The Moon has been the Earth’s nearest neighbour for around 4.5 billion years. Since humanity's earliest recorded history, it’s been a source of fascination, myth and wonder. But our constant companion is leaving Earth’s orbit... by four centimetres a year.
Our best theory is that billions of years ago the Earth had a collision with another planet and from that impact the Moon was created. (This sort of thing used to happen quite a bit in the early days of the solar system.) It was previously thought that the Moon was mostly made up of matter originating from the other planet, which astronomers call Theia. But actually, recent evidence suggests that most of the Moon was was probably formed from material that was shorn from the Earth during the collision. It's strange to think that the surface of the Moon, which looks so calm now, was once molten and fiery. It actually still has a molten core, similar to Earth's, deep underneath its surface.
If your mass is 100kg, then the weight you feel on Earth is 100kg times the effect of gravity. The Moon's gravitational pull is only 16.7% of the Earth's, so you would weigh far less if you stood on the Moon’s surface, meaning you can jump about with much less effort.
The Moon exerts a gravitational pull on the Earth. In particular, it has a really big effect on the oceans, where it causes two great bulges. We experience these bulges as tides.
How does that relate to the Moon pulling away from the Earth?
When the US Apollo missions visited the Moon, they left behind mirrors called retroreflectors that reflect laser light sent from Earth. Since we know the speed that the laser light travels at, we can accurately calculate the distance between the Earth and the Moon.
These measurements are among the most precise scientific measurements mankind has ever made. While the Moon is, on average, 384,467km away, it is now possible to measure the exact distance at any time to within a few centimetres.
and as it spins the tidal bulges pull back on the Moon. Because of the interplay between these gravitational forces, the Earth rotates incrementally slower each year and our days get a tiny bit longer. This allows the Moon’s orbit to grow that bit larger, creeping away from the Earth.
This doesn't mean that the Moon doesn’t spin; it does. But it spins around its axis once in the same time that it takes to travel around the Earth, which is why we only see one face of the Moon. Billions of years ago the Moon was not tidally locked, and spun at a much faster rate. If we had stood on the surface of a young earth, we would have been able to see the Moon’s different faces.
Tidal locking of moons is quite common throughout our solar system. But Pluto and its moon Charon are unusual because they are tidally locked to each other. On one side of Pluto, Charon is a permanent fixture in the sky, but on Pluto’s other hemisphere, Charon never appears.
The continued gravitational tugging between Moon and the tides of the spinning Earth will continue to make the days on Earth longer and longer until the length of a single day matches the time it takes the Moon to orbit the Earth.
But that would take billions of years. Long before that happens our Sun will grow to a red giant and engulf both the Earth and the Moon.
So we’ll be friends to the end.