Why are there normally two high and low tides each day?

[Spyro]

An area of no bulge, or no tide at all, is a degenerate amphidrome. Makes tidal vectors a doddle.

Like Gigha?

 

[dylanwinter]

aha

I think the idea of a big bloke under the sea breathing to create the tides is the simplest explanation so it must fit. I don't know how true it is though - how do we know his name is Neptune if nobody has actually seen him?

I have never met him

I have read about him and met a bloke who met him

I have also read about someone called the Queen

never met anyone who has ever met her though

I believe in the big wet man

I am not sure I believe in the queen thing

 

[pteron]

A balance of gravitational pull versus centrifugal (correct word?) throw.

This is it in essence.

The moon doesn't orbit the Earth, the moon and the Earth both orbit around their common centre of gravity. Because the Earth has so much more mass than the moon, that COG happens to be within the Earth but it isn't at the centre of the Earth. So the moon's gravity pulls a bulge in the sea on one side and the centrifugal force of the rotating systems 'throws' a bulge out opposite the moon.

If the Earth was perfectly smooth, you'd see two tides as it rotates inside these bulges. The non standard tides are due to the shape of the land masses changing how the water flows around.

This is a good graphic and explanation http://www.pol.ac.uk/home/insight/tidefaq.html#3

 

[MudShipper]

This is it in essence.

The moon doesn't orbit the Earth, the moon and the Earth both orbit around their common centre of gravity. Because the Earth has so much more mass than the moon, that COG happens to be within the Earth but it isn't at the centre of the Earth. So the moon's gravity pulls a bulge in the sea on one side and the centrifugal force of the rotating systems 'throws' a bulge out opposite the moon.

If the Earth was perfectly smooth, you'd see two tides as it rotates inside these bulges. The non standard tides are due to the shape of the land masses changing how the water flows around.

This is a good graphic and explanation http://www.pol.ac.uk/home/insight/tidefaq.html#3

+1 for this explanation. The shape of the land masses affects the tidal flow around them causing all sorts of interesting localised effects i.e the severn bore, and the double tides at other locations around our coast (Menai?).

By contrast, the mediteranean, being almost land locked, has very little tidal height difference.
The Gulf of Mexico has a tidal range reduced by the fact that tides follow the sun (east to west) and the water pushing into the gulf has nowhere to the west to go to. Contrast that to the tidal ranges in the Nortwest USA and Eastern Canada where the tide is pushing itself round the northern end of the land mass.

 

[boaterbaz]

I think its because all the tide books would be wrong if there were three

 

[peterb]

For anyone who is really interested in tidal theory, I would recommend reading NP120, the "Admiralty Manual of Tides". You'll need at least A-level maths, and it's an old book written in an outdated style, but it's the most complete treatment that I know.

 

[KenMcCulloch]

Thanks for the responses.

However, the moon and the sun exert gravitational pull neither produce a significant magnetic pull. The earth.s magnetic field relates to massive iron ore deposits (somewhere north of Canada?).

The graphic does show the two high/low tides each day but I would expect the water to only bulge towards the moon mostly. I don't follow why the water 'bulges' on the opposite side to the moon.

You are BenDockrell and I claim my £10.

 

[vyv_cox]

+1 for this explanation. The shape of the land masses affects the tidal flow around them causing all sorts of interesting localised effects i.e the severn bore, and the double tides at other locations around our coast (Menai?).

Menai doesn't have double tides, but it is somewhat unusual. The flood commences across Caernarfon bar from the south west, but at the same time continues much faster around Anglesey. By the time it reaches Puffin Island at the NE end of the Strait the difference in height can be as much as 15 feet. So the flood changes direction and is far stronger in the second part than the first. High tide at the Swellies occurs well over an hour after the change in direction. The ebb then carries on to the SW for almost the full duration, just the last hour or so reverting to the NE north of the bridges.

