Science
Why does the Moon change shape?
Quick answer
The Moon does not really change shape. As it orbits Earth, we see more or less of its sunlit half, which gives the phases from new moon to full moon and back in about 29.5 days.
In one sentence
The Moon's phases are the different amounts of its sunlit half that we can see as it travels around Earth.
Explained
The Moon makes no light of its own. It shines because it reflects sunlight, and the Sun always lights one half of it, just as it lights one half of Earth. The Moon takes about a month to orbit Earth. As it moves, we look at that lit half from a changing angle.
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At new moon the Moon sits between us and the Sun, so the lit side faces away and we see almost nothing. A few days later a thin curve appears: a waxing crescent. "Waxing" means growing. Next comes first quarter, when half of the face we see is lit (it looks like a half circle because we see a quarter of the whole Moon). Then the waxing gibbous, mostly lit but not quite round, and finally the full moon, when the whole face we see is lit.
After full moon the light shrinks again: waning gibbous, last quarter, waning crescent, and back to new moon. "Waning" means getting smaller.
In the northern hemisphere, a waxing moon is lit on the right and a waning moon on the left. In the southern hemisphere it is the other way round, because you see the Moon from the other side of the planet.
The Moon always shows us the same face, because it spins once for each orbit. The phases are not caused by Earth's shadow. A shadow on the Moon happens only during an eclipse. For more about our neighbour, see the Moon.
Examples
| Phase | What you see |
|---|---|
| New moon | no Moon, or a very dark disc |
| Waxing crescent | a thin curve, growing |
| First quarter | right half lit (northern hemisphere) |
| Full moon | a whole bright circle |
| Last quarter | left half lit (northern hemisphere) |
| Waning crescent | a thin curve, shrinking |
Common mistakes
- "It is Earth's shadow." Phases come from our viewing angle on the Moon's lit half.
- "The Moon only comes out at night." You can often see it in daytime, especially as a waxing or waning crescent.
- "A half-lit Moon is a half moon." It is called a quarter moon, because the Moon has travelled a quarter of the way round its orbit.
Quick quiz
Phases of the Moon
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What is a full moon?
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Related topics
The Moon
The Moon is Earth's natural satellite, a rocky ball about a quarter of Earth's width that orbits us roughly once a month. It shines by reflecting sunlight.
Day and night
We have day and night because Earth spins once around every 24 hours. The side facing the Sun has daylight, and the side facing away is in darkness.
Solar and lunar eclipses
An eclipse happens when the Sun, Earth and Moon line up. In a solar eclipse the Moon blocks the Sun; in a lunar eclipse Earth's shadow falls on the Moon.
The solar system
The solar system is the Sun and everything that orbits it, including eight planets, their moons, dwarf planets such as Pluto, asteroids and comets.
Space exploration
Space exploration began in 1957 with the first satellite, Sputnik 1. Since then people have orbited Earth, walked on the Moon and sent robots to planets, moons and beyond.
Stars
A star is a giant ball of hot, glowing gas that makes its own light. The Sun is our nearest star, and the others look like tiny points because they are so far away.