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Mercury was named
by the Romans after the fleet-footed messenger of the gods because it seemed to move more quickly than any other planet. It
is the closest planet to the Sun, and second smallest planet in the solar system. Its diameter is 40% smaller than Earth and
40% larger than the Moon. It is even smaller than Jupiter's moon Ganymede and Saturn's moon Titan. If an explorer were to
step onto the surface of Mercury, he would discover a world resembling lunar terrain. Mercury's rolling, dust-covered hills
have been eroded from the constant bombardment of meteorites. Fault-cliffs rise for several kilometers in height and extend
for hundreds of kilometers. Craters dot the surface. The explorer would notice that the Sun appears two and a half times larger
than on Earth; however, the sky is always black because Mercury has virtually no atmosphere to cause scattering of light.
As the explorer gazes out into space, he might see two bright stars. One appearing as cream colored Venus and the other as
blue colored Earth. Until Mariner 10, little was known about Mercury because of the difficulty in observing it from Earth
telescopes. At maximum elongation it is only 28 degrees from the Sun as seen from Earth. Because of this, it can only be viewed
during daylight hours or just prior to sunrise or after sunset. When observed at dawn or dusk, Mercury is so low on the horizon
that the light must pass through 10 times the amount of Earth's atmosphere than it would if Mercury was directly overhead.
During the 1880's, Giovanni Schiaparelli drew a sketch showing faint features on Mercury. He determined that Mercury must
be tidally locked to the Sun, just as the Moon is tidally locked to Earth. In 1962, radio astronomers looked at radio emissions
from Mercury and determined that the dark side was too warm to be tidally locked. It was expected to be much colder if it
always faced away from the Sun. In 1965, Pettengill and Dyce determined Mercury's period of rotation to be 59 +- 5 days based
upon radar observations. Later in 1971, Goldstein refined the rotation period to be 58.65 +- 0.25 days using radar observations.
After close observation by the Mariner 10 spacecraft, the period was determined to be 58.646 +- 0.005 days. Although Mercury
is not tidally locked to the Sun, its rotational period is tidally coupled to its orbital period. Mercury rotates one and
a half times during each orbit. Because of this 3:2 resonance, a day on Mercury (sun rise to sun rise) is 176
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