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 phases of the moon (also called lunar phases, or simply moon phases), and they follow a predictable cycle that repeats about every 29.5 days. Here is how moon phases work, the eight phases in order, and how to read them at a glance — no telescope or astronomy background needed.

How do moon phases work?

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. In short, a phase is just how much of the moon’s sunlit half is turned toward us on a given night. 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 8 phases of the moon, in order

Astronomers split the cycle into eight phases. Four are "primary" moments — new, first quarter, full and last quarter — and four are longer "intermediate" stretches between them. Here are the 8 phases of the moon in order, with roughly how much of the disc looks lit (its illumination):

• New Moon — 0% lit. The sunlit side faces away from us, so the moon is essentially invisible.
• Waxing Crescent — a growing sliver, up to about half lit, low in the western sky after sunset.
• First Quarter — 50% lit (the right half, up north), about a week after the new moon.
• Waxing Gibbous — more than half lit and still growing toward full.
• Full Moon — 100% lit. The moon sits opposite the sun and rises as the sun sets.
• Waning Gibbous — more than half lit, but now shrinking.
• Last Quarter — 50% lit again (the left half, up north), about a week after the full moon.
• Waning Crescent — a thin, shrinking sliver in the eastern sky before dawn, leading back to the next new moon.

When you can see each phase

The phase controls not just the moon’s shape but when it is up. A new moon rises and sets with the sun, so it is lost in daylight. A full moon does the opposite — it rises at sunset and is visible all night. The crescents hug the sun: a waxing crescent shows briefly after sunset, while a waning crescent appears just before sunrise. The quarter moons are the easiest to catch, riding high in a dark sky in the evening (first quarter) or the early morning (last quarter). If you are learning the phases of the moon for the first time, tracking one quarter moon across a week is the quickest way to see the cycle click into place.

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, or learn why each month's full moon has its own name.