The Moon slowly changes shape over the course of a month — from an invisible new moon to a glowing full moon and back. It's one of the oldest patterns humans have watched, yet the reason behind it is widely misunderstood. (No, it isn't Earth's shadow.) This guide explains what actually causes the Moon's phases, walks through the eight phases of its 29.5-day cycle, and shows a live chart of how much of the Moon is lit over the coming weeks.
What causes the Moon's phases?
Here's the key fact that surprises most people: the Moon's phases have nothing to do with Earth's shadow. (That's a lunar eclipse, which is rare and brief.) Instead, the Sun always lights up exactly half of the Moon — the half facing it — just as it lights half of Earth. As the Moon orbits us over the month, we see that lit half from different angles. When the Moon is on the far side of Earth from the Sun, we see its fully-lit face (full moon); when it's between us and the Sun, we see its dark side (new moon); and in between, we see a slice. The phases are simply our changing view of a Moon that's always half-lit.
The eight phases of the Moon
Astronomers divide the cycle into eight phases. Starting from darkness: new moon (invisible), waxing crescent (a sliver, growing), first quarter (half-lit, growing — confusingly called "quarter" because the Moon is a quarter through its cycle), waxing gibbous (more than half, growing), full moon (fully lit), then the reverse: waning gibbous, last quarter (half-lit, shrinking), and waning crescent, before returning to new. "Waxing" means growing toward full; "waning" means shrinking toward new. The chart above traces this as a smooth wave of illumination — peaks are full moons, valleys are new moons.
The 29.5-day cycle
One complete cycle of phases — new moon to new moon — takes about 29.5 days, a span called the synodic month. That's slightly longer than the 27.3 days the Moon actually takes to orbit Earth, because Earth is also moving around the Sun, so the Moon has to travel a bit farther to line back up with the Sun from our point of view. This 29.5-day rhythm is the basis of the word "month" and of lunar calendars used for millennia. Because it doesn't divide evenly into our 365-day year, the phases drift across the calendar from year to year.
Full moon names, supermoons, and blue moons
Each month's full moon carries a traditional name — the Wolf Moon in January, Harvest Moon near the fall equinox, and so on — inherited from Native American and colonial folklore tied to the seasons. A supermoon happens when a full moon coincides with the Moon's closest approach to Earth (its orbit is slightly oval), making it look a touch bigger and brighter. A blue moon is the modern name for a second full moon in a single calendar month — possible because 29.5 days is just short of most months — which happens roughly every two and a half years, the origin of "once in a blue moon."
Frequently asked questions
What causes the phases of the Moon?
Not Earth's shadow — that's an eclipse. The Sun always lights half the Moon; as the Moon orbits Earth, we see that lit half from changing angles, so we see anything from a full disc to none of it.
How long is the Moon's phase cycle?
About 29.5 days from one new moon to the next — the 'synodic month.' It's longer than the Moon's 27.3-day orbit because Earth also moves around the Sun.
What are the eight phases of the Moon?
New, waxing crescent, first quarter, waxing gibbous, full, waning gibbous, last quarter, and waning crescent. 'Waxing' means growing toward full; 'waning' means shrinking toward new.
What is a supermoon?
A full moon that coincides with the Moon's closest approach to Earth in its slightly oval orbit, making it appear a bit larger and brighter than usual.
What is a blue moon?
The second full moon in a single calendar month — possible because the 29.5-day cycle is just shorter than most months. It happens roughly every two and a half years.
Why don't we get an eclipse every month?
Because the Moon's orbit is tilted slightly, so at most new and full moons the Moon, Earth, and Sun aren't perfectly aligned. Eclipses only occur when they do line up.