What causes the tides?

Gravity and inertia are the major forces responsible for creating tides. The moon’s gravity and proximity to the earth pulls water toward it. The moon’s gravity causes a “bulge” in the oceans facing it. Inertia (a body in motion tends to remain in motion), acts to counterbalance the moon’s effect and causes a “bulge” on the opposite side of the earth. Together, gravity and inertia are responsible for the creation of two tidal bulges (high tides), and two low tides, one on each side of the Earth. Since the total mass of the oceans does not change when this happens, the part of it that was added to the high water regions must come from somewhere. These mass-depleted regions then experience low water levels. Hence, if water on a beach near you is advancing, you can be sure that in other parts of the world, it is receding.

The gravitational attraction between the Earth and the moon is strongest on the side of the Earth that is facing the moon. This attraction causes the water on the “near side” of the Earth to be pulled toward 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, 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 stay put, 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 farthest apart. Over the rest of the globe gravity and inertia are in relative balance, and those areas experience correspondingly lower tides. Because water is fluid, the two bulges stay aligned with the moon as the Earth rotates.

The sun also affects tides. Newton’s law of universal gravitation states that the gravitational attraction between two bodies is directly proportional to their masses, and inversely proportional to the square of the distance between the bodies. So, although the sun is much larger than the moon, its influence is significantly less (about a half) because it is so much farther away. At times, the high tides are very high and the low tides are very low. These twice-monthly, called “spring” tides, have nothing to do with the seasons. The strongest tides — usually 20 percent higher than normal — occur during the new moon and full moon when the gravitational pull of the sun and moon are aligned on one side of the Earth. Add a pinch of help from the equinox, when the sun crosses the Earth’s equator and the spring tides that follow are usually the strongest of the year.