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• Theories of tidal action
• Words to Know
• The nature of tides

Tides are distortions that occur in the shape of a celestial body. They are caused by the gravitational force of one or more other celestial bodies on that first body. In theory, any two bodies in the universe exert a gravitational force on each other. The most important examples of tidal forces on Earth are ocean tides, which result from the mutual attraction of the Moon and the Sun.

Greek geographer Pytheas (c. 380 B.C.–c. 300 B.C.) was perhaps the first careful observer of ocean tides. In about the third century B.C., he traveled outside the Strait of Gibraltar and observed tidal action in the Atlantic Ocean. Pytheas suggested that the pull of the Moon on Earth's oceans caused the tides. Although largely correct, his explanation was not widely...