MERCURY, the planet nearest to the sun. Its orbit being entirely within that of the earth, it never recedes to the point of the heavens opposite to the sun, but is always found within 29' of the sun. Hence it is only visible to the naked eye in the evening, when it is to the east of the sun, and near to its greatest distance (or elongation) from the sun ; so that the sun can descend sufficiently below the horizon to allow the planet to be seen just before its setting similarly. it is only visible in the morning (before sunrise), when it is near its greatest elongation westward of the sun. The phases of Mercury are not visible without a telescope. The apparent diameter of Mercury varies from 5 to 12 seconds ; the real diameter is '398 of that of the earth, or about 3140 miles. Its bulk is to that of the earth as 63 to 1000; its mass is reckoned at the two-millionth part of that of the sun. It revolves on an axis, the inclination of which to the ecliptic is not determined, in 24" 5r° 28.'3. It has seven times the light and heat of the earth.
The transits of Mercury, or passages of the planet over the sun s disc, take place when the conjunction (the inferior conjunction, as it is called, Mercury being between the earth and the sun) happens at the time that the planet is near its node. These transits are not so useful in the determination of the sun's parallax or the longitude of the place of observation as those of Venus : we shall therefore refer the account of such transits in general to the latter planet. The following are,
according to Delambre, the times of the transits which will yet take place in the present century (civil time at Paris) 1861 12 Nov., 74 A.M. I 1881 8 Nov.,1 a.m.
1868 5 Nov., 7 A.M. f 1891 10 May, 3 A.M.
1878 6 May, 7 r.sf. I 1894 10 Nov., Oi P.M.
Elements of the Orbit of Mercury.
Epoch, 1799, December 31, mean astronomical time at Sceberg.
Semiaxis major '3870938, that of the earth being assumed as the unit.
Excentricity .2041704; its secular increase (or increase in 100 years) '0000039.
Inclination of the orbit to the ecliptic 0'5'9; its secular increase 18"•4.
Longitudes from the mean equinox of the epoch : (1.) of the amend., iixe 15 57' tr..); its mocular increase (combined with the pre caseeieni, 1215'1; OA of the perihelion, 71• 20' its secular Mc:ewe (seeuldned with the precession), (3.) of the planet twain), 4'0" 3.
Main motion in one mean ,solar day, 1' 5' 31.".553; in 3651 It is Si' 4r 47 "u5 more than 4 cumplete revolutions; sidereal revolu tion. ts7•11692(1 mean seder days.
THANSI'r OF. Plenceliv ; Ye-Nes.] MERCI'llY,