Welcome to You Ask Andy


Gregory Mondello, age 14, of‑Longview, Tex

How does the moon cause the tides?

In the recent International Geophysical Year, the old earth was given a thorough physical examination  Many questions were answered and the answers, as usual, led to more questions  We may, for example, have to modify our old explanation of the tides in the light of new information  In the meantime, let’s look at the tidal facts in operation 

There is a general picture of world‑wide tides and a more detailed picture with countless small variations  The tidal day is about 24 hours and 50 minutes long  During this time, a high tide and low tide, high tide and low tide follow each other once around the globe  This general picture fl  :s obviously linked to the moon, which rises 50 minutes later each day.  Also, one high tide always faces the moon, lagging a little behind as the moon passes overhead 

In the detailed picture, every bay and inlet has its tidal variations, The height of the tides varies with months and seasons.  Some shores have but two tides a day and Tahiti has high tides every noon and midnight  The general picture of the lunar tides is modified by the sun, by shorelines, by the ocean depths and perhaps by the wave‑like behavior of heaving water 

The basic tide cycle is caused by the gravitational attraction between the earth and the moon  The pull of gravity exists between all heavenly bodies  The more massive a body, the greater its gravitational attraction  The sun’s gravity is strong enough to keep all the planets in their places  However, gravity decreases with distance and the sun is some 93 million miles away  The average distance of the little moon is only 239,000 miles 

Here we run into a strange mystery.  The gravitational pull of the sun upon the earth is many times stronger than that of the moon   Yet the pull of the moon on our tides is twice as strong as that of the sun  True, the big  sun is 400 times farther away than the little moon ‑ and this still may be the full explanation  But another factor is being considered

Our globe is some 8,000 miles wide  This is a mere fraction of our distance from the sun, but only one thirtieth of our distance from the moon  It well may be that the moon's gravity affects our globe on all sides 

In the past, the tidal cycle was explained by the moon overhead and the behavior of the vast body of global waters  A bulge of high tidal water is pulled up under the moon  This leaves troughs of low tidal waters  Another bulge of high water piles up on the opposite side of the troughs and the moon leads the cycle around and around the globe  This may be the full explanation  On the other hand, new studies may reveal a complex relationship between the moon s gravity on all sides of the earth  Maybe the moon pulls up one high tide and helps push up the high tide on the opposite side of the earth  But at present we cannot be sure.


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