**Today’s factismal: **We’ve been able to prove that the Earth was round for nearly 2,400 years. We’ve been able to prove that the Earth rotates for just 162 years.

One of the more frustrating things in science is having a great idea but being unable to prove it. Your idea may be simpler, easier to understand, and explain what we see better than every other theory out there, but if you can’t test it, then it just isn’t science. (Ask a physicist about string theory if you want an example of this in action.) And one of the more interesting things in science was that for nearly 300 years we knew that the Earth had to rotate but we couldn’t prove that it did so.

Proving that the Earth was round was fairly simple. In Greece, back in 350 BCE, Aristotle put together a list of ways to prove that we lived on a sphere. He pointed out that every way you traveled, things curved “down” (that’s why the last thing you see as your friend walks away on the beach is the top of his head) and the only shape that does that is a sphere. Aristotle also pointed out that the constellations move higher or lower in the sky as you move (thus setting the stage for celestial navigation) which again implies a sphere. And he pointed out that the Earth’s shadow as seen on the Moon during an eclipse is always a circle and only a sphere can do that. So we knew that the Earth was round fairly early, and could prove it.

But Aristotle and his friends all placed Earth at the center of the Universe, with everything (including the Sun) revolving around it. When that happens, there’s no need for the Earth to rotate. It took Copernicus to show that the Universe would be a simpler place if the Sun stood still and all of the planets revolved around it. And, though (almost) everyone eventually agreed that was right, it meant that the Earth had to rotate – but there wasn’t any direct proof that it did so. The motion of the stars and planets could have been explained by the old Ptolemaic system; we needed to measure the Earth’s rotation. And in 1851, we got one.

A physicist by the name of Leon Foucault had the bright idea of just using a pendulum. Though they had been around since time immemorial and had been understood since Galileo’s time, nobody before Foucault realized that a properly hung pendulum would stay swinging in one plane. This meant that the pendulum would keep swinging in one direction while the Earth rotated underneath it. To understand this, imagine a merry-go-round underneath a really tall swing; as the merry-go-round turns, people on it see the swing change its direction even though it is the merry-go-round that is actually turning. The same thing happens with a Foucault pendulum, and in 1851, he showed it. We had our first proof of the Earth’s rotation and the Ptolemaic system was finally and conclusively shown to be wrong.

Today, scientists are looking for evidence to test all sorts of ideas, ranging from simple ones (like general relativity) to complicated ones (like string theory). If you’d like to help, then why not join in the Spacewarps community?

http://spacewarps.org/