Moon Phase
The Moon's cycle from new to full and back.
Overview
The Moon's phase is a geometry problem, not a shadow problem. Half the Moon is always lit by the Sun; what changes is how much of that lit hemisphere faces Earth as the Moon moves around its orbit.
- Phases are caused by viewing geometry, not by Earth's shadow — that is a lunar eclipse, which is a different event.
- The full cycle takes about 29.5 days, which is longer than the Moon's 27.3-day orbit.
- Phase determines when the Moon is up: a full Moon rises at sunset, a new Moon is up all day.
- The same hemisphere always faces Earth because the Moon is tidally locked.
Why phases happen
The Sun always illuminates exactly half of the Moon. As the Moon orbits Earth, the angle between Sun, Moon and observer changes, so we see varying fractions of that illuminated hemisphere. At new Moon the lit side faces away from us; at full Moon it faces us directly; at the quarters we see half of it.
This is worth stating explicitly because the most common misconception is that Earth's shadow causes phases. It does not. Earth's shadow falling on the Moon is a lunar eclipse, which happens at most a few times a year and only ever at full Moon.
Two different months
The Moon completes one orbit relative to the stars in about 27.3 days — the sidereal month. But the phase cycle takes about 29.5 days — the synodic month.
The difference exists because Earth has also moved along its own orbit during that time, so the Moon must travel a little further to return to the same alignment with the Sun. The two-day gap is a direct consequence of Earth's motion and is the reason lunar calendars drift against solar ones.
Phase tells you when the Moon is up
- New Moon rises around sunrise and sets around sunset — it is in the daytime sky and invisible in the Sun's glare.
- First quarter rises around noon, is highest at sunset, and sets around midnight. Best placed for early-evening observing.
- Full Moon rises at sunset and sets at sunrise — up all night, and the worst time for faint deep-sky objects.
- Last quarter rises around midnight and is highest at sunrise, so it favours pre-dawn observers.
- A crescent Moon seen shortly after sunset is always waxing and in the west; one seen before sunrise is always waning and in the east.
Observing the Moon well
Counter-intuitively, full Moon is the poorest time to observe lunar detail. With the Sun directly behind the observer there are no shadows, and the surface appears flat and washed out. Near the quarters, sunlight strikes the terminator — the day/night boundary — at a grazing angle, throwing long shadows that make crater walls and mountain ranges stand out in relief.
Supermoons occur when a full Moon coincides with perigee, the closest point of the Moon's slightly elliptical orbit. The difference in apparent diameter compared with an average full Moon is real but modest — a few percent — and far smaller than most photographs imply. The dramatic 'huge Moon on the horizon' effect is the Moon illusion, a perceptual phenomenon: the Moon's measured angular size is not larger near the horizon.
Continue in the data
Catalogues, hubs, and reference pages that hold the underlying records for this topic.
Frequently asked
- Why does the Moon have phases?
- Because we see varying fractions of its sunlit hemisphere as it orbits Earth. The Sun always lights exactly half the Moon; what changes is the viewing angle. Earth's shadow plays no part — when Earth's shadow does fall on the Moon, that is a lunar eclipse, a separate and much rarer event.
- Why is the phase cycle 29.5 days but the orbit only 27.3?
- Because Earth moves too. In the 27.3 days the Moon takes to return to the same position relative to the stars, Earth has travelled about a twelfth of the way around the Sun, so the Moon needs roughly two extra days to return to the same Sun–Earth–Moon alignment that defines a phase.
- What is a supermoon?
- A full Moon occurring near perigee, the closest point of its slightly elliptical orbit around Earth. The Moon is genuinely a few percent larger in apparent diameter and somewhat brighter than an average full Moon, but the difference is difficult to notice by eye and far less dramatic than most coverage suggests.
- Why does the Moon sometimes appear during the day?
- Because it is above the horizon during daylight for much of the month. Only near full Moon is it up exclusively at night. A first-quarter Moon rises around noon, so it is easily visible in the afternoon sky — it simply competes with a bright background rather than being absent.