What Are Moon Phases?
Why the Moon seems to change shape every night — even though it's always a full sphere.
The Moon Doesn't Glow
Here's something that surprises a lot of people: the Moon doesn't produce any light of its own. It's a big ball of rock floating in space, and the bright glow you see at night is actually sunlight bouncing off the Moon's surface and reflecting toward Earth. The Moon acts like a giant mirror — a dusty, cratered mirror, but a mirror nonetheless.
So why does the Moon look different each night? It's not changing shape. The Moon is always a complete sphere. What changes is how much of the sunlit side you can see from Earth as the Moon orbits around us. That's what creates the phases.
The Lunar Cycle — 29.5 Days
The Moon takes about 29.5 days to complete one full cycle of phases — from new moon back to new moon. This period is called a lunar month (and it's where the word "month" originally comes from). During that time, the Moon appears to grow from a thin sliver to a full circle, then shrink back to nothing, over and over.
The Eight Phases
New Moon: The Moon is between the Earth and the Sun, with its sunlit side facing away from us. The Moon is essentially invisible in the night sky.
Waxing Crescent: A thin sliver of light appears on the Moon's right side (in the Northern Hemisphere). "Waxing" means growing — the lit portion is getting bigger each night.
First Quarter: Exactly half the Moon's face is lit. It's called "quarter" because the Moon is one-quarter of the way through its cycle, not because of how much is visible.
Waxing Gibbous: More than half but not yet full. "Gibbous" means swollen or humped — the Moon looks like a slightly flattened circle with a dark bite taken out of one side.
Full Moon: The entire face of the Moon is lit by the Sun. Earth is between the Sun and Moon, so sunlight fully illuminates the side facing us.
Waning Gibbous: The light starts shrinking. "Waning" means decreasing — the shadow creeps in from the right side.
Third Quarter (Last Quarter): Half the Moon is lit again, but the opposite half from the First Quarter.
Waning Crescent: A thin sliver remains before the cycle resets to a new moon.
Eclipses — When Things Line Up Perfectly
Occasionally, the Sun, Moon, and Earth line up in a nearly perfect straight line. When the Moon passes directly between the Earth and Sun, it blocks the sunlight and creates a solar eclipse. When the Earth passes between the Sun and Moon, Earth's shadow falls on the Moon, creating a lunar eclipse — the Moon often turns a dramatic reddish color during these events. Eclipses don't happen every month because the Moon's orbit is slightly tilted compared to Earth's orbit around the Sun.
The Moon is slowly drifting away from Earth — about 3.8 centimeters per year. Billions of years ago, the Moon was much closer, and a day on Earth was only about 6 hours long. The Moon's gravity creates tides that gradually slow Earth's rotation, making our days longer over immense timescales. In the distant future, a day on Earth will eventually be about 50 hours long.
Last reviewed: April 2026