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Why does Earth rotate?


Narrator: Welcome to Space Place Musings, where an expert answers questions from our Space Place museum partners across the nation. I'm Diane Fisher of the New Millennium Program. Our expert is Dr. Marc Rayman, a scientist at the Jet Propulsion Laboratory.

Marc, we have heard from the North Museum Planetarium in Lancaster, Pennsylvania. They have several questions about Earth's rotation period, such as why is Earth rotating? And has it always rotated at the same rate?

Rayman: Ah, yes. Earth's graceful 24-hour rotation rate is one of the traits that makes our world so friendly to life, allowing most parts of our planet to stay a nice, comfortable temperature as they are bathed in sunlight during the day and darkness at night. And while the rotation may seem slow, Earth is so big that it's turning at more than 1000 miles per hour at the equator.

Narrator: Do all the planets rotate at about this rate?

Rayman: Not at all. Each planet in the solar system has its own unique rotation rate. For example, tiny Mercury, sizzling close to the Sun, takes 59 Earth days to turn around just once. Venus, the second planet, rotates once every 243 Earth days, and in a direction opposite its orbit around the Sun.

Narrator: But why do Earth and the other planets rotate at all?

Rayman: To understand why, it will help to understand how our solar system formed. Almost five billion years ago, our solar system began as a vast cloud of dust and gas. The cloud began to collapse, flattening into a giant disk that rotated faster and faster as its gravity made it smaller. This rotation rate increased for the same reason an ice skater spins faster as she tucks 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 form the planets, moons, asteroids, and comets. The reason so many objects orbit the Sun in nearly the same plane and in the same direction is that they all formed from this same spinning disk.

Narrator: That explains why everything is orbiting the Sun, but not why the individual planets are rotating.

Rayman: While the planets were forming, clumps of matter of all sizes often collided, and either stuck together or side-swiped each other, knocking off pieces and changing each other's spins. So the rotations of the planets may have changed as they grew and were influenced by the violent conditions early in our solar system's history.

Narrator: So what do you think got Earth spinning?

Rayman: Scientists believe that a large object, perhaps the size of Mars, hit our young planet, knocking out some chunks of material that eventually collected and became our Moon. This collision may have set Earth spinning faster. Scientists estimate that a day in the life of early Earth could have been only about six hours long.

Narrator: Would such an impact explain the distance from Earth to the Moon?

Rayman: No, because 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 these tides and the turning Earth, causing the rotation to slow down just a little. As Earth slows, it lets the Moon creep away ever so gradually, but over billions of years, these tiny effects add up.

Narrator: Earth's rotation is actually slowing? Gosh, it seems like the days just keep getting shorter, instead of longer!

Rayman: I know what you mean. Actually, 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 60 miles per hour to travel only 2 inches—in other words, much less than the blink of an eye.

Narrator: Well, that tiny amount isn't going to help anyone catch up on their sleep! It's amazing that we can even measure such tiny increments of time.

Rayman: Throughout history, inventors have worked to develop more and more accurate timekeeping technologies. Extremely accurate timekeeping is essential now for operating computers and spacecraft and for many science experiments.

On that very topic, you have an interesting article and activity on The Space Place website. As I recall, the article gives some history on how our whole arbitrary timekeeping system came to be.

Narrator: Right. We seldom think about the fact that the clocks and calendars to which we are all such slaves are really arbitrary human inventions. That particular article can be found at Just type its title, which is "Reinventing Time," into the search field.

Well, you've given us some startling information today, Marc. I'm trying to imagine a six-hour day with the Moon so close it almost fills the sky. Thanks to our listeners for joining us once again. We'll talk to you soon!