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 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