About The Tides

That the tides rise and fall is something we take for granted, like the rising and setting of the Sun, the phases of the Moon, or the melting of winter into spring. But there's much more to the story of the ocean's movement than meets the eye. Astronomer Bob Berman tells that story here. If you'd like to delve deeper into the subject, visit the links below.

Photo by Stephen O. Muskie / www.outtakes.com

Why do we have tides?

Folklore often alludes to the "pull of the Moon" -- probably because coastal civilizations have always noticed that the ocean's rhythmic rise and fall mostly follows the Moon's, rather than the Sun's, motion and position. Although the Sun's gravitational influence has an immense effect on us (we orbit the Sun and not the Moon), tides move to the tune of the Moon. But why does the Sun have the greater gravitational pull and the Moon the greater tidal influence?

Tides are caused by the difference between the Moon's gravitational pull on one side of Earth compared to its pull on the other because of its proximity to Earth. The Sun is so distant -- 400 times farther away from us than the Moon -- that there isn't much difference in its gravitational pull at different points on Earth. In other words, the Sun's gravitational influence is almost the same in Russia as it is in Australia as it is in Canada, and so on. The Moon, however, is so close to us that there's a big difference between the gravitational pull on the side of Earth nearest the Moon and on the side of Earth farthest away. This difference in the Moon's gravitational effect is the tidal effect.

Since oceans are fluid and uncontained, they flow toward the side of Earth nearest the Moon. This creates a high-water bulge. A second high-water bulge occurs simultaneously on the side of Earth opposite the Moon. The second bulge exists because the Moon doesn't orbit the center of Earth. Rather, Earth and the Moon swing as a unit around the center of their combined mass. The midpoint of their weight is located much closer to Earth, which is 81 times heavier than the Moon. This center of mass, or barycenter, sits about a thousand miles beneath Earth's surface at whatever point is facing the Moon at any particular time.

As both Earth and the Moon complete their orbit around the barycenter every 27.32166 days, the side of Earth farthest from this point experiences the fastest motion. Whirled around as if on a carnival ride, the oceans there are whipped centrifugally and rise upward as if being partially hurled away, creating the second high-water bulge.

Because Earth rotates under these high-water bulges, high tides move around the world in a daily cycle: About every 12-1/2 hours, there's a new one, which gives every beach on Earth roughly 2 high tides a day. Since the Moon is also moving around Earth, the high tides arrive almost an hour later each day throughout the lunar cycle.

Thus we have two high-water bulges, and two high tides -- one on the side of Earth facing the Moon and the other on the opposite side. Neither is caused by the Moon actually pulling on water.

More About Tides

• Spring and Neap Tides: Discover how the phases of the Moon affect tidal range. Learn more!
• The Effects of Perigee and Apogee: Discover how the Moon's distance from Earth affects tidal range. Learn more!
• Beware the Proxigean Tides: Sometimes tides can cause coastal catastrophes. Learn more!
• Tides Great and Small: If you set your blanket down on the beach at the Bay of Fundy at low tide, you'll be swimming in 40 feet of water at high tide. But the tides in Tahiti rise by just one foot. Learn why!
• Ever Heard of an Air Tide? Certain lunar phases can cause atmospheric shifts that affect our weather. Learn more!
• Ever Heard of an Earth Tide? Believe it or not, the Moon causes the ground below us to move. Learn more!
• Tidal Glossary: Discover definitions for tidal terms. Learn more!