Why do some places have 2 tides a day and others 4?

10 February 2008

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Question

Why do certain parts of the world such as the gulf of Mexico have only two tides a day: one high and one low when other bits of the world have four tides a day?

Answer

First of all 99% of the world just has 2 tides a day and the reason for that is basically the moon and the sun pull on the Earth and on the water around it. If you're close to something massive it's got a stronger attraction due to gravity than something farther away. Water on the side of the Earth closest to the moon is going to get pulled the hardest and the Earth which is in the middle is doing to get pulled slightly less hard and the water on the far side is going to get pulled even less hard. So you tend to get two bulges of water: one is the bulge of water close to the moon and the other bulge of water on the other side which is getting left behind. That's the reason why most places get 2 tides a day.

Some places get 4 - the only place I know about it is Southampton, Portsmouth in the UK by the Isle of Wight. If you look very closely at the map of the Isle of Wight it has funnels on each side of the channel just north of it. As the water rushes up the channel it sort of piles into these funnels and then as it gets narrower the wave gets higher. You actually get a high tide as the water rushes up. You get another one on the other funnel as the water rushes back down the channel so you get twice as many tides as you should have.

Comments

Where does the extra 50 minutes come from on the 24 hour cycle? Thanks

The Earth is turning inside the orbit of the moon. And as the Earth turns, the gravitational pull of the moon causes water to heap up on the side of the Earth facing (and the side farthest away from) the moon; and as the planet turns, it turns inside that bulge, so the high tide happens twice, all around the world, during the day.

So far so good.

But, at the same time, remember that the moon is also in orbit around Earth, although it's making that journey quite slowly: it takes a month to complete a single orbit. So, every day, the moon has moved a bit further along its orbital path, and therefore the time of day when a particular point on the Earth's surface is directly facing, or directly opposite, the moon's surface will be about an hour later than it was the previous day. Hence the high (and low) tides occur about an hour later each day.

Nearly every place in the world gets two high tides, two lows based on the lunar position. This can be easily verified by merely looking at tide tables from around the globe.

Hey,
Good job explaining how the moon is pulling the tide on the side closest to it, creating a high tide. The high tide on the opposite side is produced by the moon physically moving the Earth as well. The tidal bulge on the back side is a result of the water resisting the Earths movement (Inertia).

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