High Tide Does Not Follow the Moon Overhead. Here Is Why.

The moon causes the tides, so you might expect high tide wherever the moon is directly overhead. It does not work that way. The moon and sun make the tides, but they do not decide when high and low tide arrive. That is set by forces much closer to home — above all, the shape of the Earth and its ocean.

The Earth seen over the moon's horizon, in a NASA image from the Apollo 8 mission. Credit: NASA.

Tides are waves

Since Sir Isaac Newton's discovery in 1687, we have known that tides are very long waves that travel through the ocean in response to forces exerted by the moon and sun. They begin in the ocean and move toward the coast, where they show up as the regular rise and fall of the sea. But the timing of that rise and fall is controlled by other, more regional forces.

If the Earth were a perfect ball

If the Earth were a true sphere covered by an ocean of the same depth everywhere, high tide really would occur where the moon was overhead, and the tidal "bulge" would travel around the planet with the moon. Our planet is not like that:

• It is not a true sphere; it bulges slightly at the Equator.
• It is dotted with large land masses — the continents.
• Higher areas, such as mountains, have a stronger gravitational pull than lower places, such as valleys or caverns.
• The depth of the ocean varies enormously.

All of these affect the height of the tides.

Tides run late

Timing matters too. There is a lag between the moon's pull and the high or low tide an observer sees, and it varies from place to place. Approaching the U.S. East Coast, for example, the rising slope of the ocean floor — the continental rise, slope and shelf — slows the tidal wave down. In some parts of the world the lag is hours, or close to a whole day. Think of the ring of waves from a rock thrown into a pond: if a tree stands in the pond, the ring bends around it. Tides wrap around obstacles in the same way.

Measuring the real thing

Because tides cannot be read from the moon's position, NOAA's Center for Operational Oceanographic Products and Services measures them with advanced acoustic and electronic equipment, at water observation stations in more than 3,000 locations around the U.S., and uses those observations to predict tides.

Sources

Based on "Are tides higher when the moon is directly overhead?" from NOAA's National Ocean Service, a work of the United States government in the public domain.