Guides
The 8 phases of the Moon, explained simply
The whole cycle is a matter of geometry: the Sun always lights half of the Moon — what changes is how much of that lit half faces you.
What causes the phases
The Moon produces no light of its own; it reflects the Sun. At every moment, half of the lunar globe is in daylight — exactly as half of the Earth is. As the Moon travels around us in 29.53 days, we see that daylit half from a changing angle: edge-on at the quarters, fully at full moon, not at all at new moon. The phases are not the Earth's shadow — that is a different and much rarer event, the lunar eclipse.
The eight phases, in order
| Phase | What you see | When |
|---|---|---|
| New moon | Nothing — the Moon stands with the Sun, lost in daylight | An instant, named for its day |
| Waxing crescent | A thin arc, visible at dusk in the west | About a week |
| First quarter | Half the disc lit — the growing half | An instant, named for its day |
| Waxing gibbous | More than half, swelling night by night | About a week |
| Full moon | The whole disc, rising as the Sun sets | An instant, named for its day |
| Waning gibbous | More than half, thinning, rising later each night | About a week |
| Last quarter | Half the disc lit — the other half | An instant, named for its day |
| Waning crescent | A thin arc at dawn in the east | About a week |
Four of the eight are instants: new moon, first quarter, full moon and last quarter each happen at a precise minute, and calendars name the day that minute falls on. The other four are the long stretches in between — crescent when less than half the disc is lit, gibbous when more than half is.
Two honest subtleties
First: the cycle of phases (29.53 days, the synodic month) is longer than the Moon's orbit around Earth (27.32 days, the sidereal month), because the Earth itself moves around the Sun in the meantime and the Moon needs two extra days to catch up with the Sun's new direction.
Second: the Moon always turns the same face toward us — the familiar seas and craters never change. What changes with the phases is only the lighting on that one face. And which way the crescent leans depends on where you stand on Earth: in the southern hemisphere the whole picture is flipped.
Questions
- How long does each moon phase last?
- The four principal phases — new, quarters, full — are instants: they last no time at all, and the calendar simply names their day. The four intermediate phases (crescents and gibbous) each span roughly a week, a quarter of the 29.53-day cycle.
- Why do we always see the same face of the Moon?
- Tidal forces long ago braked the Moon's spin until its rotation matched its orbit — one turn per revolution. This tidal locking means one hemisphere faces us permanently; the phases are changes of lighting, not of face.
- What is a blue moon?
- The popular modern meaning is the second full moon of a calendar month, which happens every two to three years. The Moon does not actually look blue — genuinely blue-tinted moons are an optical rarity caused by volcanic ash or smoke.
- Are the phases the Earth's shadow on the Moon?
- No — that is the most common misconception. The phases are just the angle from which we view the Moon's daylit half. The Earth's shadow only touches the Moon during a lunar eclipse, at most a few times a year.