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• The Earth feels firm and solid beneath your feet. Everything's calm and quiet. Right?

• We know the planet is rapidly spinning on its axis completing one full rotation every

• day.

• As we're gravitationally bound to the planet, and hurtling around in space, right along

• with it.

• We follow a circular path around the Earth at hundreds of kilometers per second.

• So, just how fast does does it spin, and how fast are we rotating around on the surface?

• Before we can talk speed, I've got to clarify how long a day is, it gets a little sticky.

• We count a day as twenty-four hours.

• This is the length of time it takes for the Sun to return to the exact same spot in the

• sky as it was in the day before.

• Astronomers call this a solar day. Here's where it gets a little complicated.

• As we're taking a full year to go around the Sun, and changing our relative position to

• our star, we have to add about four minutes every day to nudge the Sun back into the same

• spot.

• Which means if you look down at the Earth and watch it turn one complete rotation on

• its axis, you'd count twenty-three-point-nine-three hours.

• This is what's known as a sidereal day and is a more accurate measurement of the planet's

• rotation. This is the amount of time we're going to use to calculate the speed the Earth

• turns at.

• Let's assume that you're standing on the equator, the halfway point between the north and south

• pole.

• Over the course of a sidereal day you'll travel along the entire circumference of the Earth,

• and end back to your starting point.

• We know the circumference of the Earth is fourty-thousand-and-seventy-five kilometers.

• Divide twenty-three-point-nine-three hours into the circumference and you get

• one-thousand-six-hundred-and-seventy-five kilometers per hour, or four-hundred-and-sixty-five

• meters per second.

• Every second that goes by, you've hurtled almost half a kilometer through space,

• and you didn't even break a sweat.

• This spinning is even causing you to lift off the Earth a little bit, like when you

• spin a weight on a string.

• That lifting force is about zero-point-three-percent of the force of gravity pulling you down.

• If the Earth wasn't spinning, you'd weigh zero-point-three-percent more than you do

• now.

• As you travel towards the poles, your speed of rotation slows down.

• Just imagine if you were standing straight upright on the North Pole, lining your own

• axis up with the earth.

• It would take you a whole day to turn around once, which is pretty slow even by sloth standards.

• Our space agencies take advantage of the Earth's rotation to launch rockets.

• The closer you are to the equator, the less fuel you need to get into orbit, or the heavier

• That's why Cape Canaveral in Florida is a great place to launch rockets.

• Some clever people created Sea Launch, which blasts rockets off from an ocean platform,

• right at the equator.

• Which is even better at maximizing your launch benefits of planetary rotation.

The Earth feels firm and solid beneath your feet. Everything's calm and quiet. Right?

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# How Fast Does the Earth Rotate?

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Cheng-Hong Liu posted on 2015/02/18
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