Mercury (planet)

Mercury (IPA: /ˈmɝ.kjə.ɹi/) is the innermost and smallest 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°. It can only be seen in morning or evening twilight. Comparatively little is known about the planet: the only spacecraft to approach Mercury was Mariner 10 from 1974 to 1975, which mapped only 40%–45% of the planet’s surface. The period of rotation is 58.6462 days.

Physically, Mercury is similar in appearance to the Moon as it is heavily cratered. It has no natural satellites and no substantial atmosphere. The planet has a large iron core which generates a magnetic field about 0.1% as strong as that of the Earth. Surface temperatures on Mercury range from about 90 to 700 K (−180 to 430°C, −292 to 806°F), with the subsolar point being the hottest and the bottoms of craters near the poles being the coldest.

Recorded observations of Mercury date back to the time of the Sumerians, in the third millennium BC. The Romans named the planet after the Roman god Mercurius, equated to the Greek Hermes and the Babylonian Nabu. The astronomical symbol for Mercury is a stylized version of the god’s head and winged hat atop his caduceus, an ancient astrological symbol. The Greeks of Hesiod's time called it Στίλβων Stilbon (“the gleaming”) and Hermaon. 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. In India, the planet was named Budha (बुध), after the son of Chandra (the Moon). The Chinese, Korean, Japanese, and Vietnamese cultures refer to the planet as the water star (水星), based on the Five Elements. The Hebrews named it Kokhav Hamah (כוכב חמה), “the star of the hot one” (“the hot one” being the Sun). Mercury is smaller (though still more massive) than two of the natural satellites or moons in our solar system (see the list of solar system objects by radius).