William F. Denning.

Telescopic Work for Starlight Evenings online

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are excellent in their way, and the outcome of much zealous labour; but
they contain omissions and inaccuracies which it has been impracticable
to avoid. The amateur who discovers a mountain, craterlet, or rill not
depicted on his lunar maps must therefore neither regard it as a new
formation or as a new discovery; for it may have been overlooked by
some of the previous observers, and is possibly drawn or described in
a work which he does not happen to have consulted. Such differences
should, however, always be announced, as they clear the way for others
working in the same field.

A small instrument, with an object-glass of about 2½ inches, will
reveal a large amount of intricate detail on the surface of our
satellite, and will afford the young student many evenings of
interesting recreation. But for a more advanced survey of the
formations, with the view to discover unknown objects or traces of
physical change in known features, a telescope of at least 8 or 10
inches aperture is probably necessary, and powers of 300, 350, and more.

_Eclipses of the Moon._—These phenomena comprise a variety of
interesting aspects. They are less numerous, in actual occurrence, than
solar eclipses in the proportion of about 2 to 3; but they are more
frequently visible, because they may be witnessed from any part of an
entire hemisphere, whereas eclipses of the Sun are only observable from
a tract of the Earth’s surface not exceeding 180 miles in breadth. The
Moon may remain totally eclipsed for a period of 2 hours 4 minutes, and
the whole duration, including the penumbral obscuration from its first
to its last projection, is about 6 hours. Sometimes the Moon suffers
total eclipse twice in the same year, and both may be visible, as in
1844, 1877, 1964, &c. It is possible for three such eclipses to occur
within a single year, as in 1544. In 1917 there will be three total
lunar eclipses, but not all visible in England. In the latter year
there will be no less than seven eclipses, as in 1935.

On the last two occasions—Oct. 4, 1884, and Jan. 28, 1888—when the
Moon was totally immersed in the Earth’s shadow, the atmosphere was
very clear; and it is hoped equally favourable conditions will attend
the similar phenomena of Nov. 15, 1891, Sept. 4, 1895, and Dec. 27,
1898. One of the most interesting features during these temporary
obscurations of our satellite is the occultation of small stars. Prof.
Struve compiled a list of no less than 116 of these objects that would
pass behind the Moon’s shadowed limb during the eclipse of Oct. 4,
1884.

Another important effect is the variable colouring on the Moon. This
differs considerably in relative intensity as seen during successive
eclipses, and the cause is not perhaps fully accounted for. Kepler
thought it due to differences in humidity of those parts of the Earth’s
atmosphere through which the solar rays pass and are refracted to the
eclipsed Moon. The intense red hue which envelopes the lunar surface
on such occasions is due to the absorption of the blue rays of light
by our atmosphere. The sky at sunset is often observed to be similarly
coloured, and from the operation of similar causes. Sometimes the Moon
entirely disappears when eclipsed, but on other occasions remains
distinctly obvious, like a bright red ball suspended in the firmament.
On May 5, 1110, Dec. 9, 1620, May 18, 1761, and June 10, 1816, our
satellite is said to have become absolutely imperceptible during
eclipse. Wargentin, who described the appearance in 1761, remarks:—“The
Moon’s body disappeared so completely that not the slightest trace of
any portion of the lunar disk could be discerned, either with the naked
eye or with the telescope.” On Oct. 4, 1884, I noticed that the opacity
was much greater than usual; at the middle period of the eclipse the
Moon’s diameter was apparently so much reduced that she looked like a
dull, faint, nebulous mass, without sharply determinate outlines. The
effect was similar to that of a star or planet struggling through dense
haze. Yet, on March 19, 1848, the Moon “presented a luminosity quite
unusual. The light and dark places on the face of our satellite could
be almost as well made out as on an ordinary dull moonlight night.” On
July 12, 1870, Feb. 27 and Aug. 23, 1877, and Jan. 28, 1888, the Moon,
as observed at Bristol, was also fairly bright when totally immersed
in the Earth’s shadow. In explanation of these singular differences,
Dr. Burder has suggested that Kepler’s views seem inadequate, and
that the solar corona is probably implicated in producing light and
dark eclipses. He concludes that, as the corona sometimes extends
to considerable distances from the Sun, and is very variable in
brightness, it may have sufficient influence to occasion the effects
alluded to.

_Lunar Changes._—The question as to whether physical changes are
occurring in the surface-formations of our satellite is one which
offers attractive inducements to telescopic observers. Though the Moon
appears to have passed the active state, it is very possible that
trivial alterations continue to affect some of her features. In April
1787 Sir W. Herschel wrote:—“I perceive three volcanoes in different
places of the dark part of the new Moon. Two of them are already nearly
extinct, or otherwise in a state of going to break out; the third
shows an eruption of fire or luminous matter.” Schröter, however, was
correctly of opinion that these appearances were due to reflected
light from the Earth falling upon elevated spots of the Moon having
unusual capacity to return it. Schröter himself thought he detected
sudden changes in 1791. He says that, on the 30th of December, at 5^h
P.M., with a 7-foot reflector magnifying 161 times, he perceived the
commencement of a small crater on the S.W. declivity of the volcanic
mountain in the Mare Crisium, having a shadow of at least 2′ 5″. On
the 11th of January, 1792, at 5^h 20^m P.M., on looking at the place
again he could see neither the new crater nor its shadow. In this
case the disappearance was doubtless an apparent one, merely due to
the reversed illumination under which the object was examined in the
interval of 12 days.