 

[MudShipper]

Menai doesn't have double tides, but it is somewhat unusual. The flood commences across Caernarfon bar from the south west, but at the same time continues much faster around Anglesey. By the time it reaches Puffin Island at the NE end of the Strait the difference in height can be as much as 15 feet. So the flood changes direction and is far stronger in the second part than the first. High tide at the Swellies occurs well over an hour after the change in direction. The ebb then carries on to the SW for almost the full duration, just the last hour or so reverting to the NE north of the bridges.

I knew there was a good explanation! Is there a double peak for high water, or is it just held up for a bit longer? It's many years since I last went on the water up there, so memory is a bit hit and miss.

 

[nigelmercier]

The moon doesn't orbit the Earth, the moon and the Earth both orbit around their common centre of gravity. Because the Earth has so much more mass than the moon, that COG happens to be within the Earth but it isn't at the centre of the Earth. So the moon's gravity pulls a bulge in the sea on one side and the centrifugal force of the rotating systems 'throws' a bulge out opposite the moon...

While at the same time, the moon pulls the earth towards itself, further increasing the bulge on the far side. It is the gravitational friction that all this causes that has locked the moon with one face towards earth.

 

[awol]

.... but what stops the water flowing over the edge?


There's no such thing as gravity - the earth just sucks!

 

[guernseyman]

Let me help and simplify things

there are two theories to account for the tides - one old and one new

the old one is that there is a big bloke (let us call him Neptune) who lives in the sea. As he breathes in and out twice a day the volume of his massive chest changes and this makes the tides come and go

the other, even more fanciful idea, is that in some way -as yet unspecified - the molecules on the moon and the molecules of the sea are attracted to each other

this eons old love affair between different molecules a quarter of a million miles away from each accounts for the tides. We (as in the scientists) have no idea how they molecules are able to talk to each other over such vast distances

I myself like simple ideas and find the big man breathing much more credible than the stuff about talking molecules

Even Newton, who thought up the talking molecules, was a bit worried about how it was done. But Einstein showed they bend space-time around them and in that way affect distant objects.

 

[electrosys]

...This just doesn't make sense. Yes there would be less gravity on the side of the Earth away from the moon but that doesn't mean that the water would fly off into space.

But it tries to - just a little bit - as there is slightly less gravity holding it onto the Earth's surface. Why else would there be another bulge, equal and directly opposite the bulge nearest to the moon ?

It's exactly the same with the molecules that constitute our atmosphere - they likewise only stay 'attached' to the Earth because of gravitational attraction. So the further from the Earth's surface, the less gravitational attraction, and the thinner the atmosphere. QED.

And - to keep this 'nautical', that's precisely why heavy molecules (i.e. large mass = greater gravitational attraction), such as propane/ butane/ petroleum vapours will descend into your bilges, whereas light molecules such as hydrogen (minimal mass = less gravitational attraction) from your battery will ascend, ultimately right up into the ionosphere.

Indeed - if the atmosphere had a more defined and visible boundary layer, then you'd see dual bulges in the atmosphere too - but it doesn't, so you can't.

Good stuff this gravity - if it didn't exist, then we'd all be floating off into space: oceans, atmosphere - even the Archbishop of Canterbury ... Nothing would be exempt.

 

[pteron]

While at the same time, the moon pulls the earth towards itself, further increasing the bulge on the far side. It is the gravitational friction that all this causes that has locked the moon with one face towards earth.

Errrrr.... No.

 

[Pye_End]

It's all about the resolution of forces on the molecules.

Consider the molecules in line with the moon and centre of the earth - both sides of the earth. They are being pulled towards the moon, but have no force vector that will allow them to move. The have no desire to move sideways as there is no component of the forces requiring them to do so.

However, the molecules at the rim have maximum need to move towards the moon as the component is 90 degrees to the land. Everything in between has an varying component to move.

Hence a bulge on the far side, and a bulge on the near side.

