tides

[d24506291r]

When the tide is out on the east side of the country is the tide out on the west side of the country?

[Catso]

No, not quite that simple. There's a delay between E and W, but only a few hours.

Imagine the tide as a big bulge of higher water circling the globe, once every 23 hours. It'll pass one side of the UK only slightly after the other side.

Actually, perversely, I think the 'bulge' goes W to E, contrary to the sun.

[kazza12345]

well you learn summat new everyday - i always thought the tide was the same around the uk as i sort of thought it was "sucked" upwards

[mibn2cweus]

Depends (of course) on the size of the country in question. Also there are two high/low tides ~23 hours (the Moon revolves around the Earth about once a month) as they are "reflected" on opposite sides of the Earth.

tide animation

[Catso]

Yes, mibn2cweus! Every 'half of 23 hours'! Silly me :-0

[JudgeJ]

Unfortunately, whilst the animation explains quite well what happens over the course of a lunar month, it does not explain why we get twice daily �tides�.

Further, it is actually incorrect in one crucial aspect. It explains why there is a �bulge� on both sides of the earth (and hence why we get (almost) two tides per day) by attributing this to the earth�s rotation. This is not true. If the earth did not rotate there would still be two �bulges� of water in line with the moon. This is because the gravitational pull of the moon upon the earth causes a �tidal� (i.e. stretching�) effect upon the earth. It is actually caused by the moon�s gravitational effect on the side of the earth nearest the moon being greater than that on the side furthest from the moon. This effect is felt by all bodies which have near neighbours but manifests itself clearly on the earth because it is covered with a fluid.

The two bulges of high water move around the earth in the opposite direction to the earth�s rotation (but in the same direction as the apparent motion of the Sun). If the moon were stationary (i.e. did not rotate in orbit around the earth) we would have two high tides exactly 12 hours apart. However, because the moon rotates around the earth every 28 days (in the same direction as the earth rotates on its axis) high tide is displaced by about 52 minutes each day (24 hours divided by 28). This means each high tide is around 12 hours and 26 minutes later than the one before.

This tidal effect is causing the earth�s rotation to slow down and the moon�s speed in orbit to increase. Eventually the two will become �tidally locked� with the same part of the earth always facing the moon. (This has already happened to the moon � the same side of the moon always faces the earth). When this has happened there will be no more daily tide movements in the oceans.

[mibn2cweus]

JudgeJ, yes, the tides follow the predominant gravitational influence of the Moon as the Earth spins �beneath� it. The rotation of the Earth just changes the area of the Earth�s surface exposed to the Moon�s tidal influence.

Because the Moon�s gravitational force is strongest where the Earth�s surface is closest to the Moon and weakest on the opposite side in comparison to the rest of the Earth�s surface there are two high tides on opposite sides of the Earth.

The fact that the Moon rises and sets several minutes later each day makes it obvious that the Earth must turn a bit more that one revolution for a given meridian to again pass beneath the Moon. This additional time is necessary because of the Moon�s daily progress in its monthly orbit around the Earth.

Thanks for pointing out my lunacy and tidying <"? up this thread!

[violetblue]

What an interesting debate. Makes a change from some of the other stuff on B & S.