Measurement of the Earth

The word geometry literally means “measurement of the Earth.” It’s ironic that had Euclid actually taken the trouble to measure triangles on the Earth’s surface, he would have discovered that Euclidean geometry doesn’t work. The reason is that the Earth’s surface is a sphere,2 not a plane. Spherical geometry certainly has points and angles, but it’s not so obvious that it has anything that we should call straight lines. Let’s see if we can make any sense out of the words “straight line on a sphere.”

Excerpt From: Leonard Susskind. “The Black Hole War.