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Physics, Astronomy & Cosmology / Re: Why do we have two high tides a day?

« on: 26/10/2018 17:07:13 »

I was interested in this topic, as an old sailor; altho I have little knowledge in the science as discussed.
i would like to thank all those who participated in a robust discussion, in which i learned very much (altho the depth of the science remains over my head.

i was impressed by the vigor of the discussion, without anyone coming to blows ...and the tolerance of the moderator for not arbitrarily deleting comments or in any way moving them. (i have been in other forums in which the moderators enjoyed displaying their power)

i look forward to following other topics with as much to offer.

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Physics, Astronomy & Cosmology / Re: Why do we have two high tides a day?

« on: 03/08/2018 08:30:24 »

Wouldn't they only bunch on the east coasts of the emerged lands if it was the case?

No, various reasons. Firstly the pull of the moon/sun system passes over (or the earth rotates from under, depending on how you view it) and releases the bulge so it can fall back. Secondly, the sloshing backwards and forwards can due to the distance result in a wave superposition that results in the waves performing a seesaw movement, a form of resonance.  Also the movement of the bulge isn’t straight across a basin west-east but Coriolis effect turns the movement to the right (northern hemisphere). For example in the English Channel when the bulge moves west to east you get slightly higher tides on the south (french) coast and then on the north (english) when it goes back east to west. Motion in a basin is more difficult to explain without pictures, but it results in a bulge that rotates counter clockwise around a null point - look up amphidromic circulation and dynamic theory of tides for details.   

If I understand well, the tidal wave would get amplified in a basin the same way a sound wave is amplified in a trumpet. If the trumpet or the basin is longer, the frequency is lower, and vice versa. It makes sense, but it's hard to imagine.

This only happens in some very specific places where the geology works to produce a resonance - look up Bay of Fundy where length of bay causes a resonance with 12 hr tides hitting the entrance.
In many other places eg Atlantic the bulge is amplified by the wave hitting shallow water of continental shelf just like waves on a beach increasing in ht as the water shallows. It is also amplified due to amphidromic effects as the further it is from the null point, the higher the bulge will be.
As David says, the geography is responsible for the specific local tides and these will deviate from the equilibrium 2 bulges/day eg Solent has a double peak high tide due to water entering first the west then the east entrance.

However, as David pointed out in an earlier post, I think this is moving away from the objectives of the OP which is to discuss the specifics of the moon/earth effects of the equilibrium theory and the terminology used.