Why Does the Moon Change Shape? The Eight Moon Phases Explained
The Moon's shape never changes. What changes is our view of it. As the Moon orbits Earth, we see a different amount of its sunlit half each night, and those views are called the Moon's phases.
The Moon makes no light of its own
According to NASA, the only light source in our solar system is the Sun. "Moonlight" is sunlight reflecting off the Moon's surface. Sunlight always lights up one half of the Moon, the half facing the Sun. The other half is dark.
Why the view changes
Where the Moon sits in its orbit around Earth decides how much of the lit half faces us.
- At full moon, we see the entire lit side.
- After each full moon, as the Moon moves along its orbit, we see less and less of the lit part.
- At new moon, the far side of the Moon faces the Sun, so the side facing Earth is dark and we cannot see the Moon.
The eight phases, in order
- New: the Moon is not visible.
- Waxing crescent: in the Northern Hemisphere, a thin crescent of light on the right.
- First quarter: looks like a half moon.
- Waxing gibbous: between a half moon and a full moon.
- Full: completely illuminated.
- Waning gibbous: between full and half, getting smaller.
- Third quarter: a half moon again, the opposite half from first quarter.
- Waning crescent: in the Northern Hemisphere, a thin crescent on the left.
"Waxing" means getting bigger, and "waning" means getting smaller. NASA's descriptions of left and right apply to the Northern Hemisphere, such as in India. In the Southern Hemisphere the lit side appears reversed.
How long does a cycle take?
NASA gives two numbers:
- About 27.3 days for the Moon to orbit Earth once.
- About 29.5 days to go from one new moon to the next.
The cycle is longer than one orbit because Earth is also moving around the Sun. The Moon has to travel a little further to reach the same position relative to the Sun and Earth. This is why phases take longer to repeat than the orbit itself.
Common questions
Does Earth's shadow cause the phases? No. The phases come from our viewing angle on the lit half of the Moon. NASA's explanation does not involve Earth's shadow.
Why can we not see the Moon at new moon? The lit half is facing away from us.
Is the Moon the same shape every night? Yes. Only the portion we can see in sunlight changes.
Try it at home
In a dark room, put a lamp on one side and hold a ball at arm's length. Turn slowly in a circle and watch how much of the lit half of the ball you can see from where you stand. You will see the same sequence of shapes.
Facts checked October 2, 2026.
Sources
- NASA Space Place, "What Are the Moon's Phases?": https://spaceplace.nasa.gov/moon-phases/en/