|"The tides are a fight
between the Earth and moon. All water tends towards the moon,
because there is no water in the moon, and nature abhors a vacuum.
I forget where the Sun joins in this fight."
- 11-year old, from a science exam
In the oceans these swells and troughs, called tides, move as the Earth turns beneath the Sun and moon, causing the oceans to rise and fall as they pass. High water is simply called high tide; low water is called low tide. When a bulge approaches a beach or harbor, the tide "comes in," and when the bulge passes, the tide "goes out."
As tides flow in and out of harbors, they carry out some of the debris that was deposited since the last high tide, this sweeps out the ocean floor, keeping it free from excess amounts of dirt and silt by sweeping it out to sea. This leaves enough room for ships to pass. If the ocean were not freed of sand and silt each day they would quickly fill with debris and make travel to some places impossible for large ships. This cleansing action also helps to keep coastal beaches clean and healthy. Decaying material is swept off the beach during high tide and carried out to sea where it eventually settles to the bottom. But we must be responsible if we place trash in the ocean it will eventually wash back to shore it isn't like biodegradables.
There are actually four swells and four troughs on the ocean at all times. The moon causes two swells and two troughs, and the Sun also causes two swells and two troughs. The Sun's tides, however, are only about half as big as those of the moon. Even though the mass of the Sun is 27 million times greater than that of the moon, the moon is 390 times closer to the Earth so it has twice as much gravitational pull on the oceans.
When the Sun and moon line up with the Earth, their gravitational forces combine to make the tides higher than usual. These tides are called spring tides. Spring tides occur every 14 days and 18 hours. When the Sun and moon are at right angles to each other, their pull tends to cancel each other out and the tides are lower than usual. These tides are called neap tides. Neap tides also occur every 14 days and 18 hours, exactly opposite the spring tides. An entire lunar cycle takes about 29 and a half days and has two spring tides and two neap tides.
Tides always follow a predictable time schedule, which for some people is as good as a clock. The tides are so regular that some people that live near the seacoast can tell what time it is merely by looking at the sea. They know time between a high tide and a low tide is slightly less than 6 hours and 13 minutes. And they know another high tide occurs about 12 hours and 25 minutes after the previous high tide.
The 25-minute delay successive high tides is a result of the moon's rotation around the Earth. The Earth makes a half turn in 12 hours, but during those 12 hours the moon has also moved. It takes about 25 minutes for the Earth to catch up to the moon's new position.
©Copyright 1998 Elizabeth
Beckett, Holly Bernitt, and Vishwa Chandra.