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Why Does the Moon Change Shape?

New moon, half moon, full moon - what explains all these changing shapes? Watch this video to find out, or read our explanation below!


earth and moon

The Moon Revolves Around the Earth

The Moon isn’t actually changing shape - it’s just changing places, and that changes how much of it is lit up by the Sun. The Moon is in orbit around the Earth, meaning it’s constantly revolving around us. It completes one full revolution every 27 days; in fact, the word “month” comes from the same root as word Moon.

 

Only Half of the Moon is Lit up at a Time

The Earth has a day side and a side side - a side facing the Sun, and a side facing away. The Moon is exactly the same. Only one side of the Moon is lit up at a time, so that's the only side we can ever see. If you’re looking at a Half Moon in the sky, you’re seeing half of the lit-up side. The other half is still there - it’s just not lit up, so you can’t see it!

Half Moon with dark half outlined
 
phases of the Moon

The Moon’s Position Determines How Much we can See

When the Earth is between the Moon and the Sun, the side of the Moon facing the Earth is fully lit by the Sun. When the Moon is between the Earth and the Sun, the opposite is true - the side of the Moon facing us is totally dark. The other phases of the Moon - half Moon, crescent Moon, and everything in between - are what we see as the Moon moves between these two states.

Moon Facts for Kids

Questions about the Moon? Look no further!

  • Yes! Well, with a lot of help. NASA has plans to establish a lunar base within the next 20 years that would allow astronauts to live on the Moon for up to 2 months.

  • Scientists believe the Moon used to be part of the Earth! According to the leading theory, when the Earth was first forming, another small early planet the size of Mars smashed into it, combining with it and blasting debris into orbit. That debris collected together over time due to gravity and eventually formed the Moon!

  • Yes! Astronomers have discovered ice on the North and South poles of the Moon, inside cold, permanently shadowed craters.

  • No life has yet been discovered on the Moon, and astronauts have checked! Scientists analyzed Moon rocks from the Apollo mission for lunar lifeforms, but didn’t find anything.

  • The Moon’s diameter is about ¼ Earth’s diameter. That means that if the Earth were the size of a bowling ball, the Moon would be about the size of a golf ball.

  • The movement of the Moon across the sky is mainly due to the rotation of the Earth. The Moon is also moving, but the Moon takes 27 days to get all the way around the Earth. Meanwhile, it takes the Earth just 12 hours to rotate halfway around. So the Moon’s movement across the sky is just like the Sun’s movement across the sky - mainly due to the rotation of the Earth.

  • Scientists estimate that the Moon is about 4.425 billion years old - only about 100 million years younger than the Earth.

  • There isn’t any weather on the Moon, because the Moon doesn’t have an atmosphere. In other words, there’s no air, so no wind to blow around. Because there’s no air, if you dropped a hammer and a feather at the same time on the Moon, they would fall at the same exact speed. Don’t believe it? An astronaut on the Apollo 15 mission tried it in 1971, so you can watch the video to see for yourself!

  • Yes! Scientists have successfully grown plants in samples of lunar dirt (called regolith) brought back from the Apollo missions. The plants grew slowly, but the fact that it’s possible at all is promising for establishing a lasting human presence on the Moon.

  • The Moon takes 29.5 days to fully rotate, meaning that by the time a day passes on the Moon, it will have fully gone around the Earth, which only takes 27 days.

  • The Moon is farther away than most people think - you could fit 30 Earths in between the Earth and the Moon. Put another way, you could fit every single planet in the Solar System between the Earth and the Moon. And put a third way, if the Moon were a yo-yo, and your hand were the Earth, the string would be 6 feet long. The Moon is usually pictured as closer to the Earth than it really is because to picture it accurately requires making both the Moon and the Earth very small.

  • The Moon doesn’t have earthquakes, but it does have moonquakes! The strongest of them registered a 5.5 on the Richter scale - strong enough to move heavy furniture on Earth. And they continued for over 10 minutes! Astronomers still aren’t sure what causes these quakes - maybe astronauts on a Moon Base could study them further.



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