Science Questions

Why are there two high tides each day?

Wed, 25th Jan 2012

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Ola Gabrielsson asked:

Dear Naked Scientist!


I live in Jersey, Channel Islands, where we have some of the highest tides in the world. Sometimes 12 metres on a spring tide.

I can understand the reasons for the tides; the influences of the sun and the moon but one thing that has escaped me is why there are 2 high tides (and obviously 2 low) every 24 hours.

Every point on earth spins round the earth axis once in 24 hours so why is there a 'bulge' on the far side as well as the side towards the moon/sun?


Best regards


Ola Gabrielsson






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Remember the comet Schumacher-Levy that split into a string of chunks as it went past Jupiter before crashing into the same planet later on? Well, it was pulled out into a string of chunks by the gravitational pull on it which was stronger at one side than the other due to one side being nearer to the source of gravity. The same kind of thing happens when entering a black hole - even small things which are strongly held together get spaghettified by the different strength of gravity across them in such an extreme environment. The moon (and sun to a lesser degree) are also trying to pull the earth into a string in the same way, but the Earth's gravity is far too strong to let that happen. Even so, it will distort, but it's rotating too quickly for those distortions to get very far because of the solid surface and deeper stodgy material of the planet. The water on the surface, however, is able to move more freely and forms a bulge on the side nearest the moon, plus another bulge on the opposite side where the gravity from the moon is weakest - these bulges are lumps of water which are temporarily trying to follow a different orbit from the Earth as a whole.

These bulges are held in line with the moon while the Earth rotates (and there are lesser bulges caused by the sun which may amplify or damp the ones caused by the moon, depending on the alignment of Earth, moon and sun), so if you're by the sea, you'll find a bulge passing you every twelve hours, unless you are in one of the rare places where there is only one high tide a day or where there are four high tides a day. These complications come about because the bulges are blocked by the continents - they crash into the continents and rebound. The result of this is that you actually get oscillations in oceans and seas instead, and most of these result in two high tides a day because part of the oscillation is driven by a bulge being dragged across from east to west. Some of these oscillations work more naturally with just one high tide a day while others work more naturally with four - it's hard to understand why, but it will depend on the shape of the container the water is in.

These oscillations also account for high tide occuring at radically different times of day in different locations up and down the coast at the same longitude - it isn't just a simple bulge being dragged past, but a much more complex sloshing. David Cooper, Thu, 26th Jan 2012

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