Why the Moon has phases
The Moon’s phases happen because we see different amounts of its sunlit half as it orbits Earth. The Sun always lights half of the Moon; when the Moon is roughly between us and the Sun, the lit half faces away (new Moon), and when it is on the far side of Earth from the Sun, the lit half faces us (full Moon). The whole cycle, from new Moon to new Moon, takes about 29.5 days. Earth’s shadow has nothing to do with ordinary phases; it only falls on the Moon during a lunar eclipse.
Once you see the phases as geometry, a lot of everyday Moon puzzles fall into place: why the Moon is sometimes up in the afternoon, why the full Moon rises at sunset, and why a half Moon shows far more detail than a full one. For today’s phase, see the Moon.
The eight phases
The cycle is continuous, but it is traditionally divided into eight named phases. “Waxing” means the lit part is growing; “waning” means it is shrinking. “Gibbous” means more than half lit.
| Phase | What you see | Moon’s position |
|---|---|---|
| New Moon | Nothing; the dark side faces us | Roughly in the Sun’s direction |
| Waxing crescent | A thin sliver, growing | A little east of the Sun; evening sky |
| First quarter | Half lit | About 90° east of the Sun |
| Waxing gibbous | More than half, growing | Between first quarter and full |
| Full Moon | Fully lit disk | Opposite the Sun |
| Waning gibbous | More than half, shrinking | Rising later in the evening |
| Last quarter | Half lit, the other half | About 90° west of the Sun; morning sky |
| Waning crescent | A thin sliver, shrinking | Close to the Sun before dawn |
“First quarter” and “last quarter” refer to a quarter of the way through the cycle, not a quarter of the disk lit; at both, half of the disk we see is bright. The Moon phases page shows them in detail, and the Moon phase calendar gives dates for the month ahead.
When each phase rises and sets
Because each phase corresponds to a particular angle between the Moon and the Sun, each one keeps roughly the same schedule. On average the Moon rises about 50 minutes later each day, though this varies a lot with season and latitude.
- New Moon: rises and sets with the Sun, so it is in the sky during the day, lost in the glare.
- First quarter: rises around midday, is highest around sunset, and sets around midnight.
- Full Moon: rises around sunset, is highest around midnight, and sets around sunrise.
- Last quarter: rises around midnight, is highest around sunrise, and sets around midday.
These are approximate times; local rise and set times depend on your location and date. The Moon page shows them for a place.
Why you can see the Moon during the day
The Moon is above the horizon for roughly half of every day, just like the Sun, and it isn’t tied to the night. Whenever it is above the horizon while the Sun is up, and it is bright enough to stand out against blue sky, you can see it. That happens for a large part of each month:
- In the days around first quarter, the Moon is up in the afternoon, often easy to spot in the eastern or southern sky.
- In the days around last quarter, it is still up after sunrise, in the western sky in the morning.
- Near new Moon it is too close to the Sun and too thin to see, and near full Moon it is only up at night, apart from short periods around moonrise and moonset.
So a daytime Moon isn’t unusual at all. It’s simply the geometry of the phases, and the Moon is bright enough to shine through the sky’s blue glow.
The best time to observe the Moon
Many people assume the full Moon is the best time to look at the Moon. For seeing detail, it is usually the worst. At full Moon the Sun shines straight down on the side facing us, so there are almost no shadows and the surface looks flat and glaring.
The best views come along the terminator, the line between day and night on the Moon. There the Sun is low on the lunar horizon, so crater walls and mountains cast long shadows that make them stand out in relief.
- Crescent to first quarter, and a few days after: convenient evening viewing with a dramatic terminator. A thin crescent also shows earthshine, the faint glow on the dark part of the Moon, which is sunlight reflected from Earth.
- Last quarter: just as good, but you’ll need to be up late or early.
- Full Moon: best for the bright ray systems around some young craters and for the pattern of dark maria, but poor for relief.
Binoculars show the maria and larger craters clearly; a small telescope reveals countless craters, mountains and valleys. The Moon is also bright enough to view from any city. On the other hand, if you want to see faint galaxies or meteors, the Moon’s light is a nuisance; see dark adaptation and light pollution.
Common misconceptions
- “Earth’s shadow causes the phases.” No. Phases are about viewing angle; Earth’s shadow only reaches the Moon during a lunar eclipse, which can only happen at full Moon.
- “The far side is the dark side.” The far side gets as much sunlight as the near side. We never see it from Earth because the Moon keeps the same face toward us as it orbits.
- “The Moon only comes out at night.” It is up in daylight for part of most days, as explained above.