Many other observers besides Herschel have been struck with the
brightness of certain spots situated in the opaque region of the lunar
disk; but there is no doubt the cause has been uniformly one and the
same, viz. the highly reflective properties of some of the mountains
(notably of one named Aristarchus), which are distinctly visible as
luminous spots amid the relatively dark regions surrounding them. They
afford no certain evidence of existing volcanic energy, and in the
light of modern researches such an idea cannot be entertained.

On June 10, 1866, Temple noticed a remarkable light appearance,
agreeing with the position of Aristarchus, upon the dark side of the
Moon, faintly illuminated by earthshine. The object did not exhibit
a faint white light analogous to that of other craters in the dark
side, but it was star-like, diffused, in colour reddish yellow, and
evidently dissimilar to other bright spots. He wrote, in reference to
this matter:—“Of course I am far from surmising a still active chemical
outbreak, as such an outbreak supposes water and an atmosphere, both
of which are universally allowed not to exist on the Moon, so that
the crater-forming process can only be thought of as a dry, chemical,
although warm one.”

On November 17, 1866, Schmidt announced that the lunar crater Linné,
about 5½ miles in diameter, and situated in the Mare Serenitatis, had
disappeared! He averred that he had been familiar with the object as
_a deep crater_ since 1841, but in October 1866 he found its place
occupied by _a whitish cloud_. This cloud was always visible, but the
crater itself appeared to have become filled up, and was certainly
invisible under its former aspect. Such a definite statement, emanating
as it did from a diligent and experienced student of selenography,
naturally aroused keen interest, and Linné at once became the object
of wide-spread observation. But a reference to Schröter’s results,
obtained in the latter part of the last century, threw some doubt
upon the alleged change. This observer had figured Linné on November
5, 1788, as a round white spot, and there is nothing in his drawing
indicating a crateriform aspect. His description of Linné was:—“A
flat, somewhat doubtful crater, which appears as a round white spot.”
Mr. Huggins regarded Schröter’s observations as correctly expressing
the appearance of this object in 1867 under the same conditions of
illumination. On the other hand, Lohrmann (1823) and Mädler (1831)
referred to Linné as a deep crater, and in terms inconsistent both
with Schröter’s drawing and with the present aspect of the object.
The outcome of the many fresh observations that were collected was
that Linné appeared as a white cloud, with a small black crater within
a large shallow-ringed depression. But as usual in such cases, the
observers were far from being unanimous as to the details of the
formation; and certainly in regard to a lunar object this need occasion
no surprise, for slight differences in the angle of illumination
produce marked changes in the aspect of lunar features. The fact of
actual change could not be demonstrated, and the negative view appears
to have subsequently gained weight.

Another instance of alleged activity on the Moon was notified by Dr.
Klein in the spring of 1877. He saw a deep black crater about 18
miles to the W.N.W. of Hyginus, and in a particular place where he
had previously recognized no such object, though he had frequently
examined the region and was perfectly familiar with it. Forthwith every
telescope was directed to this part of the Moon. The maps of earlier
observers were eagerly consulted, and lunar photographs scanned for
traces of the new object. Many drawings were made of the district
near Hyginus and of the remarkable rill or cleft connected with it;
but amongst both old and new records some puzzling discordances were
detected. Many of the observers, instead of finding Dr. Klein’s new
formation a sharply-cut, deep crater, saw it rather in the character of
a saucer-like depression; and I drew it under this aspect on several
occasions with a 10-inch reflector. The fact, therefore, of its being
a new feature admitted of no valid and convincing proofs, and thus the
same uncertainty remains attached to this object as to Linné, nothing
being absolutely proved[16]. The problem as to whether the Moon is
still the seat of physical activity has yet to be solved.

Many circumstances are antagonistic to the discovery of changes on
the Moon. As the Sun’s altitude is constantly varying with reference
to lunar objects, they assume different aspects from hour to hour.
In a short interval the same formations become very dissimilar.
When the Sun is rising above the more minute craters they are often
distinguished in their true characters; but near the period of full
Moon they are visible as bright spots, and it is impossible to tell
whether they represent craters or conical hills. With a vertical Sun,
as at the full, all the shadows have disappeared—in fact, the entire
configuration has been transformed, and many of the interesting
lineaments displayed at the crescent phase are no longer seen. The
Moon’s libration also introduces slight differences in the appearance
of objects. And these are not the only drawbacks; for observations,
in themselves, are seldom accordant, and it is found that drawings
and descriptions are not always to be reconciled, though referring to
identical and invariable features. The lunar landscape must be studied
under the same conditions of illumination and libration, with the same
instrument and power, and in a similar state of atmosphere, if results
are to be strictly comparable. But it is very rarely that observations
can be effected under precisely equal conditions; hence discordances
are found amongst the records.

