Experience The Spinning Earth, Watch A Sunrise

Blake and Abbie Harper of Omaha got up early to watch the sun rise over Lake Superior from the Ship Canal in Duluth, Minn. this morning. Credit: Bob King
Blake and Abbie Harper of Omaha got up early to watch the sun rise over Lake Superior from the Ship Canal in Duluth, Minn. this morning. Every time the sun comes up or goes down, we can experience the rotation of the planet. Credit: Bob King

We can’t directly sense the spinning Earth but every sunrise and sunset, we get an inkling. The Earth rotates once every 24 hours but it does so soundlessly because everything — our friends, our homes, the ground, the atmosphere and us — are all moving at the same speed in the near-vacuum of space. Imagine taking an airplane to a vacation destination. After accelerating to take off, the plane levels off and assumes a constant speed of about 550 miles per hour. Because all of its precious cargo is also traveling at that speed, everything around you feels comfortable and normal. Well, maybe not that comfortable. But in any event, you can lift a plastic cup of ginger-ale to your lips without smashing it in your face or having it fly out of your hands. If you drew the window cover down, you’d have no idea you were moving at all. You might as well be sitting in a noisy box.

Now picture the plane coming to a sudden stop in the air. Everything not bolted to the jet would rush forward at 550 mph in total chaos and destruction. Like the plane, Earth spins at a constant rate, so we have no clue that it rotates. Our coffee cups, desks and cars remain where we left them because like that cup of ginger ale, they all partake of the planet’s west to east rotation. If Earth were to suddenly change its rotation rate or stop altogether, everything — the oceans, the atmosphere, forests, buildings would be torn asunder, and the entire frenzied mess would stream to the west at incredible speeds, leaving nothing but destruction in its wake.

Earth is widest around the equator and so has to spin faster there to complete one full rotation in 24 hours. North and south of the equator, the circle each city spins through during 24 hours gets smaller as you approach the poles. Moving a smaller distance in the same 24 hours of time computes to a slower speed. At the north poles, you make no circle at all so your speed is zero.
The speed at which you travel on the spinning Earth depends upon how far north or south of the equator you live.Illustration: Bob King, image from photos.com

How fast does Earth spin? Well, that depends on your latitude, the distance measured north or south of the equator. Because Earth’s circumference at its equator is 25,000 miles, and the planet rotates once in 24 hours, you can easily determine its rotation speed by simple division. 25,000 divided by 24 equals 1041 miles per hour. That’s the speed all the folks in Quito, Ecuador are moving at this very moment. As you work your way north and south of the equator however, the Earth’s rotation speed drops. At Chicago’s latitude of 42° north, its spin speed is 771 mph. Keep going north and by the time you arrive at the pole, you’re spinning in place and your velocity is a big fat zero. The same is true at the south pole.

The first hint of the sun rising this morning (July 22) looks more like a line than a circle. Credit: Bob King
The rising sun crests the horizon along the north shore of Lake Superior Friday morning  July 22. Credit: Bob King

The spin rate changes because the circle your particular city spins through in 24 hours is smaller than the one at the equator, where our planet is widest. Going a smaller distance in the same amount of time means traveling at a slower speed. It’s easy to find out how fast you’re spinning where you live. You need to know just two numbers:

* Your latitude
* The cosine of your latitude

The sun leaves a long gleam line on Lake Superior shortly after sunrise today. Credit: Bob King
The sun leaves a long gleam line on Lake Superior shortly after sunrise today. Credit: Bob King

Cosine has to do with angle ratios but don’t sweat it. To get your latitude, Google it. Type in your city’s name and the word ‘latitude’. Next, use Google again and type in the word ‘cosine’ and your latitude. For my city of Duluth, Minn. I type in cosine 47 degrees. Hit enter and copy the number that pops up into your calculator. Multiply that number by 1037.546 and that will give you the speed you’re moving at at your particular location on Earth. With Duluth as an example: cosine of 47 degrees (0.68199836) x 1037.546 = 707.6 mph. Easy (though not as tasty) as pie.

Ever wonder why NASA launches so many rockets from the swamps of southern Florida and not, say, the plains of North Dakota? The Kennedy Space Center is considerably closer to the equator than Fargo, so every rocket launched there gets an extra boost from Florida’s faster spin. That allows a satellite to reach orbital speed faster and more importantly, with less fuel. How can you not love science?

It’s a good thing we can’t directly feel Earth’s rotation as it would mean the planet and you, dear reader, would be in serious trouble. But you can sense it indirectly at every sunrise (or sunset) when the narrow line of the rising sun quickly become a beautiful, shining ball of fire in a matter of minutes. Watching it lift from the eastern horizon, you can almost feel like your being pitched forward.