Mercury is the nearest known planet to the sun. It is true that a body, provisionally named Vulcan, has been presumed to exist in the space inferior to the orbit of Mercury; but absolute proof is lacking, and every year the idea is losing strength in the absence of any confirmation of a reliable kind. Not one of the regular and best observers of the sun has recently detected any such body during its transits (which would be likely to occur pretty frequently), and there is other evidence of a negative character; so that the ghost of Vulcan may be said to have been laid, and we may regard it as proven that no major planet revolves in the interval of 36,000,000 miles separating Mercury from the sun.
Copernicus, amid the fogs of the Vistula, looked for Mercury in vain, and complained in his last hours that he had never seen it. Tycho Brahe, in the Island of Hueen, appears to have been far more successful. The planet is extremely fugitive in his appearances, but is not nearly so difficult to find as many suppose. Whenever the horizon is very clear, and the planet well placed, a small sparkling object, looking more like a scintillating star than a planetary body, will be detected at a low altitude and may be followed to the horizon.
Mercury revolves round the sun in 87 days, 23 hours, 15 minutes, and 44 seconds in an eccentric orbit, so that his distance from that luminary variesfrom 43,350,000 to 28,570,000 miles. When in superior conjunction the apparent diameter of the planet is 4″.5; at inferior conjunction it is 12″.9, and at elongation 7″. His real diameter is 3,000 miles.
Being situated so near to the sun, it is obvious that to an observer on the earth he must always remain in the same general region of the firmament as that body. His orbital motion enables him to successively assume positions to the east and west of the sun, and these are known as his elongations, which vary in distance from 18° to 28°. He becomes visible at these periods either in the morning or evening twilight, and under the best circumstances may remain above the horizon two hours in the absence of the sun. The best times to observe the planet are at his E. elongations during the first half of the year, or at his W. elongations in the last half; for his position at such times being N. of the sun’s place, he remains a long while in view.
Occasionally he presents quite a conspicuous aspect on the horizon, as in February, 1868, when I thought his lustre vied with that of Jupiter, and in November, 1882, when he shone brighter than Sirius. The planet is generally most conspicuousa few mornings after his W. elongations and a few evenings before his E. elongations.
In the course of his orbital round, Mercury exhibits all the phases of the moon. Near his elongations the disk is about half illuminated, and similar in form to that of our satellite when in the first or third quarter. But the phase is not to be distinctly made out unless circumstances are propitious. Galileo’stelescope failed to reveal it, and Hevelius, many years afterward, found it difficult. This is explained by the small diameter of the planet and the rarity with which his disk appears sharply defined. The phase is sometimes noted to be less than theory indicates; for the planet has been seen crescented when he should have presented the form of a semicircle. Several observers have also remarked that his surface displays a rosy tint, and that the terminator is more deeply shaded and indefinite than that of Venus.
The atmosphere of Mercury is probably far less dense than that of Venus. The latter being furthest from the sun might be expected to shine relatively more faintly than the former, but the reverse is the case. Mercury has a dingy aspect in comparison with the bright white lustre of Venus. On May 12, 1890, when the two planets were visible as evening stars, and separated from each other by a distance of only 2°, I examined them in a 10-inch reflector, power 145. The disk of Venus looked like newly polished silver, while that of Mercury appeared of a dull leaden hue. A similar observation was made by Mr. Nasmyth on September 28, 1878. The explanation appears to be that the atmosphere of Mercury is of great rarity, and incapable of reflection in the same high degree as the dense atmosphere of Venus.
As a naked-eye object, Mercury must necessarily be looked for when near the horizon; but there is no such need in regard to telescopic observation, which ought to be only attempted when the planetsurmounts the dense lower vapors and is placed at a sufficient elevation to give the instrument a fair chance of producing a steady image. The presence of sunshine need not seriously impair the definition, or make the disk too faint for detail.
I have occasionally seen Mercury, about two or three hours after his rising, with outlines of extreme sharpness and quite comparable with the excellent views obtained of Venus at the time of sunrise or sunset. Those who possess equatorials should pick up the planet in the afternoon and follow him until after sunset, when the horizontal vapors will interfere. Others who work with ordinary altazimuth stands will find it best to examine the planet at his western elongations during the last half of the year, when he may be found soon after rising by the naked eye or with an opera-glass, and retained in the telescope for several hours after sunrise if necessary.
Mercury was displayed under several advantages in the morning twilight of November, 1882, and I made a series of observations with a 10-inch reflector, power 212. Several dark markings were perceived, and a conspicuous white spot. The general appearance of the disk was similar to that of Mars, and I forwarded a summary of my results to Professor Schiaparelli of Milan, who favored me with the following interesting reply:
“I have myself been occupied with this planet during the past year (1882). You are right in saying that Mercury is much easier to observe than Venus, and that his aspect resembles Mars more than any other of the planets of the Solar System. It hassome spots which become partially obscured and sometimes completely so; it has also some brilliant white spots in a variable position.”
Professor Schiaparelli used an 8½-inch refractor in this work, and was able under some favorable conditions to apply a power of 400. The outcome of his researches, encouraged since 1882 by the addition of an 18-inch refractor to the appliances of his observatory, was announced in the curious fact that the rotation of Mercury is performed in the same time that the planet revolves round the sun! If this conclusion is just, Mercury constantly presents one and the same hemisphere to the sun, and the behavior of the moon relatively to the earth has found an analogy.
Spots or markings of any kind have rarely been distinguished on Mercury. On June 11, 1867, Prince recorded a bright spot, with faint lines diverging from it northeast and south. The spot was a little south of the centre. Birmingham on March 13, 1870, glimpsed a large white spot near the planet’s east limb, and Vögel, at Bothkamp, observed spots on April 14 and 22, 1871. These instances are quoted by Webb, and they, in combination with the markings seen by Schiaparelli at Milan and by the author at Bristol in 1882, sufficiently attest that this object deserves more attentive study.
One of the most interesting phenomena, albeit a somewhat rare event, in connection with Mercury, is that of a transit across the sun. The planet then appears as a black circular spot. Observers have noticed one or two very small luminous points on theblack disk, and an annulus has been visible round it. These features are probably optical effects.