Mercury, in astronomy
Mercury has the most elliptic orbit of the planets in the solar system. 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 smallest planet in the solar system, having a diameter of 3,031 mi (4,878 km); both Jupiter's moon Ganymede and Saturn's moon Titan are larger. Its mean density relatively high, a little less than that of the earth; its core is believed to occupy about 85% of its radius and to consist of a probably solid iron core, surrounded by a liquid iron layer, which is surrounded by a solid iron-sulfide layer. There is 10 times as much sulfur in its crust than is found on the surface of the earth. The planet has a dark surface that reflects relatively little light; it is believed that the surface has been darkened by the deposition of carbon-rich micrometeorites that originate from comets passing close to the sun. Mercury's 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
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 88 earth days each, with the daylight temperatures reaching as high as 800℉ (450℃). Night temperatures are believed to drop as low as −300℉ (−184℃).
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2024, Columbia University Press. All rights reserved.
See more Encyclopedia articles on: Astronomy: General