What Causes the Tides?

Most everywhere, oceans rise and fall twice a day to produce tides. The two sets of tides are the result of two oceanic bulges, one on either side of theglobe. As the Earth rotates, it passes through these heaps of water, causing the sea above any given point on the seafloor to rise when the point approaches the bulge and to fall when it leaves..

The bulges are caused by two forces: gravity and centrifugal force.

If the Earth and moon could be held in place, the moon's gravity would pull the oceans toward itself, creating a bulge of water on the side closest to the moon.

But the Earth and moon are not stationary. Though we might think of our satellite revolving aroundour planet, the Earth and moon actually both revolve around their common center of gravity. This is a point located within the Earth, about 1,070 miles from the surface (about one quarter the distance to the center).

The Earth's motion around the common center of gravity causes matter in and on the Earth, including its oceans, to move outward.

This outward-directed force is call centrifugal force.

Because the common center of gravity is located between the Earth's center and the moon, the bulge caused by the Earth's motion around this center is always on the side opposite the moon.

If the Earth moved in this way without the influence of the moon's gravity, the oceans would bulge only on the side of the Earth away from the point it spun around.

The moon's gravity, however, does have influence. The strength of this gravity depends on distance.

The Earth's surface on the side facing the moon is 8,400 miles closer to the satellite than its surface on the side opposite to the moon. So the pull of gravity is stronger on the side closer to the moon.

The moon's gravity on the nearside of the Earth is strong enough to overcome the outward-moving centrifugal force and pull the ocean towardthe moon. On the other side of the Earth, however, the moon's gravity is not strong enough to overcome the centrifugal force.That's why the oceans on the opposite side also bulge outward.

Since there are two bulges, most places on Earth experience two high tides each day as the globe rotates through each bulge. Yet at certain times, some locations on our planet may host only a single high tide in a day. See Tidal Curiosities to find out why and to learn about other tidal anomalies, such as the role the sun has on tides.