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 same face to the sun--Mercury was found to rotate once every 59 days. This would mean that to a Mercury man--if there were one who could stand its temperatures (as high as 790DEG F.)--the sun would seem to move slowly across the sky, providing daylight for about 88 days. Then it would set, bringing on a night of the same duration. It remained for a Cornell University graduate astronomy student, Steven Soter, 23, to point out that the sun's otherwise dull journey across Mercury's sky is enlivened by a solar variety of the hesitation step.

While studying Mercury's surface temperatures, Soter paused to consider the effects of the planet's elliptical orbit, which causes it to speed up as it nears its closest approach (28.7 million miles) to the sun, and to slow down as it moves away to a maximum distance of 43.6 million miles. About four days before Mercury comes closest to the sun, Soter says in the current issue of Sky and Telescope, its increased angular velocity around the sun just matches its rotational rate about its own axis. To an observer on Mercury, the sun at this point would appear to stand still in its east-to-west transit of the skies. Then, as Mercury picked up even more speed, whipped past its point of closest approach to the sun, and began to slow down as it receded farther out into space again, the sun would appear to move backward toward the east, stop again, and then resume its journey for the remainder of the day.

At longitudes where dawn could be observed during the solar reversal, the sun would seem to rise slowly until more than half of it was above the eastern horizon. Then it would set below the same horizon, and soon rise again before heading for a more normal sunset in the west.

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