Moon phases explained

The Moon does not make its own light. It reflects sunlight, and the Sun always lights one half of it. As the Moon orbits Earth, we see more or less of that lit half: that is the cycle of phases.

The eight phases

The US Naval Observatory lists eight traditional phases, starting from new Moon:

1. New Moon: the unlit side faces Earth. The Moon is not visible (except during a solar eclipse).
2. Waxing crescent: less than half lit, and the lit part is growing.
3. First quarter: half of the disc is lit, growing.
4. Waxing gibbous: more than half, but not fully lit.
5. Full Moon: the lit side faces Earth.
6. Waning gibbous: more than half lit, shrinking.
7. Last (third) quarter: half lit, shrinking.
8. Waning crescent: less than half lit, shrinking.

New Moon, first quarter, full Moon and last quarter are the primary phases: they happen at a precise moment, which is what almanacs and the Moon page list. The crescent and gibbous phases are the stretches in between, each lasting about a week.

How long the cycle takes

A full cycle of phases takes 29.5 days on average. The Moon goes around Earth in about 27.3 days, but in that time Earth has moved along its orbit around the Sun, so the Moon needs roughly two more days to return to the same position relative to the Sun. Astronomy Engine gives an average of 29.53 days between new Moons for 2026 to 2036.

Because the cycle is shorter than most calendar months, a month sometimes has two full Moons. The second is popularly called a “blue Moon”. According to the USNO this happens on average every 2.7 years.

When the Moon rises

The phase tells you roughly when the Moon is in the sky, because it depends on the angle between the Moon and the Sun. The USNO gives these rules of thumb for when the Moon crosses the meridian (its highest point):

• New Moon: about the same time as the Sun, so it is up during the day, hidden in the glare.
• First quarter: about 6 hours after the Sun: high in the evening, setting around midnight.
• Full Moon: about 12 hours from the Sun: it rises around sunset and is up all night.
• Last quarter: about 6 hours before the Sun: it rises around midnight and is visible in the morning, also in daylight.

For stargazing this matters a lot: a bright Moon washes out faint stars, the Milky Way and meteors. The nights around new Moon are the darkest. The tonight page shows when the Moon is up during your night.

Crescents and earthshine

During thin crescent phases you can often see the dark part of the Moon glowing faintly. This is earthshine: sunlight reflected by Earth onto the Moon. From the Moon, Earth is then nearly “full”, so it lights the lunar night side enough to see.

The Moon also always shows us (almost) the same face, because it is tidally locked: it rotates once on its axis in the same time it takes to orbit Earth.

Which side is lit

From the northern hemisphere, a waxing Moon is lit on the right and a waning Moon on the left. From the southern hemisphere it is the other way round, because you see the Moon “upside down” compared with the north. Near the equator, the crescent can appear lying almost flat.

Sources

• US Naval Observatory: Phases of the Moon and percent of the Moon illuminated
• US Naval Observatory: Rise, set and twilight definitions (transit times by phase)
• NASA: Moon phases (orbit period, tidal locking, earthshine)
• Phase times: Astronomy Engine, checked against the USNO. See methodology.