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Ocean water is flexible enough to allow water from each side of low tide to flow to the sides of high tide.

So for a high tide, there must be a low tide somewhere else from where water is drawn out. But if it's a small inland lake (not connected to any sea at all) that is in the part of earth closest to the moon, it cannot draw out water from any part to increase its water level even by a miniscule amount.

I feel I am missing some concept here. Any insight would be appreciated.

Curiosity
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2 Answers2

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Ocean water is flexible enough to allow water from the side where the moon is not located to flow to the other side where the moon is.

That's not how tides work. Ocean tides are complicated standing waves that slosh about in the world's major ocean basins.

https://www.youtube.com/watch?v=PSJRymZ5bJs

The oscillation is driven by the Earth's rotation and gravitational interaction with the Moon at the incredibly low-frequency of just a bit less than two cycles per day.

At such a low frequency, the wavelength of the tidal waves is huge. Ocean basins are large enough to contain waves of that length, and they can resonate with that two-cycle-per-day forcing function. Lakes just aren't big enough. The water in the lake feels the force, same as how ocean water feels it, but there's just no room in the lake for it to "slosh about" in the same way that it does in the oceans.

Solomon Slow
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The largest gravitationally driven tides in lakes are not due to the Sun and Moon acting directly on the lake water, but instead are indirect effects due to solid Earth tides "tilting" the lake. If the lake is close enough to open sea (e.g. Lago Fagnano), then ocean tides can also change the local gravitational equipotential surface at the lake by the changing the ocean weight loading on the land mass containing the lake.

Tidal amplitudes in most lakes are only a few millimetres, much smaller than other water level oscillations such as wind driven seiches that may also have daily cycles that are hard to disentangle from semidiurnal and diurnal tides. Because it is shielded from wind and temperature swings, the 18 mm amplitude tides in the subglacial Antarctic Lake Vostok seem to be a particularly clean example of lake tides.

For more details, see (for example) Melchior (1978) "The tides of the planet earth", Section 8.2 "Lake Tides".

David Bailey
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