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Why is Earth rotating? Did it always have the same rotation period? Will it always have the same rotation period?

Audio recording of Dr. Marc reading this page.

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Earth's graceful 24-hour rotation rate is one of the traits that makes our planet so friendly to life, allowing most parts of Earth to stay a nice, comfortable temperature as they are bathed in sunlight during the day and darkness at night.

Each planet in the solar system has its own unique rotation rate. Tiny Mercury, sizzling closest to the Sun, takes 59 Earth days to turn around just once. Venus, the second planet, rotates once every 243 Earth days. What's more, Venus rotates backwards from the direction of its orbit around the Sun, as do Uranus and tiny dwarf planet Pluto. Uranus even lies down on the job, rolling around with its axis of rotation pointed nearly toward the Sun.

Why do Earth and the other planets rotate at all? It will help to understand how our solar system formed. Almost five billion years ago, our solar system had its beginnings as a vast cloud of dust and gas. The cloud began to collapse, flattening into a giant disk that rotated faster and faster, just as an ice skater spins faster as she brings her arms in. The Sun formed at the center, and the swirling gas and dust in the rest of the spinning disk clumped together to produce the planets, moons, asteroids, and comets. The reason so many objects orbit the Sun in nearly the same plane (called the ecliptic) and in the same direction is that they all formed from this same disk.

While the planets were forming, there was not much peace in our solar system. Clumps of matter of all sizes often collided, and either stuck together or side-swiped each other, knocking off pieces and sending each other spinning. Sometimes the gravity of big objects would capture smaller ones in orbit. This could be one way the planets acquired their moons.

Scientists think that a large object, perhaps the size of Mars, impacted our young planet, knocking out a chunk of material that eventually became our Moon. This collision set Earth spinning at a faster rate. Scientists estimate that a day in the life of early Earth was only about 6 hours long.

The Moon formed much closer to Earth than it is today. As Earth rotates, the Moon's gravity causes the oceans to seem to rise and fall. (The Sun also does this, but not as much.) There is a little bit of friction between the tides and the turning Earth, causing the rotation to slow down just a little. As Earth slows, it lets the Moon creep away.

We can use extremely accurate atomic clocks to measure exactly how much the rotation is slowing down. One hundred years from now, a day will be about 2 milliseconds longer than today. Two milliseconds is 1/500th of a second, or how long it takes a car going 55 mph to travel only 2 inches--in other words, much less than the blink of an eye. So, if you live to be 100, you can't complain that the days are getting shorter! At this rate, though, you don't have to worry about the days getting enough longer to change things very much.

Learn more about the great variety of planets and moons in the solar system by playing Solar System Switch-a-roo.