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The Ocean's Tides Explained

The alternating pattern of rising and falling sea level with respect to land is what we know as the tides. What causes this "motion of the ocean"? In one word, gravity. Specifically, the gravitational forces of the Sun and Moon.

The key to understanding how the tides work is understanding the relationship between the motion of our planet and the Moon and Sun. As the Earth spins on its own axis, ocean water is kept at equal levels around the planet by the Earth's gravity pulling inward and centrifugal force pushing outward.

However, the Moon's gravitational forces are strong enough to disrupt this balance by accelerating the water towards the Moon. This causes the water to 'bulge.' As the Moon orbits our planet and as the Earth rotates, the bulge also moves. The areas of the Earth where the bulging occurs experience high tide, and the other areas are subject to a low tide.

Water on the opposite side of Earth facing away from the Moon also bulges outward (high tide), but for a different and interesting reason: in reality, the Moon and the Earth revolve together around a common gravitational center between them, or center of mass. Here's a rough but helpful analogy: picture yourself swinging a heavy object attached to a rope around your body as you rotate. You have to lean back to compensate, which puts the center of mass between you and the object. With the Earth-Moon system, gravity is like a rope that pulls or keeps the two bodies together, and centrifugal force is what keeps them apart. Because the centrifugal force is greater than the Moon's gravitational pull, ocean water on the opposite side of the Earth bulges outward.

The same forces are at play as the Earth revolves around the Sun. The Sun's gravity pulls ocean water toward the Sun, but at the same time, the centrifugal force of the combined Earth-Sun revolution causes water on the opposite side of Earth to bulge away from the Sun. However, the effect is smaller than the Moon, even given the greater mass of the Sun (greater mass means greater gravitational force). Why? Simply because The Sun is so far away — over 380 times farther away from the Earth than the Moon.

Because the tides are influenced by both the Moon and the Sun, it's easy to see that when the Sun lines up with the Moon and the Earth, as during a New Moon or Full Moon (a configuration also called "syzygy"), the tidal effect is increased. These are known as spring tides, named not for the season, but for the fact that the water "springs" higher than normal.

On the other hand, if the Sun and the Moon are 90 degrees apart in relation to an observer on Earth as during the First Quarter Moon or Third Quarter Moon (sometimes called half moons), then high tides are not as high as they normally would be. This is because despite its greater distance, the Sun's mass allows it to exert enough gravitational force on the oceans that it can negate some of the effects of the Moon's pull. This phenomenon of lower high tides is called a neap tide.

The height of the tides can also vary during the course of a month because the Moon is not always the same distance from the Earth. As the Moon's orbit brings it in closer proximity to our planet (closest distance within a moon cycle is called perigee), its gravitational forces can increase by almost 50%, and this stronger force leads to high tides. Likewise, when the Moon is farther away from the Earth (furthest distance is called apogee), the tides are not as spectacular.

Tides most commonly occur twice a day (diurnal). Tides can also occur as two high waters and two low waters each day (semi-diurnal). However, these periods do not happen at the same time each day. This is because the Moon takes slightly longer than 24 hours to line up again exactly with the same point on the Earth - about 50 minutes more. Therefore, the timing of high tides is staggered throughout the course of a month, with each tide commencing approximately 24 hours and 50 minutes later than the one before it.

There are many factors involved in predicting the tides. In addition to the motion of the Moon and Sun described above, timing of the tides are also affected by the Moon's declination (angular height above the equator), local geography of the coastline, topography of the ocean floor, and depth of the water, among other considerations. Thus, the tides can't be perfectly predicted solely by astronomical calculations that track the Sun and Moon. For greatest accuracy, tide prediction tables always integrate data from actual observation, often over a period of many years.