What Moves the Moon?

"Mummy, what moves the Moon?" - Question posed by Joseph (to his mummy. I'm not his mummy).

The Moon

Essentially, the question is the wrong way around: The Moon is moving because nothing's stopping it!

To understand why this is the case we need to be introduced to...

Newton's First Law of Motion

The "Newton" here is that Newton: the famous one with the apple story*. He figured out three laws of motion. The first one is...

Every body persists in its state of being at rest or of moving uniformly straight forward, except insofar as it is compelled to change its state by force impressed.

... which basically means that if an object is moving it keeps doing that until something happens to change that. Or, if an object isn't moving it stays perfectly still until something happens to change that.

We're used to things not staying in motion on Earth because they're subject to all sorts of forces: a ball stops rolling along the floor not because it isn't being given energy to keep moving but because it is robbed of its energy by things like friction.

So the Moon moves not because something is making it move, but because it has always been moving and nothing has stopped it yet.

Actually, that "always" in the previous sentence isn't strictly true: Nothing has been around forever. Most of the motion in our Solar system is left-over angular momentum from when it originally formed**.

A related and very pertinent question, though, is what keeps the Moon moving in a circle*** around the Earth? According to Newton, the Moon should just be heading off in a straight line forever...

The Moon is being affected by forces: Earth's gravitational field provides the force required to perpetually turn the Moon in its travels and keep it in orbit around the Earth. Its own momentum keeps it moving forwards, as per Newton, but Earth's gravitational influence continually changes what "forwards" means as far as the Moon is concerned.

Having said all that, there is another force acting on the Moon. You're probably aware that the tides of Earth's oceans are caused by the Moon, but the oceans themselves also affect the Moon in return! In this case, they're not slowing it down but speeding it up!

The Moon's orbit is speeding up over time, which means that it is also spiralling away from us... Not very quickly, but the effect is not just theoretical: it has been measured very accurately by firing lasers at mirrors left on the Moon by Apollo astronauts.

If you want to know more about Newton's laws (or anything else in this post) you know what to do.

*Probably not true.
** Simply put, a big cloud of gas collapsed under gravity and started to spin as it did.
*** Actually an ellipse, but I'm keeping things simple.


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