and mountains into sharp relief. Even 7×50 or 10×50 binoculars show dozens of craters.