A question about tides

[DaveGarland]

Okay, we all know that the moons gravity pulls the tides up, but on Monday while we were at the beach in Bude, the tide was high at about 18:00. However the moon wasn't overhead until around midnight. How is this so? Shouldn't high tide happen when the moon is up above? Or does the Earths rotation cause a lag?

[Knight of Clear Skies]

I find it difficult to visualise the forces, but my understanding is that the Earth's rotation displaces the tidal bulge. Tides are also strongly affected by the topography of the coastline and the Sun also raises (smaller) tides, so the end result can be quite complicated.

A consequence of this tidal displacement is that the Moon exerts torque on the Earth, slowing down its rotation and boosting the Moon's orbit. As a result the Moon recedes a few centimetres each year and our days get slightly longer. When the Earth and Moon formed a day might only have been about six hour long. If it's a clockwork universe it's running down....

Hope that helps.

Edit: (lots) more info here, the section headed Other lunar misconceptions explains how the tides are displaced by friction with the Ocean floor.

[ronin]

Tides are a problem to explain, there was a TV program that had a very good explanation of them.

The Moon and Earth form a system, the moon does not revolve about the Earth exactly, we rotate about a common centre of gravity.

This rotation of the Earth about this common CoG causes the water to actually be thrown to a bulge OPPOSITE the moon.

The Moon gravity attractes the water to it, so there is a second bulge caused by the Moons gravity.

So far 2 bulges = 2 high tides,

As it happens the Moons gravity bulge is greater then the one caused by rotation about the common CoG.

The bulges are almost static, they would circle the Earth with the moon about once a month.

However the earth itself rotates, so in effect we rotate into and out of the bulges.

So you get 2 tides owing to the Earth rotating in to and out each bulge.

The shape of the land masses comes into play and these can delay the time of a high tide, they can also delay the time of a low tide. If I recall the Solent has 4 high tides, I suppose that really it has 2 reduced low tides.

[Zakalwe]

The Moon pulls the water nearest to it up into a high tide. It also attracts water from all over the globe too. Obviously, the water thats furthest away from the Moon (ie, on the other side of the world) will experience less drag as it is further away. So it bulges into a high tide as well.

[Knight of Clear Skies]

That's the simplified kind of diagram which causes the confusion I'm afraid Zakalwe. Unintuitively, the bulge of water doesn't appear directly between the Earth and Moon. This is more representative, and explains how a high tide can be nearly six hours away from the rising of the Moon.

It also explains how the Moon's gravity can exert a torque on the Earth and slow down its rotation rate.

[DaveGarland]

Ah so the Earths rotation actually pulls the bulge ahead. Yes I see, I was wondering if it dragged it behind but that makes sense. Zakalwe that's the mental picture I had of the situation that didn't seem to match what I was seeing with my eyes. That is how I've always seen it explained though.

[Davey-T]

Seem to remember from my dim and distant school days that it's something to do with centripetal force, don't remember any more, probably nodded off

Here's a vid supposed to explain it, not sure how it relates to the tides.

Dave

[Bungielad]

Don't forget the suns pull has an effect on the tides too, combined with the moon produces high tides or what is known as "spring tide"...(nothing to do with the season).

Bungielad.

[Zakalwe]

That's the simplified kind of diagram which causes the confusion I'm afraid Zakalwe. Unintuitively, the bulge of water doesn't appear directly between the Earth and Moon. This is more representative, and explains how a high tide can be nearly six hours away from the rising of the Moon.

It also explains how the Moon's gravity can exert a torque on the Earth and slow down its rotation rate.

I stand corrected.

[Knight of Clear Skies]

Ah so the Earths rotation actually pulls the bulge ahead. Yes I see, I was wondering if it dragged it behind but that makes sense.

It seems so, although I find that quite surprising - if anything I'd expected inertia to cause it to lag behind. Even after having it explained to me I can't really visualize the stew of forces. Still, at least it isn't as bad as Feynman's inverse sprinkler.

[Kosh]

That's the simplified kind of diagram which causes the confusion I'm afraid Zakalwe. Unintuitively, the bulge of water doesn't appear directly between the Earth and Moon. This is more representative, and explains how a high tide can be nearly six hours away from the rising of the Moon.

It also explains how the Moon's gravity can exert a torque on the Earth and slow down its rotation rate.

Excellent explanation, thank you.

[AstroTux]

Both the Sun and the Moon exert gravitational pulls on the Earth's oceans, but the effect of the tidal bulge is often extremely exaggerated when people discuss the effects of tides. Even when the two pulls are together and produce the spring tide that occurs with a new moon, the actual bulge in mid-Atlantic will only be about 1cm in height. This is obviously not enough to produce the tidal effects that most people see.

Tides really only start to occur in shallow waters close to land. If you were to imagine the amount of water that 1cm in mid-Atlantic amounts to, it is quite a large body of fluid. As the wave ( not the water itself ) produced by this bulge moves towards shallower water, then the amount of fluid being displaced has more of a pronounced effect - similar to that of a Tsunami as it approaches land. The movement of this wave around the British coastline is effected by the shape of the coast which exerts a drag, causing the times of high tide to vary enormously as it moves. For example, high tide in Leith ( near Edinburgh ) was at 5:40 this morning, but in Dover it was at 1:42. A difference of nearly 4 hours.

Differences in the land and seabed topology all contribute significantly to tide times ( which were originally studied by Sir Isaac Newton using his new-found calculus ) and although the two major factors are the positions of both the Sun and the Moon, over 40 other factors producing sinusoidal effects contribute to the timing of the tides in each particular location.

Maybe not very well explained, but I hope this helps,

Alan

[Pig]

Dave,

The Sun also has an effect on the Earth’s oceans. When the Earth, Moon and Sun are all in a line the lunar and solar tides will add together creating a higher high tide and a lower low tide. This is known as a spring tide and occurs during the full Moon and new Moon

When the Earth, Moon and Sun are at right angles the solar tide will make the lunar tide smaller. This tide is called a neap tide and occurs during the first-quarter and third-quarter phases of the Moon