A couple fun facts about time

I recently finished Time: From Earth Rotation to Atomic Physics (Internet Archive), an exceptionally nerdy book. There’s a lot (probably most of it) I didn’t understand (turns out Earth’s rotation and other motion is hella complicated!), but it’s satisfying just to know that there’s so much knowledge out there. Since today is change-the-clocks day here in the US, I wanted to share a couple fun facts that stood out to me. Sometimes, assumptions need checking In 1695, Edmund Halley (of comet fame) noticed that the Moon’s motion was speeding up. For the next one-and-a-half centuries, astronomers tried to find a physical explanation for this acceleration. Finally, in 1846, Charles-Eugène Delaunay concluded that it wasn’t the Moon that was speeding up, but that Earth’s rotation was gradually slowing down. See, without mechanical clocks, the rotation of the Earth was assumed to be constant so was used to define the length of a day. Speed, however, is measured as distance (in this case…