Motion | Pages 28-29 | See Linked Version

You can experience the same process without risking an icy fall. Simply return to that desk chair and ask a friend to spin you around, except this time, hold your arms outstretched and grasp a heavy object in each hand. Two phone books will do. After you're rotating at a healthy clip, bring the books inward toward your chest. Get ready for a dizzying whirl as your moment of inertia shrinks--and don't try to read the books for a few minutes afterward.

Such changes in moments of inertia are critical in astronomy because they explain why so many things in the universe spin rapidly. When a new star forms, clumps of gas coalesce into a body that grows progressively denser and more massive. However, the gas doesn't fall in straight lines toward the center, since a collapsing cloud in space always starts with some slight spin. Material drifts inward along curved paths as the cloud's moment of inertia decreases, making it spin more quickly. After hundreds of thousands of years, the baby star grows dense enough to ignite nuclear fusion at its core. By that time it rotates as fast as once every day, a breakneck pace for a ball of gas a million miles wide.

Such changes in moments of inertia are critical in astronomy because they explain why so many things in the universe spin rapidly.

Planets spin for the same reason. They form when smaller clumps of matter congregate within the dusty disk that remains around a new star. The disk revolves in the same direction as the star, resulting in planets that spin in the same direction as their sun. However, there are two exceptions in our solar system. Uranus has an axis of rotation tilted so severely that it rolls on its side like a gaseous bowling ball. Venus spins slowly in the opposite direction, a property called "retrograde" spin. Fierce collisions or mergers with other large bodies early in the solar system's history probably created these curiosities.

Even the planets with well-behaved spins aren't perfect rotators. Earth's axis tilts 23.5 degrees from the plane of its motion around the Sun. This leads to variations in the angle of sunlight striking the ground, creating our cycle of seasons as the length of daylight changes. The rotating Earth also exhibits another more subtle behavior. The axis of a tilted rotating object wobbles in a circle if another force acts upon it, a process called "precession." For a child's spinning top, that force is Earth's gravity. The top wobbles slowly at first, then more quickly as friction slows it down and brings it tumbling to the floor. For Earth the force is the combined gravity of the Moon and the Sun. The planet's axis takes 26,000 years to precess through one circle. Today, (continued)

Variations on Rotation and Revolution

All the planets in the solar system obey the physical laws of rotation and revolution, but their individual cycles vary widely. Mercury, for example, has been greatly affected by its nearness to our home star. The Sun's immense gravity has exerted a strong tidal pull, slowing Mercury's rotation until the planet takes almost 59 Earth days to complete one full spin on its axis. This snail-like rotation, combined with the planet's highly elliptical orbit and the fact that its counterclockwise revolution around the Sun takes the equivalent of only 88 Earth days, has odd consequences, as shown here. Mercury is also unique in that its axis of rotation is not tilted with respect to the plane of its orbit. Earth, for example, is tilted 23.5 degrees to its orbital plane, and Uranus, with an axial inclination of 82.1 degrees, wallows around the Sun on its side (page 30).

Watching the Sun Rise--Twice

An observer on Mercury who has watched the sun rise once (below, 2) is pulled back into darkness (6) when the planet is at perihelion, its closest approach to the Sun. The Sun's gravitational pull causes Mercury's orbital speed to exceed its slow rate of axial rotation. Once the planet is past perihelion, the Sun pulls on Mercury from behind, slowing its orbital speed. The observer travels back across the day­night line to see the sun rise again (10 and above).

A Long (Two-Year) Day

Because of its 59-Earth-day rotation and 88-day orbital period, Mercury's days last two years (above). If an observer (flag) sees the Sun come up when Mercury is at aphelion, farthest from the Sun (1), noon occurs at perihelion (3). Sunset (5), at the next aphelion, ends one year and begins the next (6). Midnight occurs at the next perihelion (>8). Dawn (10) begins year three.