The Moon is the most familiar object in the night sky, and also one of the most changeable. Over the course of about a month, it grows from a thin silver sliver into a brilliant full disk and then shrinks back again until it disappears. People have used these changes to measure time for thousands of years; the word "month" is related to "Moon." Yet many people are unsure what causes the phases. A common belief is that Earth's shadow covers part of the Moon. That is not correct. The real explanation involves sunlight, geometry and the Moon's journey around Earth. This explainer walks through the phases step by step.
The Moon does not make its own light
The first key fact is that the Moon does not shine on its own. Like the planets, it reflects light from the Sun. At every moment, the Sun lights up half of the Moon, the side facing the Sun, while the other half is in darkness. This is exactly like Earth, where it is day on the side facing the Sun and night on the other.
What changes during the month is not how much of the Moon is lit, but how much of the lit half we can see from Earth.
The Moon's orbit
The Moon orbits Earth about once every 27.3 days relative to the distant stars. This is called the sidereal month. But because Earth is also moving around the Sun during that time, it takes a bit longer, about 29.5 days, for the Moon to return to the same position relative to the Sun and Earth. This cycle of phases is called the synodic month or lunar month.
As the Moon travels around Earth, the angle between the Sun, Earth and the Moon changes. That changing angle determines how much of the sunlit half faces us.
The eight phases
Astronomers usually describe eight phases.
| Phase | Position | What we see |
|---|---|---|
| New moon | Moon between Earth and Sun | lit side faces away; Moon nearly invisible |
| Waxing crescent | a little east of the Sun in the sky | thin crescent, growing each night |
| First quarter | 90° from the Sun | half of the disk lit |
| Waxing gibbous | between first quarter and full | more than half lit, growing |
| Full moon | Earth between Sun and Moon | entire face lit |
| Waning gibbous | after full moon | more than half lit, shrinking |
| Third (last) quarter | 90° on the other side | other half of the disk lit |
| Waning crescent | approaching new moon | thin crescent, shrinking |
"Waxing" means growing, and "waning" means shrinking. "Gibbous" comes from a Latin word meaning hump-backed, describing the bulging shape when more than half the disk is lit.
The names "first quarter" and "third quarter" can be confusing, since we see half of the disk lit. They refer to the Moon being a quarter and three quarters of the way through its cycle.
Following the Moon through a month
New moon
At new moon, the Moon lies roughly between Earth and the Sun. The sunlit side faces away from us, and the side facing Earth is dark. The Moon is also in the sky during the day, close to the Sun, so it is lost in the glare. We usually cannot see it at all.
Waxing crescent
A day or two after new moon, the Moon has moved a little along its orbit. Now we can see a thin edge of the sunlit side. In the evening, shortly after sunset, a slim crescent appears low in the western sky. Sometimes you can also see the dark part of the Moon faintly glowing. This is called earthshine: sunlight reflected from Earth onto the Moon, then back to our eyes.
First quarter
About a week after new moon, the Moon has traveled a quarter of its orbit. From Earth, we see exactly half of the disk lit. At this phase, the Moon rises around noon, is high in the sky at sunset and sets around midnight.
Waxing gibbous
Over the next week, more and more of the sunlit side becomes visible. The Moon grows into a gibbous shape and rises later each afternoon.
Full moon
About two weeks after new moon, Earth lies roughly between the Sun and the Moon. Now the entire sunlit half faces us, and we see a full, round Moon. A full moon rises at sunset and sets at sunrise, shining all night.
Waning phases
After full moon, the process reverses. The lit portion shrinks through waning gibbous, third quarter and waning crescent. The waning Moon rises later each night and is visible in the morning sky. Before dawn, a thin waning crescent can be seen low in the east, shortly before the cycle begins again with the next new moon.
Why it is not Earth's shadow
It is easy to assume that Earth's shadow causes the phases, but this is not the case. If it did, the Moon would always be close to the Sun's opposite side when partially shadowed, and the curved edge would look different. More importantly, during most of the month, the Moon is not in Earth's shadow at all.
