The Sky Is Falling--but Don’t Wait Up
There must be many children who look at the moon and wonder: Why doesn’t it fall down?
Well, it won’t. Quite the contrary. It’s moving away from us. There are other moons that are falling, however. Toward the end of 1988, three British astronomers at an observatory in the Canary Islands made measurements of the motions of Phobos, one of Mars’ moons, that put the matter beyond dispute.
Suppose we consider our own moon first. The moon moves in an orbit about the Earth, and if it were a perfect sphere and the Earth were a perfect sphere and there was no interference from outside, the moon would stay in its orbit without change for an indefinite period.
However, the moon pulls at the near side of the Earth more strongly than at the more distant far side, and this difference in pull creates the tides and is spoken of as a “tidal effect.” The moon’s tidal effect causes a bulge to appear on opposite sides of the Earth’s surface.
The moon pulls at that bulge, and the bulge pulls at the moon. However, the Earth rotates on its axis in one day, while the moon goes around the Earth in 27 1/3 days. That means that the bulge tends to be dragged along by Earth’s rotation so that it is always just slightly ahead of the moon.
This means that the moon pulls backward on the bulge, tending to slow Earth’s rotation, while the bulge pulls forward on the moon, tending to speed it up.
The effect is very tiny but it can be measured. Because of the tidal effect, Earth’s day becomes a second longer every 62,500 years. That’s not going to affect us noticeably in our lifetime, or even in the entire duration of civilization so far, but it does mount up.
Four hundred million years ago, the day was only 22 hours 13 minutes long, so that there were 395 days to the year. (The length of the year isn’t changed by the tidal effect.) Fossilized coral remains have proven that this was the case. Since the calcium deposits in coral grow daily, like the rings on trees, and grow faster by day than by night, and faster in summer than in winter, the 400-million-year-old fossils demonstrate the shorter day unmistakably.
In the same way, the moon, which is continually forced to move a bit faster by the bulge’s pull, has an orbit that bellies outward because of this faster motion. After each revolution of the moon it is about a tenth of an inch farther from the Earth. This is not large enough to notice from revolution to revolution, but it mounts up, too.
For instance, the moon looks like a disc, as seen from Earth, that is just about as large as the sun’s disc, as seen from Earth. That means that every once in a while, the moon moves in front of the sun (as seen from Earth) and we see a beautiful total eclipse. However, as the moon moves away from the Earth, its apparent disc decreases in size, while the sun’s disc does not change.
In about 750 million years, the moon will appear sufficiently small so that there will never be a total eclipse of the sun; the moon’s disc will never totally cover the sun. Still, I suppose you have to take a really long view to worry about something like that.
But what about Phobos, Mars’ nearer satellite? It is a small potato-shaped object about 17 miles in its longest diameter. It circles Mars only about 5,840 miles above its surface. It, too, produces a bulge on Mars’ surface through a tidal effect. Since Phobos is so much smaller than our moon, it produces a smaller bulge and has only a small effect on Mars. But the tiny bulge on Mars has a large effect on the tiny satellite.
Mars turns on its axis in 24 1/2 hours. Phobos, however, is so close to Mars (much closer than our moon is to us) that it revolves about Mars in only 7.65 hours. Phobos races ahead of Mars’ surface, rising in the west and setting in the east. Because it races ahead, it tends to be slightly ahead of the bulge it produces, so that its gravitational pull speeds Mars’ rotation very slightly, while the bulge pulls back on Phobos and slows it.
As Phobos’ rotation slows, it drops closer to Mars. Every year, it moves 1.5 inches closer to Mars and its time of rotation decreases by a few hundredths of a second. The measurements in the Canary Islands late last year show that in the past 10 years, Phobos has moved 14 inches closer to Mars.
The closer it moves, the larger the bulge becomes and the more rapidly Phobos loses altitude. Eventually, as it comes closer to Mars, Mars’ intensifying gravitational field will tear Phobos apart into fragments that will rain down upon the planet. Phobos has been circling Mars for billions of years, perhaps, and we now have the exciting chance to see it in the very last stages of its life.
Of course, even the last stages, short to an astronomer, would be long to anyone else. It will still take about 38 million years before Phobos breaks up and falls, so don’t hold your breath.
