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

[pteron]

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!

That's one of the best explanations I've seen.

 

[vyv_cox]

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.

I looked up the tidal curves for Menai Bridge and Caernarfon. They look normal, no double peaks or flat tops, which is how I recall them. Both are slightly asymmetrical, longer on the ebb than the flood, which I suppose is what you would expect due to the narrow entrances to the strait.

 

[MudShipper]

I looked up the tidal curves for Menai Bridge and Caernarfon. They look normal, no double peaks or flat tops, which is how I recall them. Both are slightly asymmetrical, longer on the ebb than the flood, which I suppose is what you would expect due to the narrow entrances to the strait.

Thanks for that vyv, I was canoeing last time up there, so more concerned with the flow than depth. Will need to pay more attention next time.

 

[SHUG]

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.

Totally correct. 10/10
The Centre of Mass of the Earth Moon system is about one third of the way towards the centre of the Earth and this is the axis around which the Earth -Moon system rotates.
The system is balanced by the centrepetal bulge matching the gravitational bulge thus giving two tides per 24hours.
Having lectured in this area for some years I can say that most textbooks get it completely wrong!!!