Science of the Tides

Although you don't need to know any more about Tides in order to use and enjoy your Tide Timer, we've included the following information and links for those who want to know more about the tides, and the moon-tide connection.

Helpful Links:

How the Moon Affects Ocean Tides

US Naval Observatory: Moon Phase Animation

THE MOON AND THE SUN

The moon travels through the sky each day going from east to west just like the sun, but the moon does it in 24 hours and 50 1/2 minutes instead of the 24 it takes the sun. The light we see from the moon is solely the reflection of the light of the sun and therefore the phases of the moon are determined by its relative position to the sun. Thus, full moon occurs on those days when the moon is rising just as the sun is setting, therefore enabling the full hemisphere of the moon to reflect the light of the sun. A "quarter moon" occurs when the moon is directly overhead just as the sun is setting thereby enabling only half the hemisphere of the moon to reflect light back to earth. Of course this situation exists all day.

THE MOON AND YOU

The tides obey the moon so who is to say that our bodies too don't obey this powerful force? Since ancient times, man has speculated on the effect of the moon on human emotions. Even today, many millions of people all over the globe feel that the moon affects human emotions. The surprising fact is that today, more and more scientific and medical experiments have been proving the connection between the electromagnetic and gravitational pull of the moon, and man. "Man's natural day approximates 25 hours" proclaims the SLEEP LAB at New York's famed Montefiore Hospital-as do all similar establishments in the world. If man's natural day "approximates 25 hours," this time coincides almost exactly with the moon's orbit around the earth of 24 hours and 50 1/2 minutes.

THE MOON AND THE TIDES

How did early man ever figure out that the small white disc in the sky was powerful enough to move the billions of tons of the water of the earth's oceans up and down every day-in some places by as much as 50 feet? I'll bet that the first to propose the idea came close to being burned at the stake! But of course this unbelievable gravitational strength of the moon not only moves the tides but also causes the land masses of the earth itself also to rise and fall by about 12 inches every day as the moon goes overhead...and to think that the moon's gravitational force is only one-nine millionth that of the gravity of the earth! Come to think of it, the whole idea seems incredulous even today!

The reason for earth's tides, and why high and low tides occur at regular intervals, was a subject of interest to thinkers for thousands of years, but not the good old Greeks. The Greeks live on the shores of the Mediterranean Sea. That sea happens to be relatively tideless because it is a nearly land-locked small body of water. About 325 BC, however, a Greek explorer, Pytheas of Masslia, ventured out of the Mediterranean and into the Atlantic. There he came across good pronounced tides, with two periods of high water each day and two periods of low water in between. Each month there were two periods of particularly large range between high and low tides (spring tides) and, in between, two periods of particularly small range (neap tides). What's more, the monthly variations matched the phases of the moon. The spring tides came at first quarter and third quarter. Pytheas suggested, therefore, that the tides were caused by the moon. But, his suggestion lay fallow for two thousand years. The main factor that spoiled the moon-tide connection for thoughtful scholars was the fact that there were two tides a day. For instance, suppose there is a high tide when the moon is high in the sky. That would make sense. The moon might well be drawing the water to itself by some mysterious force. If the water heaped up under a high moon, a point on the rotating earth, passing through the heap, would experience a high tide followed by a low tide. But a little over twelve hours later, there would be another high tide and then the moon would be nowhere in the sky.

In fact, it would be on the other side of the globe. If the moon were exerting an attractional force, the water on one side of the globe should be pulled downward in the direction of one's feet. There should be a hollow in the ocean, not a heap. Could it be that the moon exerted an attraction on the side of the earth nearest itself and a repulsion on the side opposite? Then there would be a heap on both sides and it would explain the two high tides each day.

The notion that the moon would pull in some places and push in other places must have been very hard to accept, and most scholars didn't try. So the moon's influence on the tides was put down to astrological superstition by the astronomers of early modern times. It was only in the 17th century that a clear explanation was given -- by Newton.

Newton argued that the gravitational attraction of the moon causes tides. This force has only a slight effect on the solid land mass (and it is detectable), but the great mass of water that makes up the oceans is free to move. Because the gravitational force decreases with the square of the distance, the moon pulls harder on that part of the earth closest to it. The water in the oceans is pulled toward the moon, resulting in a high tide on the side of the earth facing the moon. Simultaneously a high tide occurs on the opposite side of the earth. Why this second high tide? Because the pull of the moon on the far side of the earth is less (since it is farther from the moon) than the pull on the central (nearer) parts of earth, (we may visualize this as the earth being pulled away from the water on the far side, just as the water on the side of the earth near the moon is pulled away from the earth a bit) the ocean water thus tends to collect on the sides closest and farthest from the moon, and is taken from the region between, where these will be low tides. Since the moon moves relative to the earth so that it returns to the same position overhead after about 25 hours, there are 2 high and 2 low tides at any point every 25 hours (approximately 2 of each per day). Because the high tides stay more or less in line with the moon, it is as if the solid earth moved beneath the tidal bulges.