A moving experience
In my misspent youth, I had an experience that profoundly changed me and, in fact, accounts for the fact that I have been doing public work in astronomy for almost half a century.
One fine evening when I was 12 or so, I watched the moon set behind the trees. As the moon slowly disappeared, I suddenly realized to my startled amazement, that the moon wasn’t setting at all. Earth was spinning in the opposite direction. I knew, I mean really knew, at that moment that I was on a spinning ball hurtling through space.
One of the principles that governs our universe is that practically everything is spinning around — the planets, the stars and even the galaxies. To the ancients, this principle seems obvious enough. Just go out and follow the stars across the sky. The starry dome seems to turn around the North Star once every day.
Of course, they were wrong about the last part. The Earth is a ball that turns on its axis every 24 hours. The stars only seem to move because the Earth moves.
That principle violates simple common sense, and folks like to ask me about it at our programs at Perkins. After all, Earth doesn’t seem to be moving. If you throw a ball into the air, the Earth is turning under it. Why doesn’t it fall behind you?
The ancients asked the same question. Most people who thought a bit about it believed that the Earth is a sphere, but they concluded that the ball example was sufficient proof that the earth was not spinning.
As early as 350 B.C., a Greek philosopher named Heracleides suggested that our planet — and not the sky — was turning. He was widely regarded as an idiot.
It wasn’t until 1609 that Galileo pointed his primitive telescope at the sky. He discovered that the sun had spots on it, and that the spots moved slowly across the sun’s face. The sun seemed to be turning on its axis once every 27 days. If the sun moves, then why not Earth?
Such weird ideas were still unpopular. In 1633, the Catholic Church forced Galileo to publicly renounce his opinions. Galileo eventually went blind from observing the sun with a telescope. Please don’t try to repeat his observations at home!
Using more sophisticated equipment than Galileo, Gian Domenico Cassini demonstrated in 1665 that Mars turned on its axis every 24.5 hours. In 1668, he proved that Jupiter rotated once every 10 hours, a startling figure given Jupiter’s girth.
The real evidence for Earth’s rotation built slowly, however. The final proof came, I suppose, when astronauts on the moon could actually watch the Earth turn.
As it turns out, how fast you are actually moving depends on where you are on Earth. The whole planet turns once every 24 hours. At the equator, the Earth is at its fattest, so people down there travel about 24,000 miles in one day. They’re moving at 1,000 miles per hour. In Central Ohio, the circle we travel in one day is only 16,300 miles around, so we are moving at about 680 mph.
So why don’t our hats blow off? Why does the ball fall straight down? When you jump up, the Earth is turning underneath you. How is it that you come down in the same place?
Galileo took care of that little conundrum 400 years ago with a simple thought experiment. Let’s say you’re on a fast-moving sailing ship and you climb to the top on the highest mast. If you drop a tool, the ship should continue to move ahead while the ball is falling. The tool should fall into the water.
Of course, it doesn’t. Many sailors have dropped tools from many mastheads, and the tools always fall straight down.
The reason is that the sailor, the tool and the ship are all moving at the same velocity. When the sailor drops the tool, it’s still moving at the same rate and thus travels along with the ship. As you move around the Earth in Ohio, you, the ball, the air and everything else around you are moving at 680 miles per hour. When you throw the ball up in the air, it’s still moving at 680 mph.
Science doesn’t make the Earth move. It moves on its own. Still, the Earth moves for me when science helps explain it, but that isn’t quite good enough. So here is my wish for you, gentle readers. Just once, I hope that you and your children and your children’s children will go outside together and watch the moon set. I hope that you will feel the Earth move, not just in your head but in your heart where it counts.
Tom Burns is the director of Ohio Wesleyan University’s Perkins Observatory, and he would be very happy to answer your questions or sell you a ticket to one of its upcoming Friday-night programs, firstname.lastname@example.org, 740–363-1257.