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Planets As Evening And Morning Stars.—The inferior planets are evening stars from superior to inferior conjunction, and the superior planets from opposition to conjunction. During the other half of their revolutions they are morning stars.
To avoid filling the text with a multiplicity of figures, many interesting items are condensed in tables at the close of the volume.
Supposed Discovery.—Le Verrier, having detected an error in the assumed motion of Mercury, suggested, in the fall of 1859, that there may be an interior planet, which is the cause of this disturbance. On this being made public, M. Lescarbault, a French physician, and an amateur astronomer, stated that on March 26 of that year he had seen a dark body pass across the sun's disk, and that this might have been the unknown planet. Le Verrier visited him, and found his instruments rough and home-made, but singularly accurate. His clock was a simple pendulum, consisting of an ivory ball hanging from a nail by a silk thread. Bis observations were on prescription paper, covered with grease and laudanum. His calculations were chalked on a board, which he planed off to make room for fresh ones. Le Verrier became satisfied that a new planet had been really discovered by this enthusiastic observer, and congratulated him upon his deserved success. On March 20, 1862, Mr. Lummis, of Manchester, England, noticed a rapidly-moving, dark spot, apparently the transit of an inner planet. Many other instances are given of a somewhat similar character. As yet, however, the existence of the planet is not generally conceded. The name Vulcan and the sign of a hammer have been given to it. Its distance from the sun has been estimated at 13,000,000 miles, and its periodio time (its year) at 20 days.
The fleetest of the gods. Sign, a, his wand
Description.—Mercury is nearest to the sun of any of the definitely known planets. When the sky is very clear, we may sometimes see it, just after the setting of the sun, as a bright sparkling star, near the western horizon. Its elevation increases evening by evening, but never exceeds 30°.* If we watch it closely, we shall find that it again ap
* This distance varies much, owing to the eccentricity of Mercury's orbit
proaclies the sun and becomes lost in his rays Some days afterward, just before sunrise, we can see the same star in the east, rising higher each morning, until its greatest elevation equals that which it before attained in the west. Thus the planet appears slowly but steadily to oscillate like a pendulum, to and fro from one side to the other of the sun. The ancients, deceived by this, failed to discover the identity of the two stars, and called the morning star Apollo, the god of day, and the evening star Mercury, the god of thieves, who walk to and fro in the night-time seeking plunder. The Greeks gave to Mercury the additional name of "The Sparkling One." The astrologists looked upon it as the malignant planet. The chemists, because of its extreme swiftness, applied the name to quicksilver. The most ancient account that we have of this planet is given by Ptolemy, in his Almagest; he states its location on the 15th of November, 265 B. C. The Chinese also state that on June 9, 118 A. D., it was near the Beehive, a duster of stars in Cancer. Astronomers tell us that, according to the best calculations, it was at that date within less than 1° of that group. On account of the nearness of Mercury to the sun, it is difficult to be detected.* It is said that Copernicus, an old man of seventy, lamented in his last moments that, much as he had tried, he had never
* An old English writer by the name of Goad, in 1686, humorously termed this planet, " A squinting lacquey of the sun, who seldom shows his head in these parts, as if he were in debt"
been able to see it. In our latitude and climate, we can generally easily detect it if we watch for it at the time of its greatest elongation or quadrature, as given in the almanac.
Motion In Space.—It revolves about the sun at a mean distance of 35,000,000 miles. Its orbit is the most eccentric (flattened) of any among the eight principal planets, so that although when in perihelion it approaches to within 28,000,000 miles, in aphelion it speeds away 15,000,000 miles farther, or to the distance of 43,000,000 miles. Being so near the sun, its motion in its orbit is correspondingly rapid—viz., 30 miles per second. At this rate of speed, we could -cross the Atlantic Ocean in two minutes. The Mercurial year comprises only about 88 days, or nearly three of our months. Mercury revolves upon its axis in about the same time as the earth, so that the length of the Mercurial day is nearly the same as that of the terrestrial one. Though Mercury thus completes a sidereal revolution around the sun in 88 days, yet to pass from one inferior or superior conjunction to the same again (a synodic revolution) requires 116 days. The reason of this is, as already explained, that when Mercury comes around to the same spot in its orbit again, the earth has gone forward, and it requires 28 days for the planet to overtake us.
Distance From The Earth.—This varies still more than its distance from the sun. At inferior con junction it is between the earth and the snn, and its distance from us is the difference, between the distance of the earth and the planet from the sun: at superior conjunction it is the sum of these distances. Its apparent diameter in these different positions varies in the same proportion as the distances, or as three to one. The greatest and least distances vary according as either planet may happen to be in aphelion or perihelion. If at inferior conjunction Mercury is in aphelion and the earth in perihelion, its distance from us is only 90,000,000 - 43,000,000 = 47,000,000 miles. If at superior conjunction Mercury is in aphelion and the earth in aphelion also, its distance from us is 93,000,000 + 43,000,000 = 136,000,000 miles. •
Dimensions.—Mercury is about 3,000 miles in diameter. Its volume is about ^ that of the earth— i. e., it would require twenty globes as large as Mercury to make one the size of the earth, or 25,000,000 to equal the sun. Yet as it is | denser than the earth, its weight is nearly ^ that of the earth, and a stone let drop upon its surface would fall 7£ feet the first second. Its specific gravity is about that of tin. A pound weight removed to Mercury would weigh only about seven ounces.
Seasons.—As Mercury's axis is much inclined from a perpendicular, its seasons are peculiar. There are no distinct frigid zones; but large regions near the poles have six weeks continuous day and torrid heat, alternating with a night of equal length and arctic cold. The sun shines perpendio