Why the Moon has phases
The Moon does not make its own light. It shines because it reflects sunlight, and at every moment the Sun lights exactly half of it — the half facing the Sun. As the Moon travels around Earth, the angle between the Sun, Earth and Moon changes, so we see different amounts of that sunlit half. When the lit half faces away from us we see almost nothing; when it faces us, we see a full disc.
The cycle of phases
| Phase | Angle from the Sun | What we see |
|---|---|---|
| New moon | 0° | Dark side faces Earth; not visible |
| Waxing crescent | 0–90° | A thin, growing sliver |
| First quarter | 90° | Half of the disc lit |
| Waxing gibbous | 90–180° | More than half, still growing |
| Full moon | 180° | Entire disc lit |
| Waning gibbous | 180–270° | More than half, shrinking |
| Third (last) quarter | 270° | The other half of the disc lit |
| Waning crescent | 270–360° | A thin, shrinking sliver |
Waxing means growing and waning means shrinking. Gibbous means more than half. The full cycle, called the synodic month, takes about 29.5 days.
When the Moon rises
Because Earth spins once a day, the Moon rises and sets just like the Sun. Its position relative to the Sun decides when: a new moon rises with the Sun at dawn, a first-quarter moon rises around noon, a full moon rises at sunset and stays up all night, and a third-quarter moon rises around midnight. On average the Moon rises about 50 minutes later each day.
North vs. south
Observers in the Southern Hemisphere see the Moon “upside down” compared with those in the north, so the lit side is mirrored: a waxing crescent is lit on the left. Near the equator, crescents can appear lying on their backs, like a smile. The phase itself — how much is lit — is the same everywhere on Earth at the same moment.
Phases are not eclipses
Earth’s shadow does not cause the phases. Turn on Earth’s shadow in the simulator: it points directly away from the Sun, so it can only touch the Moon at full moon. Even then, the Moon’s orbit is tilted about 5° to Earth’s orbit, so it usually passes above or below the shadow. When the alignment is just right, we get alunar eclipse. Similarly, a solar eclipse can only happen at new moon.
Formulas
If θ is the angle between the Moon and the Sun as seen from Earth (0° at new moon):
Fraction of disc lit k = (1 − cos θ) / 2Day of the cycle ≈ (θ / 360°) × 29.53 daysKey terms
- Synodic month
- The time from one new moon to the next, about 29.53 days.
- Sidereal month
- The time for the Moon to orbit once relative to the stars, about 27.3 days. It is shorter because Earth also moves around the Sun.
- Terminator
- The line between the lit and dark parts of the Moon.
- Synchronous rotation
- The Moon spins once per orbit, so the same face always points toward Earth.
Real-world connections
- Calendars: many calendars, including the Islamic, Hebrew and Chinese calendars, are based fully or partly on lunar phases.
- Tides: the strongest (spring) tides occur near new and full moon, when the Sun and Moon pull in line.
- Astronomy and photography: astronomers schedule deep-sky observing around new moon, when the sky is darkest.
- Wildlife: some animals time behaviour to the lunar cycle — many corals spawn on nights linked to the full moon.
Common misconceptions
- “Earth’s shadow causes the phases.” Phases come from viewing angle; Earth’s shadow only causes eclipses.
- “The Moon is only visible at night.” It is often visible in daylight, especially around the quarter phases.
- “There is a permanent dark side of the Moon.” There is a far side we never see, but it gets as much sunlight as the near side — it is fully lit at new moon.
Key takeaways
- Half of the Moon is always lit by the Sun; phases show how much of that half faces us.
- The cycle takes about 29.5 days: new → first quarter → full → third quarter → new.
- The phase determines when the Moon rises and when it is best seen.
- Eclipses are rare because the Moon’s orbit is tilted, not because shadows cause phases.
Model note: the diagram is not to scale. The real Moon is about 30 Earth diameters away, and the Sun is about 400 times farther still.