Moon · 1 of 8

The Moon's Month

Why does a thin evening crescent grow into a full Moon?

Why the Moon changes shape: a month of evenings, a crescent to a full Moon to a morning half, and the one angle that decides them all.

You’ll be able to say what phase the Moon will be at, and when it will be up.

8 steps · about 6 min

Where and which night

Working out when this is best placed from where you are…

Where

Which night

Date

Time

Kept for this guide only, until you change it here — your map settings, and every other guide, stay as they are.

Print, play outside or present

Present on a big screen

Scan with a phone's camera, or open

Sources

Meeus, Astronomical Algorithms, 2nd ed. (1998), ch. 47 and 48 — the Moon's position, and its illuminated fraction from the phase angle
NASA Moon Fact Sheet — the 29.53-day synodic month and the 27.32-day sidereal month
NASA Science: Earthshine — sunlight reflected by the Earth onto the Moon's night side
NASA Eclipse Web Site — the new and full Moons of 2026 that this page's phase finder is tested against

← GUIDES

More below ↓

With a keyboard, focus the sky, aim it with the up and down arrow keys, and press Enter to drop the pin at the centre of the view; or press Show me.

You are here

Settings

Screen

Night mode Turns every page red, to keep your eyes adjusted to the dark.

High contrast Bolder constellation lines, larger and brighter names on the sky, and stronger text and edges everywhere. Starts on if your device asks for more contrast.

Reduce motion The camera cuts to where it is going instead of flying there, and panels and sheets appear without sliding. Starts on if your device asks for less motion.

Battery saver Draws the sky thirty times a second at most: smoothness traded for battery.

Night brightness Dims the screen below your phone’s lowest setting, down to a fifth, while night mode is on.

Clock and voice

Clock How every time is written. Until you choose, your language’s own clock decides.

D describes What D says about the view on the sky: one sentence, or in full with the field, the constellation and five names.

Sounds Quiet tones as you use the map: a tick as you drag the clock past a mark or dusk, a chime for the star you pick — higher for hotter stars — and a low swell while a figure is out of the dome. Made in your browser, and off until you turn them on.

All settings and your progress

This is tonight's Moon, in tonight's shape; the readout says whether it is up. It changes every evening and comes back to the same shape every 29.5 days. The next steps visit one month, and show that a single angle decides it.

Three evenings after new Moon, at dusk: a thin crescent low in the west. The arc runs to the Sun, just set — the Moon's angle from it is the whole story. The lit edge faces the Sun; the faint glow on the rest is earthshine, sunlight off the Earth.

Five more evenings, same hour: watch how far the Moon is from the Sun, and how much is lit.

The same month from outside; the line points to the Sun. Half the Moon is always lit, the half facing the Sun; the phase is how much of that half faces us. No shadow is involved: the dark part is the Moon's own night.

The first quarter: 90° from the Sun, at its highest at dusk, lit on the Sun's side. A quarter of the way round, but we see a half — half of the half facing the Sun. It sets around midnight, so the evening is when to look.

The full Moon: 180° from the Sun, rising as it sets. All week it grew, waxing gibbous, over half lit; now it starts to shrink: waning. It slips just past Earth's shadow; when it doesn't, that's an eclipse.

The last quarter, at dawn: 270° round, the other half lit, facing the Sun about to rise in the east. It rises around midnight, so it belongs to the morning. A week on, a thin waning crescent rises just before the Sun — then new Moon again.

The arc is tonight's angle from the Sun. South of the equator it looks mirrored.

Under 90° a crescent, over 90° gibbous
Every 29.5 days the angle comes round