Moon Phases Explained: Why the Moon Changes Shape

The Moon is the easiest thing in the night sky to find, and its changing shape is the oldest calendar there is. The cause is simple once you picture it, and it explains a lot more than the shape: when the Moon rises, when you can see it in daylight and which nights are best for stargazing.

Why the Moon has phases

The Moon makes no light of its own. As NASA puts it, moonlight is reflected sunlight. At any moment, the Sun lights exactly half of the Moon, the half facing the Sun, while the other half is in darkness.

As the Moon travels around Earth, we see that sunlit half from a changing angle:

• When the Moon is roughly between Earth and the Sun, the lit half faces away from us. We see the dark side, and that’s a new moon.
• When the Moon is on the far side of Earth from the Sun, the lit half faces us. We see a fully lit disk, a full moon.
• In between, we see part of the lit half and part of the dark half: a crescent, a half Moon or a gibbous Moon.

A common misunderstanding is that phases are caused by Earth’s shadow falling on the Moon. They aren’t. Earth’s shadow touches the Moon only during a lunar eclipse, which can happen only at full moon, and only when the Sun, Earth and Moon line up closely, so most full moons pass without one. Our eclipse page lists the upcoming ones.

The eight phases

Astronomers name four main phases, and four in-between ones. The US Naval Observatory defines the main phases by the angle between the Moon and the Sun as seen from Earth: 0° for new moon, 90° for first quarter, 180° for full moon and 270° for last quarter. In order:

1. New moon. The Moon is close to the Sun in the sky, and its dark side faces us. You can’t see it.
2. Waxing crescent. A thin sliver appears in the western sky after sunset, growing each evening.
3. First quarter. Half the disk is lit. It’s called a quarter because the Moon is a quarter of the way through its cycle.
4. Waxing gibbous. More than half is lit, and still growing.
5. Full moon. The whole face is lit, and the Moon is opposite the Sun in the sky.
6. Waning gibbous. More than half is lit, but shrinking.
7. Last quarter (also called third quarter). The other half of the disk is lit.
8. Waning crescent. A thin sliver rising before dawn, shrinking toward the next new moon.

Waxing and waning, crescent and gibbous

These four words describe every in-between phase:

• Waxing means the lit part is growing, from new moon toward full.
• Waning means it is shrinking, from full moon toward new.
• Crescent means less than half the disk is lit.
• Gibbous means more than half, but not all, is lit.

From the US and the rest of the Northern Hemisphere, a waxing Moon is lit on the right side, and a waning Moon on the left.

How long the cycle takes

The Moon goes around Earth once every 27.3 days, but it takes about 29.5 days to go from one new moon to the next. NASA explains the difference: while the Moon orbits Earth, Earth also moves along its orbit around the Sun, so the Moon needs a couple of extra days to return to the same position relative to the Sun. That 29.5-day cycle is called a lunation, or synodic month.

When each phase rises and sets

Because the phase depends on where the Moon is relative to the Sun, each phase keeps roughly the same schedule:

Phase	Rises around	Highest around	Sets around	Best seen
New moon	Sunrise	Midday	Sunset	Not visible
First quarter	Midday	Sunset	Midnight	Afternoon and evening
Full moon	Sunset	Midnight	Sunrise	All night
Last quarter	Midnight	Sunrise	Midday	Late night and morning

These times are approximate and shift with the season and your latitude. NASA’s skywatching FAQ notes that, on average, the Moon rises about 50 minutes later each day. Our Moon page shows tonight’s phase and the dates of each phase.

The same face, always

The Moon spins once on its axis in the same time it takes to orbit Earth. NASA calls this tidal locking. The result is that the same side always faces us, which is why the “man in the Moon” pattern never changes.

That means there is a far side we never see from Earth, but there is no permanent “dark side.” The far side gets just as much sunlight as the near side. At new moon, when we see none of the near side lit, the far side is in full sunlight.

Which phase is best?

It depends on what you want to see.

For stargazing, meteors and the Milky Way: pick the nights around new moon. A full moon is up all night, and its light washes out faint stars, meteors and the Milky Way. In the few days either side of new moon, the Moon is a thin crescent that sets early in the evening or rises just before dawn, leaving hours of dark sky. Our guide to the best time to stargaze covers this in more detail.

For looking at the Moon itself: pick a crescent, quarter or gibbous phase, not full moon. EarthSky points out that craters and mountains stand out best along the terminator, the line between the lit and dark parts, where the Sun is low and shadows are long. At full moon the Sun shines straight down on the face we see, so there are few shadows and the surface looks flat.

For photography: a rising full moon near the horizon, with a landscape below, is a classic shot. A thin crescent in twilight, with earthshine on the dark part, is another.

Full moon names

Each full moon of the year has a popular name in the US, such as the Harvest Moon and the Hunter’s Moon. Where those names come from is more tangled than most lists suggest. Our full moon page explains each name, its dates and who uses it.