Some nights the Moon is a round, bright ball. Some nights it is a thin smile. Some nights you cannot see it at all. The Moon has not changed shape; we see different amounts of its lit half.
Key idea
The Moon makes no light of its own; it reflects sunlight. One half of it is always lit by the Sun. As the Moon travels around the Earth, about once every 29½ days, we see more or less of that lit half. These are the phases.
| Phase | About how many days after new moon | What you see |
|---|---|---|
| New moon | 0 | nothing (the lit side faces away) |
| First quarter | 7 | the right half lit (seen from the Northern Hemisphere) |
| Full moon | 15 | the whole face lit |
| Last quarter | 22 | the left half lit |
Waxing means growing (more lit each night); waning means shrinking. Gibbous means more than half lit.
Worked example: A week later
Tonight is a full moon. What phase will it be in about one week?
- After full, the Moon wanes.
- About a week after full comes the last quarter.
Answer: Last quarter (half lit).
Common mistake
Thinking the Earth's shadow makes the phases. The Earth's shadow falls on the Moon only during a lunar eclipse, which happens only now and then, about twice a year.
Try it at home
- Draw the Moon every clear night for a month, and write the date.
- Or: in a dark room, use a lamp as the Sun and hold a ball at arm's length as the Moon. Turn slowly in a circle and watch the lit part of the ball change.
Going further. Sky and Solar System, and Moon Phases in Earth Science.