William F. Denning.

Telescopic Work for Starlight Evenings online

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[Illustration: Fig. 27.

Birt, Birt A, and the Straight Wall. 1883, Feb. 15, 6^h to 8^h 40^m.

(T. Gwyn Elger.)]

_Rill or Cleft of Hyginus._ A conspicuous example of the lunar rills,
and one which yields to very moderate instruments. Neison notes that
it is readily visible in a 2-inch telescope; while Webb remarks that
a power of only 40, in a good instrument, is enough to show it under
any illumination. The rill is about 150 miles long. It cuts through
a number of crater-pits, and Mädler found so many widenings in it
that it appeared like a confluent train of craters. The rill traverses
the large crater-pit Hyginus, which is 3-3/4 miles in diameter and
moderately deep. Other fine examples of rill-systems will be found
between Rheita and Metius and near Triesnecker and Ramsden.

_Straight Wall._ A singular structure on the E. side of the ring-plain
Thebit. It is a ridge or wall, which looks regular enough for a work
of art, according to Webb. Its average height is 450 feet (Schröter),
1004 feet (Mädler), or 880 feet (Schmidt). These several determinations
are given to show the discordances sometimes found in the measures of
good observers. This object is about 60 miles long; at one extremity
lies a small crater, at the other there is a branching mountain nearly
2000 feet high. Elger has drawn this object, under both a rising and a
setting sun, in the Liverpool Astronomical Society’s ‘Journal,’ vol. v.
p. 156, and remarks that it may be well observed at from 20 to 30 hours
after the Moon’s first quarter.

_Valley near Rheita._ South of the ring-plain Rheita, on the S.W. limb,
there is an enormous valley, which extends in its entire length over
187 miles, with a width ranging from 10 to 25 miles. There are several
fine valleys in this particular region.

_Leibnitz Mountains._ These are really situated on the further
hemisphere of the Moon, but libration brings them into view, and they
are sometimes grandly seen in profile on the S. margin. Four of the
peaks ascend to elevations of 26,000 or 27,000 feet, and one mass,
towering far above the others, is fully 30,000 feet in height, and is
unquestionably the most lofty mountain on the Moon.

_Dörfel Mountains._ Visible on the Moon’s S.S.E. limb. They exhibit
three peaks, which, on the authority of Schröter, rise to more than
26,000 feet above the average level of the limb. The loftiest mountains
on the Earth are in the Himalayas—a range of immense extent to the N.
of India. The three highest peaks are Mount Everest (29,002 feet),
Kunchinjinga (28,156 feet), and Dhawalagiri (28,000 feet). The only
lunar mountain more elevated than these is that of the Leibnitz range,
which, as we have already stated, ascends to fully 30,000 feet.

[Illustration: Fig. 28.

Aristarchus and Herodotus at sunrise. 1884, Jan. 9, 8^h 30^m to
10^h 30^m.

(T. Gwyn Elger.)]

_Apennines._ A vast chain of mountains, extending over more than 450
miles of the lunar surface. _Huygens_ is the most elevated peak, rising
to more than 18,000 feet, and on its summit it shows a small crater.
There are several other very lofty peaks in this range. The Sun rises
upon the westerly region of these mountains at the time of first
quarter, and the peaks and ridges, with their contrasting shadows,
create a gorgeous effect just within, and projecting into the darkness
beyond, the terminator. There is an immense amount of detail to be
studied here, and much of it is within the reach of small instruments.

As the lunar mountains and craters are best seen near the terminator,
it may be useful to give a table of objects thus favourably placed
between the times of new and full Moon. The summary may assist the
student, though it does not aim at exactness, only even days being
given.


_Objects near the Terminator._


Moon’s age
in days.

2 Mare Crisium, Messala, Sunrise on the Mare Humboldtianum,
Langrenus, Vendelinus, Condorcet,
Hansen, Gauss(β), Hahn, Berosus.

3(α) Craters in Mare Crisium, Taruntius, Picard, Fraunhofer,
Vega, Pontécoulant, Cleomedes(γ), Furnerius,
Petavius, Endymion, Messier(δ), Vlacq.

4 Mare Nectaris, Macrobius(ε), Proclus, Sunrise on
Fracastorius, Rheita and Metius with the intervening
valley, Guttemberg, Colombo, Santbech,
Mountainous region W. of Mare Serenitatis,
Hercules, Atlas.

5 Palus Somnii, Plana, Capella, Isidorus, Polybius,
Piccolomini, Vitruvius, Littrow, Fabricius, Posidonius,
LeMonnier, Theophilus, Cyrillus, Catharina,
Hommel.

6 Tacitus, Maurolycus, Barocius, Dionysius(ζ), Sosigenes,
Abulfeda, Descartes, Almamon, Gemma Frisius,
Plinius, Ross, Arago, Delambre, Aristoteles,
Eudoxus, Julius Cæsar, Linné, Menelaus.

α The objects for observation when the Moon’s age is from 2 to 4 days
may be suitably re-examined a few days after the full.

β An extensive walled plain, 110 miles in length.

γ A large walled plain containing a small crater, Cleomedes A.

δ A curious double crater, with comet-like rays crossing the Mare
Fœcunditatis.

ε A circular ring-plain, 42 miles in diameter.]

