Why Does the Moon Have Phases?
Some nights the Moon is a bright full circle. A week later it is half lit, and a week after that it may have vanished. The Moon itself does not change shape. What changes is how much of its sunlit side we can see from Earth, and NASA explains the whole cycle with one simple idea.
1. The Moon does not make its own light
NASA points out that in our solar system, the only object that shines with its own light is the Sun. Moonlight is sunlight that has bounced off the Moon's surface. Without the Sun, the Moon would be completely dark.
Because sunlight comes from one direction, it always lights exactly half of the Moon, the half facing the Sun. The other half is in darkness. Like Earth, the Moon has a day side and a night side.
2. Phases are about our viewing angle
As the Moon travels around Earth, we see that lit half from different angles. NASA Space Place puts it this way: our view of the illuminated part changes depending on where the Moon is in its orbit.
- When the Moon is on the far side of Earth from the Sun, its lit half faces us. We see a full Moon.
- When the Moon is roughly between Earth and the Sun, its lit half faces away from us. We see almost nothing, a new Moon.
- In between, we see part of the lit half and part of the dark half, which creates crescents, quarters, and gibbous shapes.
A simple way to see this at home: in a dark room with a single lamp on one side, hold a ball at arm's length and turn slowly in a circle. Watch how the lit part of the ball changes shape as you turn.
3. The eight phases, in order
NASA lists the phases as:
- New Moon: the lit side faces away from Earth, and the Moon is in the same part of the sky as the Sun, so it is not visible.
- Waxing crescent: a thin sliver. From the Northern Hemisphere, the light is on the right.
- First quarter: we see half of the Moon's face lit.
- Waxing gibbous: more than half, and growing.
- Full Moon: the whole face is lit, with the Moon opposite the Sun in our sky.
- Waning gibbous: more than half, and shrinking.
- Third quarter (also called last quarter): the opposite half is lit compared with first quarter.
- Waning crescent: a thin sliver. From the Northern Hemisphere, the light is on the left.
"Waxing" means growing and "waning" means shrinking. "Gibbous" describes the bulging shape between half and full.
4. Why a "half Moon" is called a quarter
NASA notes a detail that confuses many people. At first quarter, the Moon looks half lit, but you are really seeing half of the illuminated half, which is a quarter of the whole Moon. The name also reflects timing: the Moon is a quarter of the way through its monthly cycle.
5. How long the cycle takes
NASA Space Place says it takes about 27.3 days for the Moon to orbit Earth, but about 29.5 days to go from one new Moon to the next.
Why the extra two days or so? During those 27.3 days, Earth has also moved along its own path around the Sun, so the direction of sunlight has shifted. The Moon has to travel a bit farther to line up with the Sun again and return to the new Moon position.
With 8 named phases spread over about 29.5 days, each one lasts roughly 3.7 days on average. NASA notes, for example, that the Moon appears full for a couple of days.
6. Why there is not an eclipse every month
If a new Moon happens when the Moon is roughly between Earth and the Sun, why doesn't it block the Sun every month? NASA explains that the Moon usually does not pass directly between Earth and the Sun at new Moon, because its orbit is tilted by about 5 degrees. It only passes near the Sun from our point of view. Solar eclipses happen only on the occasions when the alignment is exact.
7. When to look for each phase
NASA gives rough rise and set times for most latitudes:
| Phase | Rises | Sets |
|---|---|---|
| Waxing crescent | morning, after sunrise | night, after sunset |
| First quarter | around midday | around the middle of the night |
| Full Moon | at sunset | at sunrise |
| Third quarter | around the middle of the night | around midday |
| Waning crescent | very early morning, before sunrise | afternoon, before sunset |
This explains why you can often see the Moon in the daytime. A first quarter Moon, for example, is up through the afternoon. For exact times in your location, check local predictions.
8. A bonus: earthshine
During a crescent phase, you can sometimes see the dark part of the Moon glowing faintly. NASA calls this earthshine: sunlight reflected off Earth onto the Moon. From the Moon at that time, Earth looks nearly full, so it reflects enough light to dimly light the Moon's night side.
9. Key takeaways
- The Sun always lights half of the Moon; phases come from how much of that lit half we can see.
- The eight phases run from new Moon through first quarter and full Moon to third quarter and back.
- The Moon orbits Earth in about 27.3 days, but a full cycle of phases takes about 29.5 days because Earth is moving around the Sun too.
- The Moon's 5 degree orbital tilt is why a new Moon does not cause a solar eclipse every month.
- Each phase rises and sets at predictable times, which is why the Moon is often visible in daylight.