Earth, Rotation of

Characteristics of Rotation

Although Earth appears stable to humans, it is actually spinning rapidly. The velocity of a point on the equator is 465.1 meters per second, which is faster than the speed of sound at 335 meters per second. Objects stay on the earth's surface—despite the rapid rotation—because of gravity, a force that attracts objects toward the center of the planet.

The axis around which the earth rotates is not perpendicular to the plane of its orbit around the sun. It inclines from the perpendicular by 23.5 degrees, a geometric configuration that causes seasons on Earth. When the earth's axis points away from the sun, the northern hemisphere experiences winter because the sun's light hits the hemisphere at an angle. At the same time, the sun's rays are shining more directly at the southern hemisphere, which experiences summer. The seasons in the hemispheres reverse when the axis of rotation points toward the sun.

Astronomers have discovered that the position of the earth's axis is not constant over time. Every 14 months, the earth passes through a cycle known as the Chandler Wobble. A wobble, or precession, occurs when Earth's axis of spin oscillates around another axis; Earth resembles a spinning top that begins to slow down with its axis tracing a cone around a vertical axis. Scientists hypothesize that the 6-meter wobble is caused by changes in ocean currents or the atmosphere.

The Milankovitch cycles are patterns of rotation that occur over much longer periods. The axis inclination of the earth experiences a 41,000-year cycle, during which the axis moves from 21.5° of inclination to 24.5° and back again. Astronomer Milutin Milankovitch also identified a 21,000-year cycle during which the axis of spin wobbles and a 96,000-year cycle during which Earth's orbit becomes more and then less elliptical. Some astronomers hypothesize that the Milankovitch cycles caused the ice ages because when in conjunction, the cycles can vary sunlight reaching the northern hemisphere by as much as 20%.

Changes in Rate

Earth used to rotate more than twice as fast as it does in the 21st century. During the Archaean eon, 2,500 to 4,000 million years ago, the earth completed a rotation every 10 hours. One of the forces that slows Earth's rotation is the moon's gravity, which causes tidal activity. The friction of ocean tides actually acts as a brake on the rotation of Earth, causing the length of 1 day to increase by an average of 2.3 milliseconds per century. In the past 2,500 years, day length has increased by 42 milliseconds.