A little about tides - Part 1

Yes, it is about the moon (mostly), but how?


9/25/20232 min read

OK, I will confess that I have never exactly understood how tides worked. I knew it was something to do with the moon and to a lesser extent, the sun. However, the occasions where I tried to really understand it, I always came short. IO don’t remember details but there was always something the didn’t completely add up. The other day, I came across a YouTube video that made it quite clear to me and thought I would share. However, it had a different explanation for the extremely high tides in the Bay of Fundy which will be the topic for a post in the near future.

Agreeing with my recollection, the videos says most people think the tides are caused by the moon pulling the earth’s water closer to it. That is part of it, but then why are there two high tides per day? Shouldn’t the tide be highest on the side of the earth closest to the moon and lowest on the opposite side, resulting in a roughly daily cycle? Instead we have, roughly, two high and two low tides per day.

The video reminds us that gravitational attraction varies with the square of the distance between the two bodies. Two bodies that are closer have more attraction and accelerate towards each other faster than two bodies far apart. Now, let’s imagine that the solid part of the earth is one solid body and the oceans are a whole bunch of little bodies. Then, as the earth is accelerating towards the moon at some rate, all those little water bodies closest to the moon will be accelerating faster than the center of the earth as a whole and thus bulge towards the moon (and away from the earth) a small amount causing a high tide. On the opposite side of the earth, the water is accelerating slower and thus falling behind the earth, bugling away from the earth (and the moon) causing a high tide on the other side. Viola! Two tides per day as the earth rotates.