Shedding light

The Moon is squashed, as if someone had held it at the poles between thumb and forefinger and squeezed, flattening it around its equatorial midsection.

The Moon is squashed, as if someone had held it at the poles between thumb and forefinger and squeezed, flattening it around its equatorial midsection. That is not surprising. The Moon spins, and the centrifugal force should indeed have generated a bulge as the molten magma of a young moon cooled to solid rock eons ago.

But as far back as 1799, mathematician Pierre-Simon Laplace noticed a back-and-forth wobbling because of the deformed shape of the moon. Although the flattening was slight8212;the Moon8217;s girth, 2,159 miles, is about 2.5 miles greater than its pole-to-pole height8212;it was still greater than would be expected for its current rotation period of 27 days 7 hours 43 minutes and 11.5 seconds. 8216;8216;The puzzle was the Moon was too flat,8217;8217; said Maria T. Zuber, professor of geophysics and planetary sciences at Massachusetts Institute of Technology.

Space probes of the 1960s and 1970s found a second deformity of the Moon: it is slightly elongated along the Moon-Earth axis. That is, if the Moon were sliced in half along its equator, the cross-section would not be a circle, but more like a football, with one of the narrow ends pointing toward Earth. But no one could come up with a completely convincing explanation for the Moon8217;s current shape. That is not its only mystery. Another is why its near side, which always faces Earth, is so different in material and appearance from the far side. The Moon8217;s origin still holds some uncertainties, although many scientists believe that it formed out of the debris when a Mars-size object struck Earth 4.5 billion years ago.

8216;8216;Quite a lot of the darned thing is still quite mysterious,8217;8217; said Kimmo Innanen, a professor of astronomy at York University in Toronto. In the current issue of Science, Zuber, with Jack Wisdom and Ian Garrick-Bethell, say they have a possible answer to the problem of the Moon8217;s shape. Actually, they say they have several.

Years of bouncing laser beams off mirrors left on the lunar surface by the Apollo astronauts show that each year the Moon is another 1.5 inches farther from Earth. The Moon now orbits in what astronomers call a 1:1 resonance with Earth, its orbital period equal to its rotation time so that the same side of the Moon always faces Earth. Thus, in the past, the Moon was much closer and took less time to orbit. With the 1:1 resonance, the Moon spun faster as well, possibly explaining the bulge.

One suggestion has been that the Moon by chance cooled into this somewhat odd configuration. Other solid planets like Earth are not exactly in their predicted shape either. The MIT scientists, however, say the observed distortions are larger than would be expected for chance. Instead, they suggest that in the Moon8217;s early history, it travelled in an elliptical rather than a circular orbit and that it was in a 3:2 resonance, spinning three times for every two orbits. They also found that orbits with higher resonances, like two spins for every orbit, could produce the same lunar shape. 8216;8216;It8217;s a bunch of families of solutions,8217;8217; Zuber said.

In an accompanying commentary in Science, Innanen described the proposed solution as 8216;8216;ingenious8217;8217;. But Peter M. Goldreich, an emeritus professor of astrophysics and planetary physics at the California Institute of Technology, said the MIT team did not explain how the Moon was caught in the 3:2 resonance. 8216;8216;That is a real weakness,8217;8217; he said.

KENNETH CHANG