Moon Phases: Why the Moon Changes Shape Every Night

The Moon doesn't change shape — the sunlight on it does

The Moon has no light of its own; every bit of moonlight is sunlight reflecting off its surface. Exactly half of the Moon is always lit by the Sun at any given moment, just as exactly half of the Earth is always in daylight. A phase is simply how much of that permanently-lit half happens to be facing Earth, and that fraction changes smoothly and predictably as the Moon travels around Earth once every 27.3 days (its actual orbital period, the sidereal month).

Because Earth is also moving around the Sun at the same time, it takes the Moon slightly longer — about 29.5 days, the synodic month — to return to the same phase as seen from Earth, since Earth has shifted position in the meantime and the Moon has to travel a bit farther to catch up to the same Sun-Earth-Moon alignment. This 29.5-day cycle is the one that gives us the familiar month-long rhythm of phases.

The eight phases, in order

New Moon        →  dark side faces Earth, invisible
Waxing Crescent →  a thin sliver grows on the right (N. hemisphere)
First Quarter   →  exactly half the visible disc lit
Waxing Gibbous  →  more than half lit, still growing
Full Moon       →  the whole near side lit
Waning Gibbous  →  more than half lit, now shrinking
Last Quarter    →  the other half lit
Waning Crescent →  a thin sliver, shrinking toward New Moon

Waxing means the lit portion is growing night to night; waning means it is shrinking. "Gibbous" (from Latin for "hump-backed") describes a phase more than half-lit but not full. Watching which side the light grows from is actually a reliable way to tell waxing from waning at a glance, and — in the Northern Hemisphere — it grows from the right side during waxing phases and shrinks from the right during waning.

Why we don't get an eclipse every single month

If the Moon's orbit were in the exact same plane as Earth's orbit around the Sun, every New Moon would cause a solar eclipse and every Full Moon a lunar eclipse. It isn't — the Moon's orbital plane is tilted about 5 degrees relative to Earth's orbital plane, so most months the Moon passes slightly above or below the direct Sun-Earth line at New and Full phases, missing an alignment. Eclipses only happen when a New or Full Moon coincides closely with the Moon crossing Earth's orbital plane, at points called nodes — which is why eclipses come in predictable but not monthly clusters.

The far side isn't the "dark side"

The Moon is tidally locked to Earth: it rotates on its own axis in exactly the same time it takes to orbit Earth once, so the same hemisphere — the near side — always faces us, while the far side always faces away. This is a common source of confusion with the phrase "dark side of the Moon", which is misleading — the far side receives just as much sunlight over a lunar day as the near side does; it is simply never visible from Earth, not permanently dark. This tidal locking itself is a consequence of Earth's gravity having gradually slowed the Moon's rotation over billions of years until the two periods matched exactly.

Frequently asked questions

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