Pulled by the Moon: What’s really happening

(Alan McKnight)

Cleaning out files, I found some drawings by Woodstocker Alan McKnight, whose illustrations graced several of my early books. Discovering a few I’d never used, I realized that it would be a shame not to see them published – perhaps to clarify some Night Sky columns this year. The one on this page really makes it clear how the Moon creates its three-foot tidal bulge on Earth’s oceans, even if the lunar gravity is far too weak to lift anything – even single atoms – upward in any way.

This always confuses people, who think, “If I’m made mostly of water, and the Moon pulls untold tons of seawater, why shouldn’t the Moon personally affect me?” Yet the Moon cannot budge even a gram of your bodily fluids. That’s for two reasons: First, the lunar tides arise mostly because there’s a seven-percent difference in lunar gravity between its pull on the side of Earth nearest it and the side farthest away. The difference doesn’t create the tidal effect; it is the tidal effect. And since the difference in the Moon’s gravity-strength acting on your head is exactly the same as its torque on your feet, your body is bathed in equality so far as the Moon is concerned. Nothing budges.

But on the immediate small scale, something else happens, which I think is quite interesting, and which Alan makes clear on this page. Look at the two spots in Earth’s oceans marked A and B. The Moon’s gravity is pulling upward on each drop of ocean water. The pull is not straight up, but toward the Moon, which almost always is in some direction at an angle. This lunar torque thus has a vertical and a horizontal component, each of which is depicted with a thin line.


The vertical component has no affect on the water, because it’s more than counterbalanced by Earth’s much-stronger gravity in the opposite, downward direction. But the horizontal vector is not opposed by anything. Thus, each drop of seawater is nudged sideways, toward the place on Earth beneath the Moon. Each bit of nudging is tiny, but it adds up until, at the spot beneath the Moon, the ocean piles up to a height of three feet above normal. (When the rotating Earth brings this tidal bulge toward land, the shallowing seabed accentuates it, so that the average coastal tide is five feet.)

So now you know why tides happen. Is this cool or what? Thanks, Alan!

PS: Let Jim Metzner and me tell you some very cool stuff for a few minutes. Use your smartphone or computer and go to AstoundingUniverse.com.