Back to List# Tidal Bulge

Notes and Reflections on Books and Media

by Hannah Leitheiser

Miscellanea Curiosa, Vol 1 | Edmond Halley | 1708 | Link |

2017-04-09

Finished a book by Edmund Halley -- famous for his comet. Halley is a contemporary of Newton. Book: Miscellanea Curiosa, it's a collection of papers from the Royal Society, some of which were a bit wrong-headed from modern view. Not a great text for audio, but one paper was on tides, so I'll talk about that.

When I first imagined the tidal bulge, I assumed it would be only on the side facing the moon -- that, after all, is the only side preferentially pulled toward the moon. And that would be accurate except that the system rotates. If you take a rotating frame of reference (human physics people probably won't let you do that, but you have my permission), you will find a force pulling the far side of the Earth away from the moon, which means there's a tidal bulge on both sides, and two tides per month. Note, of course, the Earth is not tidally locked to the moon, so the bulge is not actually aligned quite with the moon. Even in a reference that tracks so the Earth and moon stay put, the Earth still rotates. And there are various other factors at play, but that's the basic idea.

Was going to write a little bit of software to plot the tidal bulge, but this is a busy weekend with car failure and so forth.

To really explain this, I'd have to go over how to think about the potential energy of the ocean's surface, since the fluid is going to move to minimize its potential energy. Sufficient, I suppose, for now to think of the pulling force of gravity or acceleration drawing out the ocean a bit more than the solid land, causing tidal rise and fall.