Moon Phases Explained: How They Work & the 8 Phases

The moon looks different from one night to the next — a thin sliver, a half disc, a bright full circle. Those shapes are the moon phases, and they follow a predictable cycle that repeats about every 29.5 days. Here is what causes them and how to read them.

What actually causes the phases

The moon does not glow on its own. It reflects sunlight, and the sun always lights exactly half of it. What changes is how much of that lit half we can see from Earth. As the moon orbits us, the angle between the sun, the moon and your eye shifts, so the sunlit portion we see grows and shrinks. A common myth is that the phases come from Earth’s shadow falling on the moon — they do not. Earth’s shadow only touches the moon during a lunar eclipse, which happens a couple of times a year at most.

The eight phases, in order

• New Moon — the lit side faces away from us, so the moon is essentially invisible.
• Waxing Crescent — a thin sliver appears and grows.
• First Quarter — exactly half is lit (the right half, up north).
• Waxing Gibbous — more than half, still growing.
• Full Moon — the whole face is lit; the moon is opposite the sun.
• Waning Gibbous — more than half, now shrinking.
• Last Quarter — half lit again (the left half, up north).
• Waning Crescent — a shrinking sliver before the next new moon.

Waxing vs waning: a quick trick

Waxing means the lit part is growing; waning means it is shrinking. In the Northern Hemisphere, if the right side is bright the moon is waxing, and if the left side is bright it is waning. (South of the equator, flip that.) The two "quarter" phases are confusingly named — at first and last quarter you see a half-lit moon, because the moon has completed a quarter or three-quarters of its cycle, not because a quarter is lit.

How long it all takes

One complete round of phases — new moon to new moon — takes roughly 29.5 days, called the synodic month. That is a little longer than the 27.3 days the moon takes to orbit Earth, because Earth itself moves around the sun during that time, so the moon has to travel a bit further to line back up with the sun.

The easiest way to make this concrete is to watch it: the Moon Phase tool shows tonight’s phase and illumination, the dates of the next full and new moon, and a 30-day calendar so you can see the whole cycle at a glance. Curious about the odd ones out? Read what a blue moon and supermoon really are.