Mercury is the innermost planet in the Solar System, orbiting the Sun once every 88 days. It ranges in brightness from about −2.0 to 5.5 in apparent magnitude, but is not easily seen as its greatest angular separation from the Sun (greatest elongation) is only 28.3°, meaning it is only seen in twilight. The planet remains comparatively little known: the only spacecraft to approach Mercury was Mariner 10 from 1974 to 1975, which mapped only 40%–45% of the planet's surface.
Physically, Mercury is similar in appearance to the Moon as it is heavily cratered. It has no natural satellites and no real atmosphere. The planet has a large iron core which generates a magnetic field about 1% as strong as that of the Earth. Surface temperatures on Mercury range from about 90 to 700 K, with the subsolar point being the hottest and the bottoms of craters near the poles being the coldest.
The Romans named the planet after the fleet-footed messenger god Mercury, probably for its fast apparent motion in the twilight sky. The astronomical symbol for Mercury (Unicode: ☿) is a stylized version of the god's head and winged hat atop his caduceus, an ancient astrological symbol. Before the 5th century BC, Greek astronomers believed the planet to be two separate objects: one visible only at sunrise, the other only at sunset. The Chinese, Korean, Japanese, and Vietnamese cultures refer to the planet as the water star, based on the Five Elements.
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