Moon Phases Explained Simply (and Why They Happen)
If you have ever watched the Moon swell to a glowing circle and then quietly shrink to a sliver, you have wondered how it works. Here are the moon phases explained in plain language, so the next time you look up you will know exactly what you are seeing and why it happens.
Why moon phases happen
The Moon does not make its own light. It shines because it reflects sunlight, and the Sun always lights up exactly half of it. What changes is how much of that lit half faces you on Earth.
As the Moon orbits us over about a month, the angle between the Sun, Earth, and Moon shifts. That changing angle is the whole secret. It is not Earth's shadow falling on the Moon (that only happens during an eclipse) and it has nothing to do with clouds.
The eight phases, in order
The cycle flows smoothly from dark to full and back again. A handy rule: when the lit edge looks like the curve of a "D," the Moon is waxing (growing); when it looks like a "C," it is waning (shrinking).
- New Moon — the lit side faces away, so the Moon is nearly invisible.
- Waxing Crescent — a thin sliver returns in the evening sky.
- First Quarter — half lit, high overhead at sunset.
- Waxing Gibbous — more than half, bright and growing.
- Full Moon — fully lit, rising as the Sun sets.
- Waning Gibbous, Last Quarter, Waning Crescent — the same steps in reverse, now in the early morning sky.
Easy ways to watch the cycle
Tracking the Moon for a few weeks is one of the simplest skywatching joys, and you need nothing but your eyes.
- Sketch or photograph the Moon at the same time each night to see it change.
- Look for the crescent shortly after sunset or before sunrise, low near the horizon.
- A Full Moon washes out faint stars, so save dim targets for darker nights.
- Try binoculars near First or Last Quarter, when shadows make craters pop.
Once you know the rhythm of the Moon, the rest of the sky opens up. If you are just starting out, our guide to stargazing for beginners walks you through your first night, and you can also plan around the Moon by checking what you can see in the sky tonight.
Curious which phase is hanging over your neighborhood right now? Open the Starly sky map to see the Moon's exact phase and position from your own location, in real time.
Open the sky map with the Moon selected and see tonight’s phase from your location.
Frequently asked questions
Why does the Moon change shape?
The Sun always lights half of the Moon, but as the Moon orbits Earth the angle between the Sun, Earth, and Moon changes, so we see different amounts of the lit half.
How long is one full cycle of moon phases?
The Moon goes through all its phases in about 29.5 days, roughly one month, from one New Moon to the next.
What is the difference between waxing and waning?
A waxing Moon is growing brighter toward Full, while a waning Moon is shrinking toward New. If the lit edge looks like a D it is waxing, and like a C it is waning.