Sometimes simple questions can lead to unexpected results.
Recently a reader e-mailed me asking about the rotation of the Moon. He had heard that the Moon’s rotation (spinning around like a top) was matched so closely to its revolution (orbiting around the Earth) that it kept one side always facing the Earth. He claimed this sounded like a great coincidence, and wondered if there was more to the story.
There is indeed more. Let’s see if I can condense it down into a single column. And thanks to the reader who asked the question!
In the first place, when you talk about how the Moon (or any other body) rotates, you really have to ask “in relationship to what?” If you don’t, you’ll start arguments!
If you say “just relative to the Earth,” since we only see one side of the Moon forever, we could claim it doesn’t rotate at all. But the Earth itself is rotating; more important, it’s revolving around the Sun. A better standard of reference would be the entire universe, or at least the distant stars. Compared to them, the Moon rotates on its axis once in about 27.3 days.
This happens also to be the time it takes the Moon to orbit the Earth one time (as seen by the rest of the universe.) That’s why the Moon shows only one face to us. But it’s no coincidence.
Long ago, the Moon was spinning faster than it is today. It was also considerably closer to us than it is now, and looked much bigger in our sky. If we had been around at the time, we would have easily seen the Moon turning in our sky, revealing all its sides, even without a telescope. Total solar eclipses would have been more frequent, and they would have lasted for a longer period of time.
But the Earth had, and has, an effect on the Moon’s behavior, due to gravity. The Earth tugs harder on the near side of the Moon than it does on the far side. This difference is called the tidal force. As a result, the Moon is stretched a bit in the direction of the Earth. It has a bulge both toward and away from the Earth. The tidal bulge is small, and wouldn’t be visible to the naked eye, but it’s there. The Earth is actually raising small tides in the rock making up the Moon!
The Moon’s faster rotation in ancient times would have tried to force this bulge forward a little bit. This meant that the Earth was pulling backwards on it, acting to slow down the rate of lunar rotation. Eventually, the spin slowed down sufficiently that one face was always facing the Earth. At that point, the Moon’s rotation no longer tried to force the bulge forward; it was locked in place, pointing at the Earth. A distant observer, out among the stars, would then say the Moon was turning at exactly the same rate as it was orbiting around the Earth. A distant observer who was taking an astronomy class would say its rotation was synchronized with its revolution.
Since this wasn’t an accident, but was instead a natural consequence of the laws of motion and gravity, we would expect the same thing to happen elsewhere. And it has! Most of the major moons in the solar system have one face toward their mother planet.
Pluto (in case you still count Pluto as a planet, and for the purposes of this discussion we’ll allow it) has gone one step further. Pluto and its largest moon, Charon, are more similar in size and mass ( “weight”) than are Earth and Moon, so the tidal forces have been working on Pluto nearly as strongly as on Charon. Not only does Charon have one face always toward Pluto: Pluto always has the same face toward Charon, too! They seem like two wrestlers, gravitational hands on each other’s shoulders, circling each other on the mat, staring into each other’s eyes.
Someday the Earth and Moon will find themselves in the same relationship. This will happen because of two by-products of the Earth’s tidal force on the Moon. First, the Moon is slowly receding from the Earth, at a little over an inch per year. Right now, it’s about 239,000 miles from Earth. Eventually, it will wind up at a distance of 382,000 miles, about 1.6 times farther. It will look smaller in our sky, and we’ll no longer see any total solar eclipses.
Secondly, you may remember that Isaac Newton once said something about equal and opposite reactions. The tidal forces of the Moon on the Earth will gradually slow its rotation, until it has one face eternally pointing at the Moon, like the Pluto-Charon system. One side of the Earth will always see the shrunken Moon, the other side never will. Even more amazing, a day on Earth will be as long as 55 of our current days! Instead of having seven days in a week, in that far-off era we’ll have about seven days in a year! Think how simple this will make our calendars!
But we won’t get to this point for several billion years. In all likelihood the Sun will have become a red giant and incinerated the Earth by that time, so we really don’t have anything to worry about.
Planetarium holidays: Next Saturday is the final date for two holiday shows at the Edelman Planetarium at Rowan University. “Season of Light,” our holiday star show, plays at 7 p.m.; “Holiday Magic,” the accompanying laser show, plays at 8:30. More information can be obtained at http://www.rowan.edu/planetarium/. We have had some sold-out shows, so come a bit early; tickets go on sale at 6:30. The planetarium will then be closed for public shows until mid-February.
Keith Johnson, the director of Edelman Planetarium, faces the Moon and other celestial bodies in the star theater every day. He welcomes cosmic and comic questions at (856) 256-4389 or firstname.lastname@example.org.