 

[GrumpyOldGit]

I thought it was just the Earth breathing in and out, learn something every day me !

 

[alant]

Basic question given that the moon orbits the earth every 24 hours and 25 minutes!

There was an excellent TV programme recently.
Could have been Coast, where they visited Liverpool, which had a manual tide calculator, with which they made predictions for all the worlds harbours/ports of any note.

They showed a diagram with the Earth/Moon revolving around a centre point, hence tide on one side due to gravitational pull & tother due to centrifugal force - one of the best explanations I've seen.

Also did the Menai Strait tides!

 

[danielefua]

Originally Posted by nigelmercier
While at the same time, the moon pulls the earth towards itself, further increasing the bulge on the far side. It is the gravitational friction that all this causes that has locked the moon with one face towards earth.

Errrrr.... No.

I am afraid but nigelmercier was close to being right: not the gravitational friction (a definition I never heard about) but the "tidal friction". It is a known fact that on the long run the tidal friction tends to uniform the angular velocity of orbiting objects so that they all tend to show each other the same face. The Earth daily rotation itself, due to this phenomenon, is in fact slowing down.

Daniel

 

[timbartlett]

...Now if someone can explain why some places only get one high tide a day, I'll be interested. Otherwise I'm going to do some more YAPPing.

It's a combination of several effects... and it's quite tricky to explain without a diagram... but I'll have a go:
Imagine an Earth much like our own, but entirely covered by a uniform shallow sea.

The tides would be created as others have described, with two "bulges" of water, one under the moon and one on the other side. The bulges would be quite small -- a matter of centimetres, rather than metres.

The moon's orbit isn't perfectly lined up with the equator. So if the moon is (say) 20 degrees north of the Equator, then the bulge nearest to the moon will be north of the equator, while the bulge on the other side will be south of the equator.

If you are at 70N while the bulge is at 20S you wouldn't see a high tide -- you'd see a low tide, even though you are on the same side of the Earth as a "bulge". You will, however, see a high tide when the earth spins so that you are on the same side as the other bulge.

So you will see only one high and one low per day.

The situation is complicated by the fact that the Moon's declination changes very quickly. This month, for instance, it was S21 on 5 June, 0 on 11 June, N21 on the 18/19th, and it's now back to 0 again. So most places on our imaginary water-covered world would experience alternating patterns of daily and half-daily tides.

Tides on the real Earth are far more complicated, because of the effect of land interrupting the movement of water as it tries to flow into the "bulges" or out of the "hollows". In particular, the water sloshes about in the great ocean basins and major seas. The North Atlantic resonates to the half-daily cycle, so most places on the North Atlantic actually experience half-daily tides even when the moon's declination suggests they should be getting daily tides -- because the water has a half-daily slosh!

Chunks of the Pacific naturally slosh to the daily cycle so they have daily tides even when the moon's declination says they should be on a half-daily cycle.

The land effect also explains why the real tides in some places are vastly bigger than others, and various other strange effects:

Think of each bulge as a huge wave (literally a "tidal wave") travelling round the earth.
Now imagine an ordinary, common-or garden wind-generated wave running into shallow water: it gets steeper, with a higher crest and deeper trough. Same thing with a tidal wave when it hits the continental shelf. Or think of an ordinary wave meeting a beach at an angle: it turns towards the beach -- so does a "tidal" wave.

Does any of this make the slightest bit of sense? I hope so! But it's difficult without being able to include sketches!

 

[pteron]

I am afraid but nigelmercier was close to being right: not the gravitational friction (a definition I never heard about) but the "tidal friction". It is a known fact that on the long run the tidal friction tends to uniform the angular velocity of orbiting objects so that they all tend to show each other the same face. The Earth daily rotation itself, due to this phenomenon, is in fact slowing down.

Daniel

I failed to trim my quote (bad me!). I was referring to the "While at the same time, the moon pulls the earth towards itself, further increasing the bulge on the far side."