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As you’re probably aware, the Earth rotates on its axis, turning in space as it orbits around the Earth. But the rotation of the Earth is actually pretty interesting, and there are a few surprises.
Seen from above the north pole, the Earth rotates in a counter clockwise direction. This is why regions in the east see the Sun before regions in the west. If you could fly in space above the Earth and time its rotation, you’d be surprised to find out that the Earth only takes 23 hours, 56 minutes and 4.09 seconds to turn once on its axis. Astronomers call this a sidereal day.
So why does a day take 24 hours? What we think of as a day is actually how long it takes for the Sun to return to the same point in the sky; that’s called a solar day. There’s a 4 minute difference between the sidereal day and a solar day because the Earth is actually orbiting around the Sun. And because of this speed of its orbit, the Sun appears 4 minutes earlier each day. Go ahead and divide 24 hours (1440 minutes) by 365 days – you’ll get about 4 minutes.
Because of its rotation, the Earth isn’t a perfect sphere. Instead it’s an oblate spheroid – a squished ball. When you’re standing on the equator, you’re actually traveling at a speed of 1,674.4 kilometers per hour in a circle. The velocity causes the middle of the Earth to bulge out so that points on the equator are a few kilometers more distant from the center of the Earth than the poles. Rocket scientists use this to their advantage to launch rockets. Since they’re already traveling at a velocity of 1,674.4 km/h when they blast off, they need less energy to get into orbit. Or it allows rockets to carry heavier payloads with less fuel.
The rotation of the Earth comes from the fact that we were once just a cloud of elements floating in the Solar System. 4.6 billion years ago we were part of the solar nebula, which collapsed down into the Sun and all the planets. Conservation of angular momentum of all the molecules in the Solar System set everything spinning: the Sun, the Earth and the rest of the planets. And without any friction, we’ve kept on spinning for billions of years.
We’ve also recorded an entire episode of Astronomy Cast just about the Earth. Listen here, Episode 51: Earth.