Tides - What Creates Them and Determines Their Timing

The Sun and Moon Affect the Oceans

A sailboat stranded on the beach at low tide


Thomas Pollin / Getty Images 

The gravitational pull of the moon and the sun creates tides on the earth. While tides are most commonly associated with oceans and large bodies of water, gravity creates tides in the atmosphere and even the lithosphere (the surface of the earth). The atmospheric tidal bulge extends far into space but the tidal bulge of the lithosphere is limited to approximately 12 inches (30 cm) twice a day.

The moon, which is approximately 240,000 miles (386,240 km) from the earth, exerts a greater influence on the tides than does the sun, which sits 93 million miles (150 million km) from the earth. The strength of the sun's gravity is 179 times that of the moon's but the moon is responsible for 56% of the earth's tidal energy while the sun claims responsibility for a mere 44% (due to the moon's proximity but the sun's much larger size).

Due to the cyclic rotation of the earth and moon, the tidal cycle is 24 hours and 52 minutes long. During this time, any point on the earth's surface experiences two high tides and two low tides.

The tidal bulge that occurs during high tide in the world ocean follows the revolution of the moon, and the earth rotates eastward through the bulge once every 24 hours and 50 minutes. The water of the entire world ocean is pulled by the moon's gravity. On the opposite side of the earth simultaneously there is a high tide due to the inertia of the ocean water and because the earth is being pulled toward the moon by its gravitational field yet the ocean water remains left behind. This creates a high tide on the side of the earth opposite the high tide caused by the direct pull of the moon.

Points on the sides of the earth between the two tidal bulges experience low tide. The tidal cycle can begin with high tide. For 6 hours and 13 minutes after high tide, the tide recedes in what is known as ebb tide. 6 hours and 13 minutes following high tide is low tide. After low tide, the flood tide begins as the tide rises for the next 6 hours and 13 minutes until high tide occurs and the cycle begins again.

Tides are most pronounced along the coastline of the oceans and in bays where tidal range (the difference in height between low tide and high tide) is increased due to the topography and other factors.

The Bay of Fundy between Nova Scotia and New Brunswick in Canada experiences the world's greatest tidal range of 50 feet (15.25 meters). This incredible range occurs two times ever 24 hours 52 minutes so every 12 hours and 26 minutes there's a single high tide and a low tide.

Northwestern Australia is also home to very high tidal ranges of 35 feet (10.7 meters). Typical coastal tide range is 5 to 10 feet (1.5 to 3 meters). Large lakes also experience tides but the tidal range is often less than 2 inches (5 cm)!

The Bay of Fundy tides are one of 30 locations worldwide where the power of tides can be harnessed to turn turbines to produce electricity. This requires tides greater than 16 feet (5 meters). In areas of higher than usual tides a tidal bore can often can be found. A tidal bore is a wall or wave of water that moves upstream (especially in a river) at the onset of high tide.

When the sun, moon, and the earth are lined up, the sun and moon are exerting their strongest force together and tidal ranges are at their maximum. This is known as spring tide (spring tides are not named from the season but from "spring forward") This occurs twice each month when the moon is full and new.

In the first quarter and third quarter moon, the sun and moon are at a 45° angle to each other and their gravitational energy is diminished. The lower than the normal tidal range that takes place at these times are called neap tides.

Additionally, when the sun and moon are at perigee and are as close to the earth as they get, they exert a greater gravitational influence and produce greater tidal ranges. Alternatively, when the sun and moon as far as they get from the earth, known as apogee, tidal ranges are smaller.

The knowledge of the height of tides, both low and high, is vital for many functions, including navigation, fishing, and the construction of coastal facilities.

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Rosenberg, Matt. "Tides - What Creates Them and Determines Their Timing." ThoughtCo, Aug. 27, 2020, thoughtco.com/what-are-tides-1435357. Rosenberg, Matt. (2020, August 27). Tides - What Creates Them and Determines Their Timing. Retrieved from https://www.thoughtco.com/what-are-tides-1435357 Rosenberg, Matt. "Tides - What Creates Them and Determines Their Timing." ThoughtCo. https://www.thoughtco.com/what-are-tides-1435357 (accessed June 9, 2023).