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There's a tidal effect that we can clearly observe in oceans, which is the effect of gravity from the Sun and the Moon. If gravity affects everything equally, why don't lakes have tides?

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You probably got voted down cause this can easily be google searched, but the simplest way to explain it is that a tide happens because the lunar tug on one side of the ocean is measurably more than on the other side of the ocean and as the earth rotates the tidal "bump" follows the moon so you get 2 high tides and 2 low tides a day.

A tide is effectively one very large wave. The distance from Peak (High tide) to Trough (low tide) is 1/4 the circumference of the Earth.

Lakes do have tides, but since all lakes are much smaller than 6,000 miles across, it's nothing like ocean tides. The wavelength still applies to all bodies of water, but the lake is so small compared to the wavelength that even a big lake would have a tidal rise and fall of maybe 1/3rd of an inch - too little for most people to notice.

Your bathtub also has tides, but now we're talking about maybe the height of a few atoms. Even the Mediterranean sea isn't large enough for significant tides. It has small ones.

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    $\begingroup$ This is nice, but it is not correct. That wave you described does not and cannot exist. That would be a shallow water wave (a wave whose wavelength is much greater than depth). The depth of the ocean dictates the speed of a shallow water wave, and that speed is much less than the Earth's rotation rate. There is no tidal bulge. $\endgroup$ Commented Aug 27, 2015 at 13:54
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    $\begingroup$ If I may suggest an edit for this post (in bold): "...most lakes are much smaller than 6,000 miles $\text{across}^{\textbf{[citation needed]}}$..." $\endgroup$
    – Jim
    Commented Aug 27, 2015 at 14:52
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    $\begingroup$ @CarlWitthoft That's a great lake example $\endgroup$
    – Jim
    Commented Aug 27, 2015 at 15:17
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    $\begingroup$ @Jim you left out "...try the veal" $\endgroup$ Commented Aug 27, 2015 at 15:18
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    $\begingroup$ This question explores the myth of the globe-encircling tidal bulge. $\endgroup$
    – user10851
    Commented Aug 27, 2015 at 17:44

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