Science Wire
technology news beyond the headlinesabout science wirescience wire webcastspast science wires

sign with surfer carrying board

Hanging Ten: Surfing the web, the surfing the waves
waves The science of surfing tides rain links

Dance of the Tides p.2
Think About This!
Stop the world, stop the moon too, and there will still be tides.

To understand the basic reason for the tides, stop the moon in its orbit and let it begin to fall to earth. While we’re stopping things, stop the earth from spinning, too. The earth’s gravitational pull accelerates the moon toward the earth. At the same time, the gravitational pull of the moon accelerates the earth toward the moon. Take a snapshot of these falling bodies and examine what’s happening on the earth.

Tides are created by the moon's gravity pulling the earth. Water flows more easily than solid ground and so responds more to the pull of tidal forces. High tide would be occuring on the sides of this globe that are in the "lunar bulge."

It’s about 13,000 kilometers from one side of the earth to the other—a considerable distance. Because the gravitational pull of the moon decreases with distance, a chunk of earth closest to the moon feels a stronger gravitational attraction by the moon than a chunk at the center of the earth, and both feel a larger force than a chunk on the far side of the earth. If it were free of the earth, the chunk nearest the moon would accelerate faster and be pulled away from the center of the earth by the greater attractive force, while the chunk on the far side of the earth would be left behind by the rest of the earth.

Because of the earth’s gravity, which pulls things toward the center of the earth, the various chunks aren’t free to fall separately toward the moon. Instead, the gravitational attraction of the moon creates tidal bulges. There are bulges in the solid rock of the earth, although they are small and difficult to notice. But most of the earth’s surface is covered by water, which can move about more easily than rock. The water on the side of the earth nearest the moon is pulled up by the moon’s gravity about half a meter, creating a tidal bulge or a high tide, with the surrounding water pulled toward the bulge. The water on the opposite side of the earth is "left behind," creating a second high tide.

Why do the tides get later each day?
See how the sun creates tides, too!


copyright Exploratorium