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Tides are actually waves, the biggest waves on the planet, and they cause the sea to rise and fall along the shore around the world. Tides exist thanks to the gravitational pull of the Moon and the Sun, but vary depending on where the Moon and Sun are in relation to the ocean as Earth rotates on its axis. The Moon, being so much closer to Earth, has more power to pull the tides than the Sun and therefore is the primary force creating the tides.

What Causes The Tides?

The Moon’s gravitational pull causes water to bulge on both the side of Earth closest to the Moon and on the opposite side of the planet. The Moon’s gravity has a stronger pull on the side of Earth that is closest to it, which makes the ocean bulge on that side, while on the opposite side of the planet the centrifugal force created by the Moon and Earth orbiting around one another pulls the ocean water out. Centrifugal force is the same force that smooshes riders to the outside walls of spinning carnival rides.

Meanwhile, Earth continues to spin. As Earth rotates, the water bulges stay in line with the Moon while the planet’s surface moves underneath it. A specific point on the planet will pass through both of the bulges and both of the valleys. When a specific place is in the location of a bulge it experiences a high tide. When a specific place is in the location of a valley it experiences a low tide. During one planetary rotation (or one day) a specific location will pass through both bulges and both valleys, and this is why we have two high tides and two low tides in a day. But, while Earth takes 24 hours to complete one rotation, it must then rotate an additional and 50 minutes to catch up with the orbiting Moon. This is why the time of high tide and the time of low tide change slightly every day.

The Sun also has a part to play in causing the tides, and its location in relation to the Moon alters the strength of the pull on the ocean. When the Sun and Moon are in line with one another they reinforce each other’s gravitational pulls and create larger-than-normal tides called spring tides. This happens when the Moon is either on the same side of Earth as the Sun or directly on the opposite side of Earth. Smaller-than-usual tidal ranges, called neap tides, occur when the gravitational force of the Sun is at a right angle to the pull from the Moon. The two forces of the Sun and Moon cancel each other out and create a neap tide.


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