“Tides are created because the Earth and the moon are attracted to each other, just like magnets are attracted to each other.” By Keith Cooley. The surface of the ocean will rise and fall in a predictable manner according to the position of the moon and the sun. To a somewhat lesser effect lakes are affected in the same way. The moon, which is approximately 237,000 miles from earth, exerts the most gravitational force on Earth’s oceans. The sun, being about 93 million miles away, is about 40 % as strong as the moon in affecting Earth’s tides. Sir Isaac Newton, by Energy Quest, was the first man to explain tides in 1686.
Ocean water moves in response to the moon and the sun. When the tide reaches the coast it can be measured as a rise and fall of the sea surface. The high water mark is known as the high tide. The lowest water mark is the low tide or trough. The difference in these two tides is called the tidal range. The horizontal movement of water that accompanies tides is the tidal current. Tides flowing into rivers and bays is called a flood tide. When the moon moves away and these waters recede it is called the ebb tide. Currents that occur between the tidal flood and the ebb tide are called slack tides.
The gravity of the moon causes the Earth’s water nearest the moon to be drawn towards it. Inertia attempts to keep the water in place. This water becomes the tide and it creates a “bulge” in the earth. On the side of the earth opposite the moon, another bulge is created where inertia wins because it is stronger than the moon’s gravity. The sun plays a part in this by exerting its gravitational force and making the tides stronger, weaker or canceling out the moon’s pull. The Earth’s crust is also affected by the moon and rises with the tides, according to the Penn State Surveying Program. The moon makes the Earth’s crust rise several dozen yards over the entire 27,000 mile circumference.
Spring tides occur during the full moon and the new moon. At this time the sun and the moon are in a line and both pull on the same side. These tides are the highest on the high end and the lowest on the low end.
Neap tides happen during quarter moons. Neap tides are very weak tides because the sun and the moon are at right angles to each other. In this position the gravitational forces cancel each other out.
The proxigean tide is a rare tide that occurs at high tide when the sun and the moon are at their closest to the Earth. This tide happens about once every 1.5 years.
The highest tides in the world occur in the Minas Basin. According to Wolfville, Nova Scotia, “The average tide range is 12 metres and can reach 16 metres when the various factors affecting the tides are in phase.” The reason for these tidal extremes is the farther from the equator you are the greater the tidal range. There are two high tides and they are about 12 hours and 25 minutes apart.
Tides have been with the Earth since the moon was formed. The moon is falling away from the Earth at the rate of about 3 centimeters per year. One day in the far distant future there will be no moon orbiting the Earth. At that point there will still be tides caused by the gravitational pull of the sun but they will not be as romantic.
