Supposed planet, “Vulcan.”—Visibility of Mercury.—Period &c.—Elongations.—Amateur’s first view.—Phases.—Atmosphere of Mercury.—Telescopic observations.—Schiaparelli’s results.—Observations of SchrÖter and Sir W. Herschel.—Transits of Mercury.—Occultations of Mercury.

Supposed planet “Vulcan.”—Mercury is the nearest known planet to the Sun. It is true that a body, provisionally named Vulcan33, has been presumed to exist in the space interior 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. Certain planetary spots, observed in motion on the solar disk, were reported to have been transits of this intra-Mercurial orb. Some eminent astronomers were thus drawn to take an affirmative view of the question, and went so far as to compute the orbital elements and predict a few ensuing transits of the suspected planet. But nothing was seen at the important times, and some of the earlier observations have been shown to possess no significance whatever, while grave doubts are attached to many of the others. 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.

Visibility of Mercury.—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.

Period &c.—Mercury revolves round the Sun in 87^d 23^h 15^m 44^s in an eccentric orbit, so that his distance from that luminary varies from 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 3000 miles.

Elongations.—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 E. and W. 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. It is unfortunate that when the elongation approaches its extreme limits of 28° the planet is situated S. of the Sun, and therefore not nearly so favourably visible as at an elongation of only 18° or 20°, when his position is N. of the Sun.

I have seen Mercury on about sixty-five occasions with the naked eye. In May 1876 I noticed the planet on thirteen different evenings, and between April 22 and May 11, 1890, I succeeded on ten evenings. I believe that anyone who made it a practice to obtain naked-eye views of this object would succeed from about twelve to fifteen times in a year. In a finer climate, of course, Mercury may be distinguished more frequently. 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 conspicuous a few mornings after his W. elongations and a few evenings before his E. elongations.

Amateur’s First View.—The first view of Mercury forms quite an event in the experience of many amateurs. The evasive planet is sought for with the same keen enthusiasm as though an important discovery were involved. For a few evenings efforts are vain, until at length a clearer sky and a closer watch enables the glittering little stranger to be caught amid the vapours of the horizon. The observer is delighted, and, proud of his success, he forthwith calls out the members of his family that they, too, may have a glimpse of the fugitive orb never seen by the eye of Copernicus.

Phases.—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. Galilei’s telescope failed to reveal it, and Hevelius, many years afterwards, 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.

Atmosphere.—The atmosphere of Mercury is probably far less dense than that of Venus. The latter being farthest 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.

Telescopic Observations.—As this planet is comparatively seldom to be observed under satisfactory conditions, it is scarcely surprising that our knowledge of his appearance is very meagre, or that amateurs consider the planet an object practically inaccessible as regards the observation of physical peculiarities, and one upon which it is utterly useless to apply the telescope in the hope of effecting new discoveries. Former attempts have proved the extreme difficulty of obtaining good images of this planet. The smallness of the disk, and the fact that it is usually so much affected by the waves of vapour passing along the horizon as to be constantly flaring and moulding in a manner which scarcely enables the phase to be made out, are great drawbacks, which render it impossible to distinguish any delicate features that may be presented on the surface.

These circumstances are well calculated to lead observers to abandon this object as one too unpromising for further study; but I think the view is partly induced by a misconception. The planet’s diminutive size is a hindrance which cannot be overcome; but the bad definition, resulting from low altitude, may be obviated by those who will select more suitable times for their observations and not be dismayed if their initiatory efforts prove futile. 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 planet surmounts the dense lower vapours 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 equatoreals should pick up the planet in the afternoon and follow him until after sunset, when the horizontal vapours will interfere. Others who work with ordinary alt-azimuth 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. He may sometimes also be brought into the field before sunset (at the eastern elongations in the spring months), by careful sweeping with a comet-eyepiece, especially when either the Moon, Venus, or Jupiter happens to be near, and the observer has found the relative place of the planet from an ephemeris.

Schiaparelli’s Results.—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 Prof. Schiaparelli, of Milan, who favoured 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 has some spots which become partially obscured and sometimes completely so; it has also some brilliant white spots in a variable position. As I observe the planet entirely by day and near the meridian I have been able to see its spots many times, but not always with the necessary distinctness to make drawings sufficiently reliable to serve as a base for a rigorous investigation. It is remarkable that the views taken near superior conjunction have been more instructive for me than those taken when the disk is near dichotomy, the defect in diameter being compensated by the possibility of seeing nearly all the disk, which, under those conditions, is more strongly illuminated. I believe that by instrumental means, such as our 8½-inch refractor at Milan gives, it is possible to prove the rotation-period of Mercury and to gain a knowledge of the principal spots as regards the generality of their forms. But these spots are really very complicated, for, besides the difficulties attending their observation, they are extremely variable.”

Prof. Schiaparelli used an 8½-inch refractor in this work, and was able, under some favourable 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, has been recently 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 behaviour of the Moon relatively to the Earth has found an analogy. But these deductions of the eminent Italian astronomer require corroboration, and this is not likely to be soon forthcoming owing to the obstacles which stand in the way.

Observations of SchrÖter and Sir W. Herschel.—SchrÖter observed Mercury with characteristic diligence between 1780 and 1815. In 1800 he several times remarked that the southern horn of the crescent was blunted, and fixed the planet’s rotation-period at 24^h 4^m. He also inferred the existence of a mountain 12 miles in height. But elements of doubt are attached to some of SchrÖter’s observations; and Sir W. Herschel, whose telescopic surveys of both Mercury and Venus were singularly barren of interesting results, pointed out their improbability. But the great observer of Slough was not very amicably disposed towards his rival in Germany. His strictures appear, however, to have been not without justice if we consider them in the light of modern observations.

Surface-markings.—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 N.E. and S. The spot was a little S. of the centre. Birmingham, on March 13, 1870, glimpsed a large white spot near the planet’s E. 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.

Amateurs with moderately large instruments would be usefully employed in following this planet at the most opportune periods and making careful drawings under the highest powers that can be successfully applied. Mercury has been persistently neglected by many in past years, and no doubt this “swift-winged messenger of the Gods” has eluded some of his would-be pursuers; but there is every prospect that a patient observer, careful to utilize all available opportunities, would soon gather some profitable data relating to his appearance.

Transits of Mercury.—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 the black disk, and an annulus has been visible round it. These features are probably optical effects, and it will be worth while to remember them on the occasion of future transits, of which the subjoined is a list:—

• 1891, May 9.
• 1894, Nov. 10.
• 1907, Nov. 12.
• 1914, Nov. 6.
• 1924, May 7.
• 1927, Nov. 8.
• 1937, May 10.
• 1940, Nov. 12.
• 1953, Nov. 13.
• 1960, Nov. 6.
• 1970, May 9.
• 1973, Nov. 9.

The first observer of a transit of Mercury appears to have been Gassendi, at Paris, on Nov. 7, 1631.

Occultations of Mercury.—There was an occultation of Mercury by the Moon on April 25, 1838. It occurred on the day of the planet’s greatest elongation E., and at a time in the evening when it might have been most suitably witnessed, but cloudy skies appear to have frustrated the hopes of intending observers. There was a repetition of the event on the morning of May 2, 1867, and it occurred, curiously enough, less than 24 hours after an occultation of Venus.