What causes the tides?

High tides and low tides come and go, as the level of the sea goes up and down. This cycle of two high tides and two low tides occurs most days on most of the coastlines of the world.


Ocean bulges upward on side of Earth that faces Moon.

Tidal forces are based on gravitational attraction. Newton’s law of universal gravitation says that the gravitational attraction between two bodies is directly proportional to their masses. That means the more massive the objects, the greater the gravitational attraction between them.

On the other hand, Newton's law also says that the gravitational force between bodies gets weaker the farther away they are from each other. For example, if the distance between two objects were to double, the gravitational force between them would be only 1/4th as strong. If the distance between them were to triple, the gravitational force would be just 1/9th as strong.

The Moon has only about 1/100th the mass of Earth. Even so, it has enough gravity to affect us. As Earth rotates, the Moon exerts its gravitational pull on Earth. The land masses don't pay much attention to this little tug, but the oceans, being much more "flexible," respond by bulging "up" toward the Moon. The bulge stays on the Moon-facing side as Earth turns "beneath" it.

The effect of distance on tidal forces is seen in the relationship between the sun, the moon, and Earth’s waters.

Our sun is 27 million times larger than our moon. Based on its mass, the sun's gravitational attraction to the Earth is more than 177 times stronger than that of the moon. However, the sun is 390 times farther from the Earth than is the moon. Thus, the sun’s tide-generating force is about half that of the moon.

That explains one high tide per day as Earth rotates on its axis, but what about the other high tide?

Ocean bulges on side of Earth opposite Moon too. But why?

The ocean also bulges out on the side of Earth opposite the Moon.


If the Moon's gravity is pulling the oceans toward it, how can the ocean also bulge on the side of Earth away from the Moon?

Gravity is a major force responsible for creating tides. Inertia, acts to counterbalance gravity. Inertia is the tendency of moving objects to continue moving in a straight line. Together, gravity and inertia are responsible for the creation of two major tidal bulges on the Earth.

The gravitational attraction between the Earth and the moon is strongest on the side of Earth facing the moon, simply because that side is closer to the moon. As gravitational force acts to draw the water closer to the moon, inertia attempts to keep the water in place. But the gravitational force exceeds it and the water is pulled toward the moon, causing a “bulge” of water on the near side toward the moon.

On the opposite side of the Earth, or the “far side,” the gravitational attraction of the moon is less because it is farther away. Here, inertia exceeds the gravitational force, and the water tries to keep going in a straight line, moving away from the Earth, also forming a bulge.

In this way the combination of gravity and inertia create two bulges of water. One forms where the Earth and moon are closest, and the other forms where they are furthest apart. Over the rest of the planet, gravity and inertia are more balanced. Because water is very "flexible," the two bulges stay aligned with the moon as the Earth rotates.