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

Wikipedia is your friend.

The Moon orbits the Earth every 28(ish) days, not 24(ish) hours, by the way.

Briefly, because the ocean on the side of the Earth adjacent to the Moon experiences a gravitational pull greater than the average Earth-Moon value, so bulges towards the moon. On the opposite side of the Earth the value is less than the average so the ocean bulges away from the moon. These are the two high waters, and the Earth rotates under them.
 
Tides

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.
 
The amount of gravitational force between two masses depends on the distance apart; the closer they are the more pull.

1) The water nearest the moon gets pulled towards the moon; high tide.

2) Next nearest is the lumpy rocky bit in the middle, that gets pulled a bit too, but less because it's a bit further away.

3) Furthest away is the water on the other side. That doesn't get pulled so much and is left behind a bit by the lumpy bit in the middle; high tide there too.

This also explains why the two high tides per day are not equal in height.

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.
 
AngusMcDoon said:
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.
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Ask a Scientist :D
"In many places in the Gulf of Mexico there are diurnal tides, for instance along much of the west coast of Florida and northern Gulf coast. The positions of the Moon and Sun and how their tides combine or detract from each other control whether tides are diurnal or semi-diurnal."
 
wfe1947 said:
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.
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The oceans would love to fly off into space, but are only held onto the Earth's surface (as indeed we are) by gravitational attraction.

Now - suppose you had a 10ft tide on the side of the Earth nearest to the moon ...
The Earth's centre of gravity would then be shifted 5ft in the direction of that bulge ... yes ?

Such a shift in the Earth's centre of gravity would then result in a more-or-less* equal reduction in gravitational attraction towards the oceans on the side of the planet opposite to that bulge.

So you then get a 10 ft bulge on t'other side, which restores the Earth's CofG to it's original position - and all remains well with the world, and we can sleep easy in our beds.

* = the mean tidal 'bulges' will only be exactly equal if the barometric pressures on both sides of the Earth are also exactly equal - something which is next to impossible as the tidal bulges 'move around the globe' as the Earth rotates.
 
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electrosys said:
The oceans would love to fly off into space, but are only held onto the Earth's surface (as indeed we are) by gravitational attraction.

Now - suppose you had a 10ft tide on the side of the Earth nearest to the moon ...
The Earth's centre of gravity would then be shifted 5ft in the direction of that bulge ... yes ?

Such a shift in the Earth's centre of gravity would then result in a more-or-less* equal reduction in gravitational attraction towards the oceans on the side of the planet opposite to that bulge.

So you then get a 10 ft bulge on t'other side, which restores the Earth's CofG to it's original position - and all remains well with the world, and we can sleep easy in our beds.

* = the mean tidal 'bulges' will only be exactly equal if the barometric pressures on both sides of the Earth are also exactly equal - something which is next to impossible as the tidal bulges 'move around the globe' as the Earth rotates.
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...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.

Water isn't compressable like a gas an regardless of the gravitational attraction of the earth the water won't levitate.
 
Bloater said:
.... Water isn't compressable like a gas an regardless of the gravitational attraction of the earth the water won't levitate.
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Water is compressible, just not a lot, but it does compress. So technically if gravity was magically removed in an instant, then any force caused by gravity that contributed to the compression, would be released. The covalent bond would then spring back to its new balanced state. This would cause a reaction against the crust of the earth. The water (and air above it, due to the same process) would start to migrate up, up and away.

From Wikipedia

Compressibility
The compressibility of water is a function of pressure and temperature. At 0 °C, at the limit of zero pressure, the compressibility is 5.1×10−10 Pa−1.[28] At the zero-pressure limit, the compressibility reaches a minimum of 4.4×10−10 Pa−1 around 45 °C before increasing again with increasing temperature. As the pressure is increased, the compressibility decreases, being 3.9×10−10 Pa−1 at 0 °C and 100 MPa.
The bulk modulus of water is 2.2 GPa.[29] The low compressibility of non-gases, and of water in particular, leads to their often being assumed as incompressible. The low compressibility of water means that even in the deep oceans at 4 km depth, where pressures are 40 MPa, there is only a 1.8% decrease in volume.[29]
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So the forum believes in 2 bulges? What about amphidromes, the places where there never is a bulge at all?
350px-M2_tidal_constituent.jpg
 
Are you really saying that the sea at the furthest point from the moon is a different density to say that at the poles? I can't believe that this is the reason for the smaller high tide around the back of the Earth.
 
Utter nonsense

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
 
Porthandbuoy said:
An area of no bulge, or no tide at all, is a degenerate amphidrome. Makes tidal vectors a doddle. :D
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The absence of bulge or change in tidal height doesn't necessarily mean no tidal flow. Just means everywhere else is either uphill or downhill.

The real answer is the world wobbles on its axis so the water sloshes backwards and forwards just like a big bath. That's why when it's high tide in UK, it's low tide in New York.
 
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?
 
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