Tidal forces are physical strains caused by the uneven pull of gravity near a massive object, such as a moon or planet. An object’s gravitational pull weakens with distance. The moon’s gravity, for example, pulls most strongly on the part of Earth that is nearest to the moon. It pulls more weakly on the part of Earth that is farthest away. In fact, the moon’s gravity pulls slightly differently on each part of Earth. This variation creates strains in Earth that are commonly referred to as tidal forces.
Tidal forces tend to stretch an object out of shape, causing it to bulge slightly toward, and away from, a massive object. Tidal forces generated by the moon, for example, cause the side of Earth nearest the moon to bulge slightly toward the moon. They also cause the side of Earth farthest from the moon to bulge slightly away from the moon. Scientists call these bulges tides. As Earth rotates, the moon’s relative position changes, causing these tides to move around Earth.
On Earth, the most prominent effect of tidal forces occurs along the coasts. There, the level of the ocean rises noticeably and then falls again as a tide caused by the moon passes. Earth’s crust also rises and falls with the passing tides. These movements are slight, however, because the rigid crust cannot move as easily as the water in the oceans can.
Any massive body can create tidal forces in nearby objects. Just as the moon’s gravity generates tides in Earth, Earth’s gravity generates tides in the moon. These tides cause the moon’s crust to bulge slightly toward and away from Earth. Earth also experiences tidal forces from the sun. However, tides caused by the sun are much smaller than those caused by the moon because the sun lies much farther away than the moon does.
As tides travel around a rotating body, they stretch and squeeze the body, causing it to heat up. This loss of energy as heat causes the body’s rotation to slow. Eventually, the rotation becomes so slow that the same side of the body always faces the object causing the tides. Scientists call this situation tidal locking because the once-moving tides now remain “locked” in position. The moon, for example, is tidally locked to Earth, so that the same side of the moon always faces Earth.
Within a certain distance of a large body, tidal forces become intense enough to tear some smaller objects apart. Astronomers call this distance the Roche limit after Edouard Roche, the French astronomer who first calculated it in 1848. Inside the Roche limit, tidal forces tend to tear apart smaller objects that are held together by gravity alone. Tidal forces also prevent small objects inside the Roche limit from coming together through gravity to form larger objects. Most planetary rings lie within their parent planet’s Roche limit. There, tidal forces prevent the small particles that make up the rings from coming together to form larger bodies.
Tidal forces from two or more nearby bodies can compete against one another, producing a “tug of war” that strongly heats an object’s interior. Astronomers call this effect tidal heating. Jupiter’s moon Io, for example, experiences tidal forces from the enormous gravity of Jupiter and from the weaker pulls of the moons Callisto, Europa, and Ganymede. The resulting tidal heating causes molten material to form inside Io and erupt violently onto its surface.
See also Tide .
