The one idea that explains all of it
The Moon makes no light of its own. We only ever see the part of it the Sun happens to be lighting. As the Moon goes around Earth once every 29.5 days, our angle on that lit half changes, and so the shape we see grows and shrinks. The Moon isn't changing. Our view of it is.
The eight phases, in order
- New Moon. The Moon sits between Earth and Sun with its lit side facing away, so the sky is moonless. The best nights of the month for faint stars and the Milky Way.
- Waxing Crescent. A sliver shows in the west just after sunset and grows each night.
- First Quarter. Half lit. It's called a quarter because the Moon is a quarter of the way through its cycle. It rises around noon and sets around midnight, so it owns the evening sky.
- Waxing Gibbous. More than half lit and filling out toward full.
- Full Moon. Earth sits between Sun and Moon and we see the whole lit face. It rises at sunset and sets at sunrise, up all night.
- Waning Gibbous. Shrinking now, and rising later each evening.
- Last Quarter. Half lit again, but the other half. Rises around midnight. An early riser's moon.
- Waning Crescent. A thinning sliver in the east before dawn, and then back to new.
Waxing or waning? The one-second check
In the Northern Hemisphere, if the right side of the Moon is lit, it's waxing, or growing. If the left side is lit, it's waning. In the Southern Hemisphere the rule flips. Another way to remember it: a waxing crescent curves like a D, a waning crescent like a C. D for developing, C for closing.
Why full moons have names
Harvest Moon, Wolf Moon, Strawberry Moon and the rest come from old North American and European farming calendars, tied to what people were doing that month. A supermoon is just a full moon that lands when the Moon is at the near end of its slightly oval orbit, so it looks a bit bigger and brighter. A blue moon is the second full moon in one calendar month, which happens every two or three years because 29.5 days doesn't quite fit our months.
Using the Moon for stargazing and photos
Full moon is the worst night for astronomy, not the best. Its glare washes out everything faint. Planets don't mind, but galaxies, nebulae and meteor showers want the darkest sky you can get, so plan those within a few days of new moon. If you want to photograph the Moon itself, the quarter phases are better than full. Sideways sunlight throws the craters into sharp shadow along the line between light and dark, and you see texture the flat-lit full moon hides.
Check tonight's phase, plus sunrise and sunset for any city, on the Sun & Moon page.