ζ The interior of this crater exhibits some interesting features as
the Sun rises higher above it.

7 Ptolemæus, Albategnius, Manilius[17], Hyginus and
its rill-system, Hipparchus, Autolycus, Aristillus,
Cassini, Alpine Valley, W. C. Bond, Walter,
Miller, LaCaille, Apennines, Triesnecker and the
rills W. of it.

8 Mare Frigoris, Arzachel, Alphonsus, Alpetragius,
Bode, Pallas, Archimedes, Plato, Maginus[18],
Mösting[19], Thebit, Saussure, Moretus, Straight
Wall, Lalande, Kirch.

9 Tycho, Clavius, Eratosthenes[20], Stadius and the craters
running to N.E., Timocharis, Pitatus, Gruemberger,
Teneriffe Mountains, Straight Range[21],
Formation W. of Fontenelle[22], Gambart.

10 Sinus Iridum, Copernicus, Hesiodus and the rill
to E., Wilhelm I., Longomontanus[23], Heinsius,
Pytheas, Lambert, Helicon, Wurzelbauer.

11 Bullialdus, Campanus, Mercator, Reinhold, Riphæan
Mountains, Hippalus, Capuanus, Blancanus, Tobias
Mayer.

12 Mare Imbrium, Gassendi[24], Aristarchus and sinuous
valley to the N.E., Herodotus, Marius, Flamsteed,
Letronne, Schiller, Mersenius, Doppelmayer.

13 Schickhard, Wargentin, Grimaldi, Byrgius, Phocylides,
Hevelius, Seleucus, Crüger, Briggs,
Segner, Sirsalis.

14 Mare Smythii, Bailly, Inghirami, Bouvard, Riccioli,
Olbers, Hercynian Mountains, Cardanus, Krafft,
Cordilleras[25], Pythagoras[26].

_Occultations of Stars._—Among the various phenomena to which the lunar
motions give rise none are more pleasing to the possessors of small
telescopes than occultations of stars. Several of these occurrences
are visible every month. If the amateur has the means of obtaining
accurate time, he will engage himself usefully in noting the moments
of disappearance and reappearance of the stars occulted. This work
is efficiently done, it is true, at some of our observatories, and
therefore little _real_ necessity exists for amateurs to embark in
routine work which can be conveniently undertaken at establishments
where they have better appliances and trained observers to use them.
The mere watching of an occultation, apart from the registry of exact
results, is interesting; and there are features connected with it which
have proved exceedingly difficult to account for. The stars do not
always disappear instantaneously. On coming up to the edge of the Moon
they have not been suddenly blotted out, but have appeared to hang on
the Moon’s limb for several seconds. This must arise from an optical
illusion, from the action of a lunar atmosphere, or the stars must be
observed through fissures on the Moon’s edge. The former explanation
is probably correct; for it has happened that two observers at the
same place have received different impressions of the phenomenon. One
has seen the star apparently projected on the Moon’s limb for about 5
seconds, while the other has witnessed its sudden extinction, in the
usual manner, as it met the Moon’s edge. New observations, made with
good instruments and reliable eyes, and fully described, will doubtless
throw more light on the peculiar effects sometimes recorded.

_Visibility of the new and old Moon._—It is an interesting feature
of observation to note how soon after conjunction the Moon’s thin
crescent is observable with the naked eye. A case has been mentioned
in which the old Moon was seen one morning before sunrise and the new
Moon just after sunset on the next day. At Bristol, on the evening of
March 30, 1881, I saw the new Moon at 7^h 10^m, the horizon being very
clear in the west. She was then only 20^h 38^m old. On June 4, 1875, I
observed the Moon’s crescent at 9^h 10^m, or 22^h 49^m after new Moon.
Dr. Degroupet, of Belgium, saw the old Moon on the morning of Nov. 22,
1889, between 6^h 47^m and 7^h 22^m G.M.T., or within 18^h 22^m of the
time of new Moon.


FOOTNOTES:

[16] In September 1889 Prof. Thury, of Geneva, reported a change in the
centre of the crater Plinius. With a 6-inch refractor he saw, instead
of the usual two hills in the interior, a circular chalk-like disk
“with a dark spot in its centre like the orifice of a mud-volcano.”

[17] A fine ring-plain, 25½ miles in diameter.

[18] Mädler says “the full Moon knows no Maginus,” meaning that this
object is invisible under a vertical Sun.

[19] Mösting, Lalande, and Herschel form a fine triangle when the Sun
has attained a great altitude. Mösting is a ray-centre.

[20] A ring-plain 37½ miles in diameter, with very irregular terraced
walls.

[21] A range of mountains, with intervening valleys.

[22] Mädler describes this as a square enclosure with rampart-like
boundaries, which “throw the observer into the highest astonishment.”

[23] A great walled plain, 91 miles in diameter.

[24] A walled plain, 55 miles in diameter, in which Schröter suspected
changes.

[25] An extensive mountain-range on the E. by S. limb.

[26] A walled plain, 95 miles in diameter, and probably the deepest in
the N.E. quadrant, for the S.E. side of its wall rises to nearly 17,000
feet.

After the full the same objects should be re-examined under the
reversed illumination.




CHAPTER VII.

_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



Online LibraryWilliam F. DenningTelescopic Work for Starlight Evenings → online text (page 12 of 32)