Ask Glenn

Where does all the water go at low tide?

Into Space! Well – kind of. The Earth and its seas are egg-shaped, and water doesn’t cover the Earth evenly. High tides happen as we spin through the deep bits. Low tides happen as we spin through the shallows.

What? The Earth is an egg?! I thought it was a ball...

Nope. It’s almost spherical, but not quite. It’s squashed outwards round the middle, as if someone is pinching it with a giant thumb and finger at each pole. But in fact, it isn’t being squashed at all. It’s being stretched out of shape by tidal forces.

Tides with moon orbiting the Earth

Hang on – doesn’t ‘tide’ mean ‘when the sea goes in and out’?

It does. But the tide has to be caused by something, right? Well that something is a tidal force. This is the pulling, stretching and squashing of one large mass by another one, due to gravity. In this case, it’s the Earth being pulled out of shape by the Moon and, to a lesser extent, by the Sun.

Err... what?

Let’s backtrack a bit. We know gravity is a force that pulls objects together, and the bigger the object, the bigger the pull. We also know that the closer together the objects are, the bigger the pull between them. That OK?

OK...

The Moon is a big object, and it’s reasonably close to the Earth. So the Moon’s gravity constantly tugs at the Earth. But if you think about it, one side of the Earth is a lot closer to the Moon than the other side – about 6,000 miles closer, in fact, since this is how wide the Earth is. So there’s a far stronger pull on the Moon-ward side than the opposite side. This stretches the Earth in the direction of the Moon, making the Earth slightly oblate (or egg-shaped), rather than a perfect sphere.

But how does that make the tides happen?

The Moon doesn’t just pull on the Earth – it pulls on all the water stuck to the outside of the planet, too. The seas and oceans of the Earth form a watery shell round it, but this shell isn’t spherical, either. The water on the Moon-ward side is even closer to the Moon than the Earth it sits on, so it gets pulled towards the Moon more than the water on the opposite side. This makes an egg-shaped watery shell in Space, and the Earth itself is pulled into the middle of it. So now you have a squished Earth spinning within a squished ball of water.

Weird.

Slightly, yes. Now here’s the clever bit: if you imagine (or better yet, draw) these shapes, you’ll see that you get an uneven depth of water around the planet. If the water forms an egg shape pointing towards the Moon, then it’s deepest at the tip and base of the egg, and shallowest round the sides. Because the Earth spins round once a day, this means one point on it (say, Hawaii) moves alternately through the deeps and shallows twice a day – going deep-shallow-deepshallow in the course of one turn.

So the island is in deep water twice a day (high tide) and shallow water twice a day (low tide). And the water at low tide doesn’t really ‘go’ anywhere – it just gets shifted around in Space.

Cool. But what about the Sun? You said that it pulls on the Earth too?

The Sun does pull on the Earth, and it’s much bigger than the Moon. But because it’s so much further away, it doesn’t have as much of a tidal, squashing effect on the planet as the Moon does. It does pull on the oceans a little bit, though, and we see the effects in spring tides and neap tides. This is when you get extra-high and extra-low tides twice a year, as the Earth moves round the Sun, and the pull from the Sun reinforces that of the Moon. If you live by the sea, you may have noticed this before.

So if the Moon wasn’t there, we’d have no tides?

Pretty much. No tides, no big waves and no surfing.

Bummer.