Most everyone on the planet knows that the Earth is rotating, but not so many think about how fast the Earth is moving, or why it's rotating in the first place.
It seems as if astronomy likes to play with our intuition. Looking at the Sun, Moon and stars rising, crossing the sky and setting, intuition would tell us that it's the sky which is rotating but we know that isn't so. We know it's because the Earth is rotating.
But about that rotation? Again, our intuition tells us that an object needs to be pushed (or pulled) by some force to keep it in motion, if not, the object will slow and cease moving. But again, this is wrong. It's one of Isaac Newton's basic laws, that objects in motion will stay in motion unless some force causes it to slow down or speed up. Likewise, an object at rest will stay at rest unless some force causes it to move (I can verify this just by watching my cat.)
Here on Earth, it's the force of friction which causes things like cars and skateboards to slow down. For the Earth though, the only force acting on it to slow it down is the tidal effect from the Moon, and that's working very, very slowly. This means the Earth doesn't need any force to keep it rotating. But how did Earth get moving in the first place?
To understand this, we have to go back to the birth of the Earth (and the rest of the Solar System.) The Sun and planets formed from the collapse of a very large, dense interstellar cloud. This cloud was probably thousands of light years across and it collapsed quite quickly to the size of the Solar System. The cloud was rotating as it collapsed and the collapse made it rotate even faster. Just think of an ice skater spinning and pulling their arms in to make themselves spin faster. The rotation from the cloud was transferred to the Sun and planets. The force that caused the spin is angular momentum and without a force to stop it, the Earth just keeps spinning.
So that's why the Earth is rotating, but we still haven't answered the question about how fast it's rotating. The Earth rotates once (with respect to its center) every 23 hours, 56 minutes and 4.091 seconds. Our planet is a slightly flattened sphere with a circumference of 40,075 km at the equator. This means that at the equator, the Earth is spinning with a speed of 460 m/s, about 1,070 mph. This speed decreases as you move to higher latitudes because the circle is smaller. Speed decreases by the cosine of your latitude so that at a latitude of 45 degrees, cos(45) = .707 and the speed is .707 x 1670 = 1180 kilometers/hr. You can use this formula to find the speed of rotation at any latitude.
Of course, there's more motions than Earth's daily “spin” on its axis to consider.Besides rotating on its axis, the Earth is moving around the Sun, the whole Solar System is moving around the Galaxy and the Galaxy is moving through space!
We know the Earth is moving in nearly circular orbit around the Sun. We are moving along in this orbit at almost 30 kilometers per second, or just over 67,000 miles per hour. Sound fast? Hang on: it gets even faster!
Our Solar System is tucked away in an arm of the Milky Way Galaxy. Our galaxy is rotating, and we are rotating around the galaxy's center at about 220 kilometers per second (490,000 miles per hour!)
Our Milky Way galaxy is part of a cluster of galaxies we refer to as the “local group.” Together, they are cruising through space at an astounding 1,000 kilometers per second. It seems that we're in a hurry to get wherever we're going, so the next logical question would be “where are we going?”
To talk about where we are going, we must have a reference point, and for that we will choose the beginnings of the universe – the Cosmic Background Radiation (CBR) left over from the fires of creation. We are moving away from that, as are all things in the universe. But which direction? Go out on a spring night and find the constellation Leo the lion. The Earth is moving towards Leo at a speed of 390 kilometers per second.
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Kathy Miles, Author, and Chuck Peters, Systems Administrator
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