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Mercury, in astronomy, nearest planet to the sun, at a mean distance of 36 million mi (58 million km); its period of revolution is 88 days. Mercury passes through phases similar to those of the moon as it completes each revolution about the sun, although the visible disk varies in size with respect to its distance from the earth. Because its greatest elongation is 28°, it is seen only for a short time after sunset or before sunrise. Since observation of Mercury is particularly unfavorable when it is near the horizon, the planet has often been studied in full daylight, with the sun's light blocked off. Mercury has the second most elliptic orbit in the solar system, only Pluto's being greater among the planets. Its great eccentricity of orbit and its great orbital speed provided one of the important tests of Einstein's general theory of relativity. Mercury's perihelion (its closest point to the sun) is observed to advance by 43´´ each century more than can be explained from planetary perturbations using Newton's theory of gravitation, yet in nearly exact agreement with the prediction of the general theory. Mercury is the second smallest planet in the solar system, having a diameter of about 3,000 mi (4,800 km). Its mean density is comparable to that of the earth. Its small mass and proximity to the sun prevent it from having an appreciable atmosphere, although a slight amount of carbon dioxide has been detected. The surface of Mercury is much like that of the moon, as was shown during the Mariner 10 spacecraft flyby in 1974. Most of its craters were formed during a period of heavy bombardment by small asteroids early in the solar system's history. It was long thought that Mercury's period of rotation on its axis was identical to its period of revolution, so that the same side of the planet always faced the sun. However, radar studies in 1965 showed a period of rotation of 58.6 days. This results in periods of daylight and night of 90 earth days each, with the daylight temperatures reaching as high as 800°F (450°C). Night temperatures are believed to drop as low as -300°F (-184°C).
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