Tides Essay

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Tides are the twice-daily rising and falling of the ocean at the shores of continents and islands. Over an approximately 12-hour cycle the tide in a locality flows out to low tide (ebb tide) and flows in to high tide (flood tide). The cycle is repeated again over the next 12 hours so that each day and each night there is a high and a low tide.

Tides also occur in all bodies of water on the earth. In some freshwater lakes, such as the Great Lakes, the tide may be only an inch or two, as most tides are too small to be noticed without instruments. In some places, tides are relatively shallow; in other places, the turning of the tide creates tidal rushes that are many feet in depth. Tidal highs and lows vary between localities because of local topological conditions, the strength of winds blowing on or off shore, and the position of the earth relative to the sun and the moon.

Tides in all places display the enormous amounts of energy that it takes to create them. The forces creating tides are the gravitational pull of the moon and the sun, the rotation of the earth, and the drag of the earth’s uneven surface upon the movement of the oceans’ waters.

The envelope of water on the surface of the earth is attracted by the moon’s gravitational pull toward the moon. As the moon rises and sets relative to an observer on the earth, the earth rotates on its axis as well. High tide will be on the side of the earth facing the moon and on the side of the earth opposite to the moon. Low tide will be on the two sides of the earth at 90-degree angles to the moon. The moon’s gravity pulls the water closest to it toward itself causing high tide. In addition, it causes high tide on the side of the earth opposite to the moon by also pulling the solid earth toward itself.

The earth rotates on its axis once every 24 hours. However, the moon always faces the earth with the same side as it rotates around the earth in every 29 and a half days. When the moon and the sun are aligned with the earth, the sun’s gravity combines with the moon’s to create the highest tides (spring tides) of the month. The tides occur at the full of the moon and at new moons when the pull of the moon and sun combined are at their greatest. The gravitational pull of the sun is 46 percent less than the moon’s gravitational pull. The weaker gravitational pull of the sun is due to its distance from the earth, though its mass is many times greater.

Twice each month, the moon’s orbit takes it to a position that is at a 90-degree angle to the earth and the sun. This is the time of neap tides, which are the lowest tides of the month. These occur at the times of the first and third quarter of the moon when the moon’s gravitational pull is most out of line with the gravitational pull of the sun.

Tides are constantly changing as the sun, moon, and earth change. Tides are also affected by the winds of the seasons and by the local topography of the land next to the seaside. In estuaries and bays that are broad and open, the tide will be less than in those places where the entrance to the land is narrow and confined. In some places, such as the Bay of Fundy, the tidal range may be as much as 50 feet (15 meters) between tides.

The tides in the Atlantic and Pacific are different because of the size of the oceans. Pacific tides can be so-called “mixed” tides. Some islands have mixed tides where the tidal flows are such that one ebb tide is slight and then after the next high tide the following ebb tide is great. In a few localities in the Pacific, there is only a daily tide of a high and a low tide.

Tides provide natural cleansing of the estuaries and bays and renew the nutrient levels in the marshes. People who live beside the sea will usually regulate their activities according to the tides. Ships come and go with the tide using high tide in harbors to avoid shallows or underwater obstacles. Digging clams, crabbing, or some kinds of fishing (flounder) are done at low tide. There are also tides in rock and tides in the atmosphere, but these are too slight to detect without instruments.


  1. John D. Boon, Secrets of the Tide: Tide and Tidal Current Analysis and Predictions, Storm Surges, and Sea Level Trends (Horwood Publishing Limited, 2004);
  2. James Greig McCully, Beyond the Moon: A Conversational, Common Sense Guide to Understanding the Tides (World Scientific Publishing Company, , 2006);
  3. David T. Pugh, Changing Sea Levels: Effects of Tides, Weather, and Climate (Cambridge University Press, 2004);
  4. John Wright, ed., Waves, Tides, and Shallow-Water Processes (Elsevier Science & Technology Books, 2000).

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