The Moon makes no light of its own. Sunlight always lights the half of the Moon that faces the Sun, just as it lights the day side of Earth. As the Moon orbits Earth, about once every 29.5 days from new moon to new moon, we see different amounts of that lit half. These changing shapes are the phases.
| Phase | About how many days after new moon | What we see (Northern Hemisphere) |
|---|---|---|
| New moon | 0 | Nothing: the lit half faces away from us |
| Waxing crescent | 4 | A thin sliver, lit on the right |
| First quarter | 7 | The right half lit |
| Waxing gibbous | 11 | More than half, growing |
| Full moon | 15 | The whole face lit |
| Waning gibbous | 18 | More than half, shrinking |
| Third quarter | 22 | The left half lit |
| Waning crescent | 26 | A thin sliver on the left |
Key idea
Half of the Moon is always lit by the Sun. The phase depends on how much of that lit half faces Earth, which changes as the Moon moves around its orbit.
Why it works
Hold a ball at arm's length in a room with one lamp, and turn slowly in a circle. When the ball is between you and the lamp you see its dark side (new moon). A quarter turn later you see half lit and half dark (a quarter moon). With the lamp behind you, the whole face you see is lit (full moon). Nothing blocked any light; only your view of the lit half changed.
Worked example: What comes next?
Tonight the Moon is at first quarter. What will it look like in about a week?
- From first quarter it keeps growing (waxing) toward full.
- First quarter is about day 7; a week later is about day 14 or 15.
Answer: A full moon
Worked example: Rising time
A full moon is on the opposite side of Earth from the Sun. About when does it rise?
- Opposite the Sun means it comes up as the Sun goes down.
- So a full moon rises around sunset and sets around sunrise.
Answer: Around sunset
Common mistake
Thinking Earth's shadow makes the phases. Earth's shadow points straight away from the Sun, and it falls on the Moon only during a lunar eclipse, a few times a year. Check: a crescent moon is near the Sun in the sky, where Earth's shadow could never reach it.
Try it in the Night Sky Lab: step through a month and watch the Moon's shape and rising time change.