The Moon is one of the most familiar objects in the sky – but how much do you know about it? There are many moons in the solar system, but our own is one of the most remarkable. It’s also had a huge influence on conditions here on Earth.
In fact, without it the chances of life evolving and surviving here would have been a lot lower.
Age: 4.5 billion years
Circumference at equator: 10,921km
Mass: 7.342×1022 kg
How big is the Moon?
We already know that the Moon has a diameter of 3,474.2km, but how does it compare to other objects in the solar system? Well, it’s a lot smaller than Earth – just over a quarter of the diameter, and only around 1/80th of the mass.
That’s still large for a natural satellite, though. In fact it’s the largest in size relative to the planet it orbits. Three of the four Galilean moons of Jupiter are larger – Ganymede has more than twice the mass – but compared to Jupiter itself they’re tiny.
The Moon is far larger than Pluto, which until 2006 was classed as a planet. Its diameter is greater by around 1,200km and it’s almost six times as massive. Even the largest dwarf planet, Eris, is barely more than a fifth of the Moon’s mass.
If our familiar satellite ever escaped and set off on its own orbit round the Sun it would probably be a full-fledged planet rather than a dwarf.
Could the Moon escape?
The Moon could escape – the solar system can be a violent place, and an impact from a large enough object could knock the Moon out of its orbit. That isn’t very likely though, because most bodies large enough to affect the Moon have long since settled down into orbits of their own.
A dramatic escape probably won’t happen any time soon, but the Moon is slowly moving away from Earth. The way the Moon interacts with our tides gradually transfers momentum from Earth. One effect is that Earth’s rotation is getting slower by about 23 millionths of a second every year; at the same time the Moon, with more momentum, shifts its orbit outwards by around 38mm.
However it won’t keep moving away until it finally escapes; eventually it would stabilise in a higher orbit that takes it around Earth every 47 days, compared to 27.32 days now. That would take about 50 billion years though, and by then the Moon and Earth will both have been incinerated by the expanding Sun.
Where did the Moon come from?
The Moon probably came from Earth. Most scientists believe that about 4.5 billion years ago, not long after Earth itself formed, it collided with a stray planet about the size of Mars.
The stray hit Earth at a shallow angle and bounced off, but the impact was violent enough to tear away a large chunk of Earth’s crust and throw it into space. Gradually the debris clumped together until its own gravity pulled it into a sphere.
This theory seems to be the most likely, because the Moon’s rocks are almost identical to Earth’s crust – but not to the deeper mantle.
Does the Moon help us?
Without the Moon life on Earth would be very different, and might not exist at all. It’s the Moon that causes tides, and tides could have been very important for the beginning of life.
Many biologists believe that life began in shallow tidal pools along coastlines. Warmed by the Sun, and fed a fresh supply of nutrients every time the tide came in, these would have been ideal incubators for the first living cells – but that couldn’t have happened without the Moon.
The Moon has also done a lot to protect Earth. Look at it through binoculars or a telescope and you’ll see how scarred and battered its surface is. The Moon has almost no atmosphere, so there’s no weather to erode impact craters. Those craters tell the story of thousands of objects that hit the Moon, instead of being dragged into Earth’s gravity well and hitting us instead.
The Chicxulub crater in Mexico, caused by the impact that killed the dinosaurs, is about 180km wide. The largest crater on the Moon is 2,240km wide. What would have survived if that object hit Earth instead? Probably not much.
What is it like on the Moon?
The Moon is a completely dead world. Its gravity is too weak to hold on to much of an atmosphere; the one it does have is incredibly thin – less than one trillionth as dense as Earth’s – and mostly made up of gas released from the rock.
The surface is covered in a layer of fine dust created by micrometeorite impacts. The interior has mostly cooled and solidified, although there’s a thin layer of molten rock remaining around the small iron core. But although it’s lifeless the Moon is still important to us, and it’s also one of the easiest astronomical objects to study.