Like a Diamond in the Sky

Bright with reflected sunlight, and spread across 169,300 miles of space, the rings of Saturn gleam through telescopes as one of the most glorious sights in the sky. They seem as solid and substantial as Saturn itself. But astronomers know better: the great rings are really next to nothing at all. Stars shine right through them, and when they turn edge-on toward earth they vanish completely. This should not be surprising, say Drs. Allan Cook and Fred Franklin of the Smithsonian Astrophysical Observatory at Cambridge, Mass. The beautiful rings, as the two astronomers see them, are less than 8 in. thick.

When the earth is between the sun and Saturn and sunlight is falling on the rings from over the earth's shoulder, the rings get suddenly brighter. This effect can be explained by an assumption that the rings are made of small particles, probably ice, and that the nearer ones cover the shadows that they cast on others. Cook and Franklin measured the rate of brightening with precise modern instruments and decided that about one-twentieth of the rings' volume is filled with particles of ice-fog that are about one one-thousandth of an inch in diameter. Only if they are arranged in a sheet less than 8 in. thick will those tiny bits of ice cover one another's shadows just enough to cause the rings' sudden brightening.

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