The Moon, our nearest neighbor in space. Shining brightly with reflected sunlight from its surface the Moon is as ancient as the Earth. Humans have always wondered about it, written songs and stories about it, set their calendars by it and worked by its light.
Image credit: Apollo 11, taken on return trip to Earth (NASA).
If you flew around the Earth ten times you would have gone the same distance as the Moon is from us, about 240,000 miles. It is our last great wilderness, a high desert with no air and extremes of low and high temperatures.
Feel the Moon!
The Moon is so close to the Earth that its gravity pulls strongly on us. This nearby and there is a noticeable difference between how strongly the Moon pulls on something on the side of Earth facing it, and the opposite side, facing away. This also means that the oceans get pulled up towards the Moon as it passes overhead. If you’ve ever spent a day at the beach then you’ve seen this happen – the tides that bring water up the beach and then down again every 12 hours are mostly because of the Moon !
Tides are also the reason that from Earth we only ever see one side of the Moon. That’s because billions of years ago the gravity of the Earth pulling on the Moon made it slow its spin until it took the same amount of time to spin around once as it takes for it to orbit around the Earth once – what we now call one month. Today the gravity of the Moon, and the tides on Earth are gradually slowing down how fast the Earth spins. Eventually the Earth will slow its spin until only one side of the planet always faces the Moon – but this will take many billions of years.
It’s incredible to think, but humans have only seen the far side of the Moon in the last fifty years – since we’ve been able to send spacecraft there. It looks different, and has many big craters across its surface.
Image: Far side of the Moon, seen by Apollo 16 (NASA).
The surface of the Moon
The Moon is a strange place. It is drier than the driest deserts on Earth. It has no atmosphere worth mentioning. If you stand on the Moon it is just like standing in space itself. In the daytime it can be as hot at about 250F – that’s enough to cook food if you could! At night it is as cold as -390F, far colder than anywhere on Earth. The Moon is only about 1/10th the mass of the Earth, and about a quarter the diameter. Standing on the Moon you would feel as if you weighed 1/6th as much as you do on Earth, and when things fall they fall much slower than on Earth.
It is also covered in craters, big and small holes from where asteroids, and comets, and smaller rocks from space have crashed into it. Over billions of years these craters have covered every square inch of the Moon. Every time something crashes into the Moon it grinds the rocks into smaller pieces. Most of the Moon is covered in thick, dark grey dust from all this grinding. The Moon only looks bright to us because it is so close!
Billions of years ago some parts of the Moon had huge floods of lava – hot, molten rock. Today these appear as the big dark patches on the surface of the Moon. These great flat plains across the Moon are know as “mare” (mar-ray), which comes from the Latin word for “sea”.
There are mountains on the Moon, caused by great collisions with asteroids or comets sometime in the past. The highest mountain is almost 3 miles high. Can you imagine how big the explosion must have been that made a mountain like this?
At the south pole of the Moon is an incredibly enormous crater called the Aitken Basin. It is about 8 miles deep, and is one of the biggest craters anywhere in the solar system. There are places inside the Aitken Basin where it is always in the shadow, sunlight never warms it up. Scientists think that there might be frozen water in these dark locations, and so they might be very interesting to visit.
Image: The south pole of the Moon, taken by the Clementine mission (NASA).
The Moon is the only other place in the universe that humans been to in person. In 1969 NASA’s Apollo 11 spacecraft took three astronauts to the Moon. While one of them, Mike Collins kept watch in orbit, two others – Buzz Aldrin and Neil Armstrong – became the first human beings to set foot on another world. Apollo 11 stayed for about a day, there were then five other Apollo missions that landed on the Moon, with the last one – Apollo 17 – in 1972.
Before and after humans visited the Moon there have been many other spacecraft and robotic landers that have gone there. Many have mapped out the surface of the Moon, studied the types of rocks it is made of, and even searched for frozen water deep inside craters near its north and south poles.
The origin of the Moon
Of all the rocky planets, only the Earth has a large Moon. Where did it come from? Four and a half billion years ago the Earth was probably hit by a huge object called a proto-planet. This object was about the size of Mars! Imagine a collision like that, it would have almost destroyed the planet.
When this object hit it broke up and tore away a lot of the outside rock of the Earth. All this rock and rubble would have ended up in a great ring orbiting the young Earth – a bit like the rings around Saturn. Pretty quickly though all this rocky, dusty material would have stuck together because of gravity – perhaps in as little as a month! This is what made the Moon, all the left-over stuff from that great collision. This is why the inside of the Moon is very different from the inside of the Earth – it is made mainly from all the lighter rock that ends up on the outside of planets, and not so much of the heavy elements like iron that tend to sink down to the middle of planets.
So part of the Moon is really a part of the Earth, and a part of our planet’s history!
Image: Earth seen from Moon, taken by Apollo 8 (NASA)
Seeing the Moon
From the Earth the Moon appears to change every night. Sometimes we see a thin crescent, sometimes we see a full circle, and sometimes we can’t see it at all! This is because it takes the Moon about 27 days to orbit around the Earth – but the Earth spins once every 24 hours, and so each night we see the Moon in a slightly different place.
The way we see the Moon is by the sunlight that reflects from its surface. So sometimes we can only see a little bit of the Moon’s dayside, and it looks like a great crescent, or arc. When the Moon is closer to the Sun than we are then we usually can’t see it at all because we can only see its night side, during our daytime. These changes are called the phases of the Moon.
Image: the phases of the Moon (Wikimedia Commons)
Sometimes, when the Moon is between us and the Sun it can line up so that it casts a shadow onto the Earth! This is called an eclipse. The Moon can even block out all of the Sun for a short time, which is called a total eclipse. The Moon can even block out all of the Sun for a short time, which is called a total eclipse.
Think about it
The Moon is like a remote desert, it’s almost a part of the Earth, but we’ve only explored it a tiny bit because it’s so hard to get to. It also changes the way things work here on Earth – it helps make the tides in the oceans. How many creatures can you think of that rely on the tides to help them survive? The Moon has been around for as long as there has been life on Earth, shining in the night sky. What might ancient humans have thought about the Moon? Some animals seem to respond to the phases of the Moon, why do you think that is?
The Moon is a beautiful thing in the sky, but it’s also a huge world of its own.