The Marvel of Our Changing Tides

A DAY at the beach! The little boy can no longer wait. Ahead of the family, he speeds through the palm trees on to the beach. But he stopped dead in his tracks. The glee on his face turns to dismay.

“Dad!” he shouts, “the sea is all gone! Where is all the water?”

“Don’t worry, Son,” replies the father. “It’s just the tide. It’s gone out. You’ll see it come right back in again after lunch.”

A short way up the shore, a group of young snorkelers are glad that the tide is out. For without the big waves at high tide, the sand settles gently to the bottom, leaving the water crystal clear, warm and safe. Swimming slowly through the shallow, azure-blue waters around the reef, they gaze in wonder at a whole new world of corals, shells, greenery and myriads of brightly colored fish.

A couple of hours later the flood tide is in. The little boy, now atop a high cliff jutting out into the sea, watches intently as the majestic rollers pound on the rocks below, shooting spray and mist high into the air. Crash​—there goes another one, surging and churning the water foamy white.

Mesmerized by the brute power of this awesome spectacle, he can no longer contain his curiosity.

“Dad, where does all the water come from? Why does it go out and then come in again?”

Can you explain how this gigantic task is accomplished? What is it that moves those billions of tons of water to within inches of a predictable “waterline”?

An Incredible Feat

For thousands of years, the daily rise and fall of the sea had been a mystery to men. Some thought it was the earth breathing. It took a mind like Isaac Newton’s to come up with an answer. In his thesis on gravitation he included an explanation of the tidal movement.

Tides are essentially the result of the gravitational pull of the sun and moon on the sea. But since the moon is so much nearer, its pull is more than twice that of the sun. It attracts, or pulls, the water directly beneath it into a high spot, or tidal “bulge.” In the middle of a large ocean, this bulge may only be a foot high. But when the tidal bulge is pushed up against a broad landmass, as at Nova Scotia’s Bay of Fundy, the incoming tide is a sight to behold. Helped by the unique combination of a wide inlet and converging shores there, the difference between extreme high and low tides can reach 50 feet (15 m). So when the moon is overhead, you can expect a high tide.

As the moon moves on, it pulls the water away with it. The tide begins to ebb, or go out, until the moon has moved one quarter of the way around the earth from you. The tide has now fallen to its lowest point, or low tide. Since it takes the moon 24 hours and 50 minutes to travel around the earth, low tide usually occurs about six hours after high tide.

Now, you may be wondering why low tide occurs when the moon is one quarter the way around the earth from you rather than when it is on the opposite side of the earth. Again, we look to the law of gravity for the answer. Simply stated, the closer an object is to a source of gravity, the more strongly it is attracted. When the moon is creating a high tide on one side of the earth, the water on the other side is 8,000 miles (12,800 km) farther away and so experiences a much weaker pull. It is, in effect, left behind and forms itself into a bulge almost identical to the one beneath the moon.

Thus, when the moon has gone half way around the earth, or 12 hours and 25 minutes after the first high tide, you will see a second high tide. This is followed by another low tide about six hours later when the moon is three quarters of the way around the earth from you. So, in most places, there are two high and two low tides each day, and they come about 50 minutes later each day than the day before.

But remember that the sun also exerts an influence on the tides. Depending on their relative positions, the sun may add its pull to that of the moon or subtract from it. Every two weeks, at new moon and full moon, the sun and moon are pulling along the same axis and their forces are combined. On such days the high and low tides reach their extremes, and these are called spring tides. During the first and third quarter phases of the moon, when the two luminaries are pulling at right angles to each other, we get neap tides, which neither come in very high nor go out very far.

A Never-Ending Variety

Although the sun and the moon provide the force behind the tides, it is the actual configuration of the shoreline, the sea bottom and many other factors that determine the final outcome. Thus, tides are strictly local affairs, and there is a never-ending variety.

Where the coastal seabed is very shallow and flat, the low tide can recede many hundreds of yards, leaving large areas exposed. For example, the famous Mont-Saint-Michel in Normandy is left high and dry at low tide, allowing people to walk out to it over the sand. But when the tide comes in, this medieval fortress is completely surrounded by water, except for a built-up road linking it to the mainland. So the next time you walk out a long distance to explore an exposed reef, make sure you know when the tide will return. A fast incoming tide can leave you stranded and out of your depth a long way from shore.

At the estuaries of the Amazon, the Severn in England and a number of other rivers, the incoming high tide is held back by the rapid outward flow of the river. The water level builds up outside the entrance until the river can no longer hold it back. Then, in a huge, gushing, wall-like wave​—a tidal bore—​the water rushes back up the river. On the Chien-tang River in China, this rushing wall of water can reach a height of 25 feet (7.6 m)!

A few islands in the Pacific have only one high and one low tide each day instead of the usual two. And some places in the Gulf of Mexico experience two high and two low tides for part of the month and then for the remainder, only one. This is because the moon does not always orbit the earth directly over the equator. The center of its pulling effect moves to the north and the south in a constantly changing pattern. Also, large bodies of water in motion have a momentum of their own. All of this alters their reaction to the influence of the sun and moon.

Although tides differ greatly from place to place, the exact levels they will reach at a particular spot can be predicted with great accuracy more than a year in advance. These are published in ‘tide books,’ which also tell us when the sun and the moon will be closest to the earth, bringing the highest and lowest tides of the year.

Salvage tugs, for example, are greatly aided by these different tides. For when a ship goes aground, it is often necessary to wait for the next spring tide to ‘float it off.’ But woe to the ship that goes hard aground during a high spring tide! It may have to be left where it is​—abandoned. Remember, never moor your boat with a short rope at high spring tide. Otherwise, when the tide goes out, your boat may still be there, securely moored, but dangling high and dry.

Low spring tides are the times when fishermen mend their traps, which are half submerged at other times, or drive in poles for a new jetty, or complete slips or other near-the-water construction jobs. High tides bring us driftwood to be made into table lamps and other ornaments. They allow ships safely in and out of harbors. And man is now even learning how to harness the immense power of high tides to generate electricity.

Silent Testimony

Have you ever sat on the beach on a moonlit night, enchanted by the gentle whisper of the waves as they slither up and down the sand? Take a walk along the shore​—it is best at dawn. Feel the fine, clean sand swishing between your toes. All around, there are only your footprints to see. Overnight, the tide has washed away all evidence of yesterday. It is as if there never was anyone there before. Do you not feel a deep sense of peace and contentment?

Just think​—all this beauty and harmony is brought to us silently day after day by the sun and the moon out there in space. But is it only by the sun and the moon? Is it not, rather, by the One who laid down the laws that govern the movement of these heavenly bodies? The marvel of our ever-changing tides is a silent testimony to the love and thoughtfulness of our Creator, “for his invisible qualities are clearly seen from the world’s creation onward, because they are perceived by the things made, even his eternal power and Godship.”​—Romans 1:20.

[Diagrams on page 21]

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Spring Tides

Sun

New Moon

Pull of the moon

Pull of the sun

High

Low

Earth

Low

High

Pull of the moon

Full Moon

During new and full moon each month, earth, moon and sun are in line, and the sun and moon combine their strength to produce extra high and low tides called spring tides

Neap Tides

Sun

Pull of the sun

First Quarter

Pull of the moon

Earth

Pull of the moon

Last Quarter

At the first quarter and last quarter of the moon, when the moon and sun are pulling at right angles to each other, the tidal range is not so great and we get neap tides