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 from the viewpoint of a person on earth, 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.

[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