Astronomy: Mercury's Double Dawn

Because Mercury is so close to the sun, astronomers long believed it was the victim of captured rotation. Just as the earth's gravity has locked the near side of the moon toward it, they theorized, the sun's enormous gravity had caused the same hemisphere of Mercury to face the sun perpetually, with never a dawn or sunset.

Within the past two years, however, radar observations of Mercury's surface have disproved the theory. Instead of revolving once on its axis during the 88 days it takes the planet to complete one solar orbit—as it would have to do to present the...

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