How the Tides Dance to the Moon’s Embrace
In 1970, I moved to Larchmont, a village on Long Island Sound in New York’s Westchester County. I joined the local beach, which was named Horseshoe Harbor because of its unique shape. One day, I biked the mile from my house and as I entered the beach, I heard a few lifeguards laughing. I asked them what the joke was.
One said to me, “See that woman over there, walking toward the rocks? It’s low tide now. She came in, saw the water was out about 50 feet further away than at high tide, and asked, ‘Where is all our water?’ I pointed across the Sound to Port Washington and replied, “That town across from us asked for our water this morning. So, we lent it to them. They promised to give it back in six hours. She said, “Fine!” and walked away, satisfied with my explanation.”
I noticed a tide table posted on the wall outside the locker room. It showed the times when high tide and low tide would occur each week. I noticed that there were two high and two low tides daily, advancing each day by about a half hour. Some days there were extreme high and low tides, while other days there wasn’t much difference between in height between high and low tides. I wondered for the first time how the tides actually worked.
Newton’s Law of Universal Gravitation
I knew it had something to do with the moon’s gravity. I went to the library and read that in 1687, Isaac Newton published a paper using his new law of gravitational attraction to explain the tides. This universal law became one of the most important in the history of science. Newton said that bodies attract each other with a force proportional to their masses, so that the greater the masses the stronger the attraction. The attraction decreases when the masses move apart and strengthens the closer they get.
How the tides work
The bodies attracting the Earth and its oceans are the moon and the sun. Both cause the tides. Let’s first consider the moon’s effect on the Earth.
Three events happen simultaneously:
(1) The moon attracts the water on the side of the Earth facing it (see diagram above) and it bulges out toward the moon, causing a high tide.
(2) The moon also attracts the Earth as a whole, which moves toward it leaving a bulge of water behind.
(3) The water on the opposite of the Earth is also attracted to the moon, but because it is further away the attraction is smaller. Thus, that water is left behind. Because the Earth has moved away from it, it causes a second bulge of water, a high tide.
Here’s another way to look at this: The earth as a whole moves a little bit toward the moon, because of their mutual gravitational attraction. The water on the other side of the earth doesn’t move much toward the moon, because it’s further away. The gravitational attraction weakens with distance. So, when the earth moves toward the moon, it leaves a bulge of water on the far side too. That’s why there are two high tides on opposite sides of the earth. And the low tides occur at 90 degrees from the high tides, a quarter of the Earth’s circumference away, because the high tides were taking the lion’s share of the water!
Why the tides come later each day
I also learned why the tides occur later each day. It takes the moon a month (a “moonth”) to orbit the Earth. As the moon advances in its orbit, the two-sided bulge of water follows it. The Earth spins inside the bulge of water. The next high tide comes when our position on the Earth once again is under the moon. If the moon were stationary, this would take half a day or 12 hours. But because the moon continually advances in its orbit, it takes another 26 minutes for our position on Earth to come under the moon again. This happens twice a day. That’s why the tides advance by 52 minutes a day.
The Sun also affects the tides
Sir Isaac Newton also explained why there are extreme high and low tides. This is the result of the sun’s attraction to the earth and its oceans. The force of gravity depends on the masses of the attracting bodies and distance between them. The sun is much more massive than the moon but much further away from the Earth, so it exerts about the same gravitational attraction on the water as the moon. When the sun and moon are in line with each other, they both pull in the same direction and cause very high tides. When they’re at right angles to each other their gravitational attractions partly cancel each other and the net pull on the water is weak. Hence the extreme high and low tides.
Surprising effects of the moon’s gravitational attraction
Tidal locking
Have you ever wondered why the moon shows the same face to us? It’s because tidal friction has slowed the moon’s rotation over the eons. The Earth’s gravity stretches the moon much as the moon’s gravity stretches the earth. The tidal friction caused by the Earth has slowed the moon’s rotation until the moon’s “day” is as long as its year. This means that the moon rotates only once as it goes around the earth, thus keeping the same side pointing at us. Astronomers call this phenomenon a “tidal lock.”
Tidal friction has slowed the length of our day
The tidal bulges caused by the moon’s gravity cause friction between the water and the land. This slows the Earth’s rotation, gradually lengthening the duration of a day. The year lengthens by only about .002 seconds per century, but over geologic time that adds up. About 400 million years ago, a day was 22 hours long. A year had nearly 400 days.
Tidal heating
When you squeeze a tennis ball between your hands, after a while you can feel the heat you’ve generated. Similarly, the gravitational attraction of Jupiter on its innermost four moons squeezes each moon and causes them to heat up. Io, one of the moons, experiences continuous volcanic activity and lava flows which reshape its surface as a result of tidal heating.
Tidal friction powers the geysers of water that jet into space from the surface of Saturn’s moon Enceladus. Title stretching from Saturn causes faults in the moon’s icy covering to rub back and forth. This produces enough heat to melt the ice and cause plumes of water vapor and crystals to jet into space. Scientist believe that the icy covering hides a liquid ocean, probably not made entirely of water. It is tantalizing to think that alien beings might live in such an ocean.
The moon’s effect on the land
The force of the moon’s gravity is much stronger than you might expect. When it passes over the Great Plains, the land itself is attracted and rises several inches toward the moon. Clams and oysters in fish markets also feel the moon’s pull and open their shells as it passes overhead.
The moon’s possible effect on human behavior
Since antiquity, people have believed that the moon affects human behavior and mood. It may be that the varying intensity of moonlight as the moon cycles though its phases affect our brains. The word “lunatic” comes from the Latin word lunaticus, which means “moonstruck.” There is anecdotal evidence that outdoor crime increases when the moon is full. There is also anecdotal evidence that the full moon makes psychiatric patients more violent, but researchers have found no proof of this. There is, however, scientific evidence that the time it takes to fall asleep and the duration of sleep is affected by the lunar cycle.
Watching the Tide Come In
I moved my beach chair to the edge of the water and watched the tide come in over the next few hours. I thought about the nearly four centuries that have passed since Newton’s discovery. And I realized how much I had learned in the last few hours about the world around me. Just knowing how the tides worked give me a sense of power over my environment. I and every other human being who ever lived did so in the shadow of the of the moon’s gravitational field.
That evening I watched the moon rise over New York City on the other side of the Hudson River from my apartment house. I couldn’t feel its pull, but I knew that the water in the river was rising to the moon as it reached its zenith. How remarkable, I thought, that this small former piece of the Earth is orbiting us at only eight Earth diameters away. And how remarkable that with Newton’s laws to guide us, we have walked on our moon. Without Newton’s achievement, we would simply be gazing at it in wonder in the night sky.
At the day’s end the tide came in and the woman who had asked the question noticed this. She said to me, “It’s about time they gave us back our water!”
