_MERCURY._
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.
“Come, let us view the glowing west,
Not far from the fallen Sun;
For Mercury is sparkling there,
And his race will soon be run.
With aspect pale, and wav’ring beam,
He is quick to steal away,
And veils his face in curling mists,—
Let us watch him while we may.”
_Supposed planet “Vulcan.”_—Mercury is the nearest known planet to
the Sun. It is true that a body, provisionally named Vulcan[27],
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.
[Illustration: Fig. 29.
1882 Nov. 5, 18^h 49^m. 1882, Nov. 6, 18^h 55^m.
Mercury as a morning star. (10-inch Reflector; power 212.)]
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.
FOOTNOTES:
[27] Chambers, in his ‘Descriptive Astronomy,’ 4th edition, 1889,
devotes a chapter to the discussion of facts having reference to
Vulcan; and the reader desiring full information will find it here.