Mysteries of Mercury

Astronomers last week were having learned fun with the transit of Mercury--the first in 13 years. When the small broiled planet (seared on one side, cold on the other) passes across the face of the sun, it always leaves a flurry of problems.

The transit itself was not spectacular, for planets are insignificant on the solar scale. Mercury looked like a grain of bird shot creeping across a pie. What interested the astronomers was the timing of Mercury's appearance. It never keeps appointments exactly, and they have learned a great deal by figuring out what makes it early or late for a date.

Mercury's orbit is sharply elliptical, and its long axis wheels around the sun. The wheeling motion was too fast to fit astronomical theories, and astronomers tried to account for this speedup in ingenious ways: e.g., the influence of an undiscovered planet between the sun and Mercury. None of their explanations worked. But in 1915, Albert Einstein published his General Theory of Relativity. Then all was --relatively-- simple. According to Einstein, a body gains mass as it gains speed. When Mercury is approaching the sun on its elliptical orbit, it speeds up a lot. This makes it slightly more massive and makes its orbit wheel faster. Now the astronomers feel that Mercury is a friendly witness. They time its transits carefully, regarding them as visible evidence of the truth of relativity.

But even after the "Einstein correction" has been allowed for, Mercury does not keep appointments accurately. This year Mercury crossed the sun about 20 seconds too soon, and the experts are now trying to figure out why. Astronomer Gerard P. Kuiper of the University of Chicago believes that the chief reason is the inaccuracy of man's fundamental timepiece, the revolution of the earth on its axis. For many reasons, including the drag of the tides and the little-understood motions of fluids in its interior, the turns of the earth vary slightly. This makes the earth a capricious clock which can be checked only by comparing its turning with the motions of independent bodies such as Mercury.

Astronomers believe that since 1900 the earth's turning has fallen about 30 seconds behind schedule, which would account, roughly, for Mercury's overpromptness. If a discrepancy still remains even after this effect has been allowed for, astronomers may find evidence for some principle even deeper than relativity.

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