How moon phases work — the geometry behind the 29.5-day cycle

The Moon is always half lit by the Sun; what changes is how much of the lit half faces Earth. That single sentence is the whole of lunar phases, and from it follow the eight names, the 29.5-day rhythm, and the trick for telling whether tonight's Moon is growing or shrinking. The moon phase tool shows today's; this guide explains it.

The geometry

Sunlight illuminates the half of the Moon facing the Sun. As the Moon orbits Earth, we see that lit half from changing angles: with the Moon between Earth and Sun, the lit side faces away and we see nothing (new moon); with Earth between, we see the whole lit face (full moon); in between, a fraction. The phases are not Earth's shadow — that is a lunar eclipse, which happens only at full moon and only when the alignment is exact. The line between light and dark on the Moon is the terminator, and it is where craters cast long shadows and the surface looks most dramatic through binoculars.

The eight phases

1. New moon — invisible, near the Sun in the sky.
2. Waxing crescent — a sliver in the evening sky after sunset.
3. First quarter — half lit, a quarter of the way round the orbit; high at sunset.
4. Waxing gibbous — more than half, growing.
5. Full moon — rises at sunset, sets at sunrise.
6. Waning gibbous — shrinking; rises later each night.
7. Last (third) quarter — half lit, the other half; high at sunrise.
8. Waning crescent — a sliver in the dawn sky.

"Quarter" refers to the orbit, not the appearance — a quarter moon looks half full. The illuminated fraction runs 0% → 50% → 100% → 50% → 0% through the cycle.

Why 29.5 days

The Moon orbits Earth in 27.3 days relative to the stars (the sidereal month). But Earth has moved along its own orbit in that time, so the Moon needs about two more days to return to the same position relative to the Sun and repeat the phase: 29.53 days, the synodic month. Twelve of those are 354 days — why lunar calendars (Islamic) drift 11 days a year against the seasons, and why lunisolar calendars (Hebrew, Chinese) add a leap month every few years. It is also why a "blue moon" — a second full moon in one calendar month — happens about every 2.7 years: 29.5 into a 30- or 31-day month occasionally fits twice. Why Easter is on a different date every year follows the same arithmetic into the Christian calendar.

Reading the sky

In the northern hemisphere, a Moon lit on the right is waxing (the lit edge points toward the Sun, which set to the west) and one lit on the left is waning; in the southern hemisphere it's reversed. The mnemonic "DOC" — D-shaped waxing, O full, C-shaped waning — works north of the equator. Timing helps too: an evening Moon is waxing, a Moon still up after dawn is waning, and a full Moon is opposite the Sun all night. The sunrise and sunset tool gives the Sun's side of the sky; How sunrise and sunset times are calculated — latitude, the seasons, and the equation of time the geometry.

What phases do and don't affect

Tides: yes — the Sun and Moon align at new and full moon, producing the larger spring tides, and pull at right angles at the quarters, producing smaller neap tides. Brightness: a full Moon is far brighter than a half, because the lit surface faces us directly with no shadows. Human behaviour, births, hospital admissions, crime, sleep: large studies find no effect or effects too small to matter, despite the persistence of the belief. Gardening by the Moon has tradition behind it and no evidence. The tool gives the phase, the illumination and the next full and new moons — the astronomy, which is quite enough.