By Rachel D | Updated March 24, 2022

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While the Moon’s orbit is continuous, each named phase is an instant point in its 29‑day, 12‑hour, 44‑minute cycle. To the naked eye, the full moon can seem to linger for several days, but astronomically it is a fleeting moment before the lunar disk begins to wane.

Phases of the Moon

The Moon has eight distinct phases: new moon, waxing crescent, first quarter, waxing gibbous, full moon, waning gibbous, last quarter, and waning crescent. These phases are not evenly spaced; the illuminated fraction of the Moon’s disk changes continuously, so the duration of each phase is effectively instantaneous.

Primary vs. Intermediate Phases

The primary phases—new moon, first quarter, full moon, and last quarter—occur when the Moon’s position relative to the Sun creates a 0°, 90°, 180°, or 270° angle. The intermediate phases (crescent and gibbous) are the transitional periods between the primaries.

When the Phases Occur

The timing of each phase is governed by the relative geometry of the Earth, Moon, and Sun. A new moon happens when the Moon lies between Earth and Sun, while a full moon occurs when they are on opposite sides of Earth. The first and last quarters mark the points when the Moon has traveled one‑quarter and three‑quarters of its orbit, respectively.

Interesting Moon‑Phase Facts

• Because the lunar cycle is slightly shorter than a calendar month, a particular phase usually repeats before the month ends.
• Every 2.7 years, a full moon appears twice in the same calendar month—a phenomenon called a "Blue Moon." This rarity gave rise to the idiom "once in a blue moon," meaning something that occurs very infrequently.
• In Islamic culture, the beginning of a month is marked by the first visible waxing crescent.
• Some cultures consider it taboo to observe the new moon through a window or glass, believing it hides the celestial body from direct view.

For more detailed calculations and a visual representation of the lunar cycle, consult NASA’s Lunar Phases page.