UtilityBase2 min read
The 8 Phases of the Moon, Explained
Why the Moon appears to change shape, the eight phases in order, and a simple trick for telling whether tonight's Moon is waxing or waning at a glance.
Why the Moon seems to change shape
The Moon doesn't produce its own light — it reflects sunlight. Exactly half of it is always lit, but as the Moon orbits Earth we see that lit half from different angles, so the sunlit portion facing us grows and shrinks. That changing view is what we call phases.
A common misconception is that phases are caused by Earth's shadow falling on the Moon. They aren't — that's a lunar eclipse, which is rare. Phases are simply about geometry: where the Moon, Earth, and Sun sit relative to one another.
The eight phases in order
The cycle runs: new moon (no lit side visible), waxing crescent, first quarter (half lit, growing), waxing gibbous, full moon (fully lit), waning gibbous, last quarter (half lit, shrinking), and waning crescent, before returning to new.
'Quarter' refers to the Moon being a quarter of the way through its cycle, even though we see a half-lit disc. 'Gibbous' means more than half lit but not full. The whole cycle from one new moon to the next — the synodic month — takes about 29.5 days.
Reading tonight's Moon
You can tell where the Moon is in its cycle with a quick look, especially in the Northern Hemisphere.
- 1Notice how much of the disc is lit — a sliver, a half, a bulge, or full.
- 2Check which side is lit: in the Northern Hemisphere, a right-lit Moon is waxing (growing), a left-lit Moon is waning (shrinking).
- 3Combine the two: right-lit sliver is a waxing crescent; left-lit bulge is a waning gibbous.
- 4Remember the hemispheres are mirror images — the lit side flips south of the equator.
Frequently asked questions
How can I tell if the Moon is waxing or waning?
In the Northern Hemisphere, if the right side is lit, the Moon is waxing (heading toward full); if the left side is lit, it's waning (heading toward new). South of the equator it's reversed. Waxing means the lit area is growing night to night.
Why don't we get an eclipse every full moon?
The Moon's orbit is tilted about 5 degrees relative to Earth's orbit around the Sun, so most months the full moon passes slightly above or below Earth's shadow. Only when the alignment is close enough do we get a lunar eclipse.
How long is one full cycle?
About 29.5 days from one new moon to the next — this is called the synodic month. It's slightly longer than the Moon's orbit around Earth because Earth is also moving around the Sun, so the Moon has to travel a little farther to line back up.
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