The small and rocky planet Mercury is the closest planet to the
Sun; it speeds around the Sun in a wildly elliptical (non-circular)
orbit that takes it as close as 47 million km and as far as 70 million
km from the Sun. Mercury completes a trip around the Sun every 88
days, speeding through space at nearly 50 km per second, faster than
any other planet. Because it is so close to the Sun, temperatures on its
surface can reach a scorching 467 degrees Celsius. But because the
planet has hardly any atmosphere to keep it warm, nighttime temperatures
can drop to a frigid -170 degrees Celsius.
Because Mercury is so close to the Sun, it is hard to see from Earth except
during twilight. Until 1965, scientists thought that the same side of
Mercury always faced the Sun. Then, astronomers discovered that
Mercury completes three rotations for every two orbits around the Sun. The length of one Mercury day (sidereal rotation) is equal to 58.646 Earth days.
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