How does the moon affect tides?

Any why do we have two tides a day?
05 December 2017

Full moon

Full moon over a forest



How does the moon impact tides, especially when it comes to supermoons?


Astrophysicist, Matt Middleton, gave Chris Smith a super answer to this question from Tom. 

Matt - A great question. The Moon and the Earth interact through gravity and it’s what we call a ‘tidal force.’ The clues in the name it’s a tidal force, we’re talking about tides. Now because the Earth is a crust; it’s solid at least on the outside and then you’ve got all these oceans… the Moon can try and pull that crust but it’s not going to go anywhere but the oceans, on the other hand can, because it can slosh around. What you find is that there’s a bulge of water which tracks the motion of the Moon.

In a supermoon, we have the Moon being as close as possible to Earth in it’s elliptical orbit - it’s what we call perigee. A supermoon, when we see it big and bright in the sky, and on a supermoon it’s about 14% bigger than when it’s at its smallest, which is roughly the same difference between a 1p and a 2p piece. When that happens we have an alignment of the Earth, the Moon, and the Sun - it’s what we call perigee syzygy.

Chris - Can you spell that?

Matt - I can now, yes. S-y-z-y-g-y. So you can’t do that in scrabble.

Chris - Wow. That was quick.

Matt - I know, it’s remarkable isn’t it? I just have these things in my head. So when you have perigee syzygy, which should create an elf out of nowhere, then you have this supermoon. Because it’s closer to the Earth, you have a stronger tidal interaction so you have stronger tides. When the Moon is a very long way away, it’s called a mini moon. It might be a micro moon rather than a mini moon.

Chris - When I was at school, one of my colleagues got caught outside at night - it was a boarding school - and the house master said to him “why have you been sent to me with this note saying you were doing a moonie on the lawn outside the boarding house - what’s a moonie?” So the kid turned round and said “it’s seven star jumps, sir.” Got away with it - brilliant.

A quick question for you then: why are there two tides a day? Because you’ve got the Moon very close to the Earth and I can see that that’s going to pull a bulge of water on that side of the Earth facing the Moon towards the Moon. But then on the opposite side of the Earth coming round 12 hours later you’ve got a different tidal time. So why are we getting two tides a day?

Matt - Absolutely. That’s because, essentially, you’re shielding the Moon from the water using the Earth in the middle, so you’ll end up with a bulge on that side as well. And obviously these things are going round so you have two a day because they keep going round.


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