Sunday, July 21, 2013

Why are there two tides a day?

Think about it.  Shouldn't there just be one tide on the side facing the Moon?  Counter to what we expect, there is a high tide on the side of Earth facing the Moon -and- a high tide on the opposite side.  Why is that?  Well, explanations abound and many of them are rife with misconceptions.  It's been said that the high tide on the opposite side of the Moon is caused by centrifugal motion since the moon is large enough and close enough that the center of mass between the two off sets the Earth's rotation (think of a binary star system--they really orbit each other).  Another common explanation is that the moon pulls the Earth away from the water on the opposite side of the Earth, bunching it up together.  

The real explanation, as best I understand, is that almost the entirety of the effect is due to gravity (although, I imagine multiple causes are at play).  The diagram illustrates.  On the side closest to the moon, there are two main gravities pulling--the Earth pulling towards its center of mass, and the moon pulling towards itself.  The net effect is a well of water.  On the top and bottom (and right angle sides) of Earth the two gravity effects are the Earth pulling down and the Moon's gravity pulling the water towards the high tide.  This effect causes low tide.  On the opposite side of Earth from the Moon, the two gravity pulls of Earth and Moon are working in concert to cause the water to slightly bunch up.  Add this effect to the fact that low tides exaggerate the appearance of high tides and we now have two high tides a day.
*Please send additional resources.  I am open to debate and correction.


  1. According to your explanation the lowest tide should be on the side opposite the the moon because both the gravitational pull of earth and moon would would be towards the center of the earth there.
    The actual answer is that the earth and moon both rotate around the center of mass of the two bodies which is located off the earth's center towards the moon. The moon's gravitational pull creates the high tide under the moon ( actually a bit offset ahead of the location of the moon because of the relatively fast rotational speed of the earth - this is what causes the moon to move very slowly away from the earth ). Because the earth is not rotating around its center there is there is a centrifugal force on the side opposite the moon which creates the lesser high tide.

  2. Jack, thanks so much for the comment. Intuitively, I want to agree with you but want to see more re/sources if you are aware of any. The site that I included above directly says the inertial explanation for the "second" tide is false. The summary seems to be that the rotational displacement is not sufficient to fully create it.


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