Earth's shadow does fall on the Moon during a lunar eclipse, but that only happens occasionally, at full moon, when the Sun, Earth and Moon line up very precisely. The phases happen every month because of the changing viewing angle, not because of shadows.
Why we don't have eclipses every month
If the Moon passes between Earth and the Sun every new moon, and Earth passes between the Sun and Moon every full moon, why are there not solar and lunar eclipses every month? The reason is that the Moon's orbit is tilted by about 5 degrees relative to Earth's orbit around the Sun. Most months, the Moon passes slightly above or below the line between Earth and the Sun. Eclipses only occur when a new or full moon happens near the points where the Moon's orbit crosses Earth's orbital plane.
Why we always see the same side
Another fascinating fact is that we always see the same side of the Moon. This is not because the Moon does not spin. It does spin, but exactly once for every orbit around Earth. This is called synchronous rotation or tidal locking.
Over billions of years, Earth's gravity raised tidal bulges in the Moon's rock. The pull on these bulges gradually slowed the Moon's rotation until its spin matched its orbital period. Many moons in the solar system are tidally locked to their planets.
The side we never see is sometimes called the "dark side of the Moon," but that name is misleading. The far side receives just as much sunlight as the near side; it is only dark during our full moon, when the near side is fully lit. The far side was first photographed by a Soviet spacecraft in 1959, and China landed a rover there in 2019.
Because the Moon's orbit is slightly elliptical and tilted, we can actually see a little more than half of its surface over time, about 59 percent, thanks to small wobbles called librations.
How to tell the phase at a glance
In the Northern Hemisphere, a simple trick helps:
- If the right side of the Moon is lit, it is waxing, growing toward full.
- If the left side is lit, it is waning, shrinking toward new.
A memory aid: when the Moon looks like the curve of a "D," it is developing; when it looks like a "C," it is coming to an end. In the Southern Hemisphere, this is reversed, because observers see the Moon "upside down" compared with the north. Near the equator, crescents can appear to lie on their backs, like a smile.
The Moon's influence on Earth
The Moon's phases are closely linked with tides. Ocean tides are caused mainly by the Moon's gravity. When the Sun and Moon line up at new and full moon, their combined pull creates larger tides, called spring tides. At first and third quarter, when they pull at right angles, tides are smaller, called neap tides. The term "spring" has nothing to do with the season; it comes from the idea of the water "springing up."
The Moon's phases have also shaped calendars across cultures. Islamic, Hebrew and Chinese calendars, among others, are based fully or partly on lunar months.
Supermoons and blue moons
Because the Moon's orbit is slightly elliptical, its distance from Earth varies between about 221,500 and 252,700 miles (356,500 and 406,700 kilometers). A full moon near its closest point, called perigee, appears slightly larger and brighter. This is popularly called a supermoon. The difference is noticeable in measurements but subtle to the eye.
A "blue moon" usually refers to the second full moon in a calendar month, which happens about every two to three years. It is not actually blue.
Questions readers ask
What causes the phases of the Moon? The changing angle between the Sun, Earth and the Moon as the Moon orbits Earth. Half of the Moon is always lit by the Sun; we see different portions of that lit half.
How long does it take to go through all the phases? About 29.5 days, called a synodic or lunar month.
Why can I sometimes see the Moon during the day? The Moon spends about half of each day above the horizon, and for much of the month it is bright enough to be visible in daylight.
Why is the full moon so bright? At full moon, we see the entire sunlit side facing us. Moonlight is still only a tiny fraction as bright as sunlight.
Is there really a dark side of the Moon? There is a far side that we never see from Earth, but it receives sunlight just like the near side.
In short
The Moon's phases come from geometry, not shadows. The Sun always lights half of the Moon, and as the Moon orbits Earth every 29.5 days, we see more or less of that sunlit half: from a dark new moon through crescents, quarters and gibbous phases to a bright full moon and back again. Because the Moon spins once per orbit, we always see the same face. Next time you look up, notice which side is lit and how high it is at sunset, and you will know exactly where the Moon is on its monthly journey.