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© Copyright 1997, Jim Loy
Neat title, huh? OK, some people believe in astrology because it is so scientific. Boy, that left an ugly taste in my mouth. I'm OK, don't worry. These people may point out a phenomenon like the 20 foot high (or so) tides in the Bay of Fundy. They may say, "Hey, the human body is mostly water. If the moon can lift water 20 feet, just think what it can do to a newborn baby." Thank goodness that the moon doesn't lift a baby 20 feet, at least not prematurely.
Maybe you would like to figure out how far the gravitational pull of the moon could raise one drop of water. Put the drop of water on a table, right under the moon, and the water doesn't budge. The moon will not raise the water. The earth is pulling on the drop of water, way more than the moon is. So, why are there tides?
The moon does not pull water toward it. It's more complicated than that. Instead, the gravity of the moon deforms the earth. And it's not the pull of the moon's gravity, but the difference in that pull at different places on earth. Different places on earth are at different distances from the moon, at any given time. When the Bay of Fundy (in Canada) is relatively close to the moon, other places (a ways out to sea, for example) are farther away. This difference in distance means that there is a difference in the gravitational force on the water (and ground) at the two places. Because of this difference in force, water moves horizontally from out to sea into the Bay of Fundy. Once some water gets to the Bay of Fundy (or the sea coast anywhere else), the water has no place to go, and builds up vertically. And we have what we call tides.
The Bay of Fundy has the highest tides in the world. It is the shape of the Bay of Fundy which magnifies its tides. It is shaped so that the water, coming from out to sea, is really trapped. The water builds up vertically, just like on any beach, but more so.
While we are having high tides on the side of the earth closest to the moon, we are also having high tides on the side of the earth farthest from the moon. The difference in distance, and thus the difference in gravitational pull, causes the same exact movement of water which I described above. So, there are two high tides every day.
Well, how about tides in the human body. Sorry, they are not measurable. They are there, but they are miniscule. A baby gets more gravitational pull (and actually more tidal forces) from his/her mother, the doctor, the hospital building, and the building next door, than he/she gets from the moon. The distances that cause the tides on earth (distances of miles, and more) are not there in the human body.
Addendum:
I received email informing me that the tides are 6 hours (90 degrees) off from what I describe above. Here is my response:
The causes of the tides are as I described. But, at any place on earth, there is a delay, because the spinning Earth carries the water past the moon, because it takes a while for water to move from one place to another, and because of the geometry (dimensions) of the various underwater basins. This delay may be minutes in some places, but it is usually hours, a little over six hours [see next message below] on the average. In many places the delay is more than eleven hours. Some places can have high tide while low tide is being experienced just a few miles away.
The amount of this delay (and the variations in the heights of the tides, which can be small or large) cannot be predicted very accurately from the geometry of the situation, because it is too complicated. But it can be predicted with remarkable accuracy based upon the past history of tides at that place.
I knew of this delay, and forgot to mention it in my article. But I didn't realize that it varied so drastically.
After further research:
Sorry about the 6 hours; that was a misreading of the 6 hours 13 minutes that the low tide follows high tide. I was also confused by the fact that my tidal charts do not give negative times (when the tide precedes the moon), but gives the next high tide, which may be up to 12 hours and 25 minutes after the high tide that we are interested in.
Looking at Honolulu, I find that the high tide is roughly 4 hours after the moon passes over. Midway Island is roughly 3 hours. Easter Island is roughly 0.5 hours. Christmas Island is roughly 4.5 hours. Tahiti is roughly 0.5 hours ahead of the moon. South Georgia is roughly 6 hours off from the moon. Any other islands that you're interested in?
There are indeed many ports where the high tides are roughly six hours away from the moon, as you suggest. From the sound of your emails, it would seem that you are proposing a force of repulsion between the water and the moon?
The astronomy and astrophysics books that I have found so far all say the high tide is directly below the moon, with a delaying effect because the water cannot flow freely (because of several causes), and with local variations (sloshing of water across bays and other basins) which often heavily outweigh the position of the moon.