Where tides eventually go

Dear Matthew Alice, When the tide goes out, where does it go? — Jennifer in P.B.

You were expecting maybe the movies? Dancing? Well, strictly speaking the tide doesn’t go in and out so much as it goes around and around. Or the tide stays where it is and P.B. goes around and around, if you prefer. That sounds like the P.B. I’ve always known, come to think of it.

Our daily tides are just the gravitational pull of the moon (and to a lesser extent the sun, which we’ll ignore for the moment) on the ocean’s water molecules that create two clumps of water, two extremely long waves. One always faces the moon, the other always faces directly away from it (because of centrifugal force of the rotating earth-moon system). Since ail that clumped-up water has to come from someplace, two troughs are created between the tidal bulges. As the earth rotates, P.B. is once a day in the strongest pull of the moon (and the corresponding clumped-up water) and once a day is directly opposite that point (ditto). Voila, you’ve got your high tides. As P.B. continues to rotate, it passes out of the area of strong moon-gravity influence into the area of low water between the tidal bulges — and as you put it, the tide goes out.