The strange physics of tides

So there's a few things going on.

The first thing is you get two tides a day rather than one tide because while the moon is influencing the oceans, it's also influencing the earth underneath the oceans.

So it's pulling on the earth very, very slightly as well.

So if you imagine the oceans as kind of like an envelope for the earth, they're getting stretched a little bit by the moon and then the earth inside is getting pulled along a little bit as well.

So you get this bulge on the moonward side of the earth, if you like, and then also on the opposite side.

So that's why you get two high tides every day.

There's the bulge sort of facing the moon and then the bulge opposite the moon.

but then also the oceans aren't just like a perfectly even bowl of water on the earth's surface right they're really complicated there's a lot of land interfering with how they move um currents temperature all of those sorts of things so the oceans aren't going to move in a really perfectly uniform way either as the moon pulls on them it's not this really sort of neat expansion and contraction it varies a little bit depending on both what the moon is doing and also how the the earth and the land the ocean is shaped

there are terms that are thrown out regularly with tides the spring tide and a king tide

what do those

mean

So I've been talking about the moon's effect on the tides.

The sun also has a very small effect on the tides.

It's about half that of the moon.

So when the sun and the moon line up, the sun kind of enhances the moon's gravitational pull.

So you get really, really dramatic high and low tides, really big differences.

We call those spring tides, and they happen sort of twice a month around the new moon and the full moon.

People also talk about king tides, which are really, really dramatic spring tides.

So there's not really a formal scientific definition of what a king tide is.

It kind of varies place to place depending on where you are and what's happening.

But you get really dramatic tidal changes at some points in the year.

Sometimes when the full moon is particularly close to Earth, sometimes when the Earth is a little bit closer to the sun, all of those different gravitational influences can make the tides move a lot more than they normally would.

And then sometimes you get the opposite effect.

In South Australia, we talk about dodge tides around Adelaide, where you get almost no tidal movement.

And that happens when there's a half moon sometimes.