The whole of the Moon’s visible sphere exhibits striking imprints of
convulsions and volcanic action in past times, though no such forces
appear to operate now. The surface seems to have become quiescent,
and to have assumed a rigidity inconsistent with the idea of present
energy. But we cannot be absolutely certain that minute changes are not
taking place, and, being minute, the prospect of their detection is
somewhat remote. Students of lunar scenery will probably have to watch
details with scrupulous care and for long periods before an instance of
real activity can be demonstrated.

_Lunar Formations._—The Moon abounds in objects of very diversified
character, and they have been classified according to peculiarities
of structure. The names of eminent astronomers have been applied
to many of the more definite features—a plan of nomenclature which
originated with Riccioli, who published a lunar map at the middle of
the seventeenth century. The following brief summary comprises many of
the principal formations:—

_Mare._ A name applied by Hevelius to denote the large and relatively
level plains on the Moon, which present some similarity to terrestrial
seas. They are visible to the naked eye as dusky spots, and in a
telescope show many craters, hills, and mounds, and some extensive
undulations of surface.

_Palus_ (Marsh) and _Lacus_ (Lake) were titles given by Riccioli to
minor areas of a dark colour, and exhibiting greater variety of detail
and tint than the _Maria_.

_Sinus_ (Bay) has been applied to objects like deep bays on the borders
of the _Maria_.

_Walled Plains_ extend from 40 to 150 miles in diameter, and are
commonly surrounded by a terraced wall or mountain-ranges. The
interiors are tolerably level, though often marked with crater-pits,
mounds, and ridges.

_Mountain-Rings._ These represent rings of mountains and hills,
enclosing irregularities, possibly furnished by the debris of the
crumbling exterior walls, which, in certain instances, appear to have
fallen inwards.

_Ring-Plains_ are more circular and regular in type than the walled
plains, and consist of a moderately flat surface surrounded by a single
wall. _Crater-Plains_ are somewhat similar, and seldom exceed 20 miles
in diameter. They “rise steeply from the mass of debris around the foot
of their walls to a considerable height, and then fall precipitously
to the interior in a rough curved slope, whilst on their walls,
especially on the exterior, craterlets and crater-cones often exist in
considerable numbers.”

_Craters, Craterlets, and Crater-Pits._ Usually circular in form, and
severally offering distinctions as to dimensions and shape. The craters
are surrounded by walls, rising abruptly to tolerable heights, and
pretty regular in their contour. When the Sun is rising the shadow of
the walls falls upon the interior of the craters, and many of these
dark conspicuous objects are to be seen near the Moon’s terminator.
With a high Sun some of the craters are extremely bright. In proof
of the large number of these objects, it may be noted here that in
Mädler’s lunar map (1837) 7735 craters are figured, while in Schmidt’s
(1878) there are no less than 32,856!

_Crater-Cones._ Conical hills or mountains, visible as small luminous
spots about the period of full Moon. They are from 1/2 to 3 miles in
diameter, and show deep central depressions. It is somewhat difficult
to distinguish them from the ordinary mountain-peaks and white spots,
and they are not unlike the cones of terrestrial volcanoes.

_Rills or Clefts._ These are very curious objects. They were first
discovered by Schröter in 1787, and some of them are to be traced over
a considerable extent of the lunar surface, their entire length being
200 or 300 miles. They have the appearance of cuttings or canals, and
are sometimes straight, sometimes bent, and not unfrequently develop
branches which intersect each other. They apparently run without
interruption through many varieties of lunar objects. The bottoms of
these rills are nearly flat, and look not unlike dried riverbeds. Some
observers have regarded them as fractures or cracks in the Moon’s
surface; but their appearance and circumstances of arrangement are
opposed to such a view. Our present knowledge includes more than 1000
of these rills.

_Mountain-Ranges_ are chains of lofty peaks and highlands, sometimes
divided by rills and numerous ravines and cross valleys. Some of these
ranges are of vast magnitude, and the summits of the mountains reach
altitudes between 15,000 and 20,000 feet, and sometimes even more.

_Mountain-Ridges_ are to be found scattered in the greatest abundance
in the most disturbed localities of the lunar surface. They sometimes
connect several formations, or surmount ravines or depressions of large
extent. Peaks attaining altitudes of more than 5000 feet rise from
them, and they range in several cases over 100 miles.

_Ray-Centres._ Systems of radiating light-streaks, having a
mountain-ring as the centre of divergence, and stretching to distances
of some hundreds of miles round. Tycho, Copernicus, Kepler, Anaxagoras,
Aristarchus, and Olbers are pronounced examples of this class.

In Beer and Mädler’s chart of the Moon the names are attached to the
various formations, as they are also in Neison’s maps and in some other
works. One of these will be absolutely necessary to the student in
prosecuting his studies. He will then have a ready means of acquainting
himself with the various formations, and making comparisons between his
new results and the drawings of earlier selenographers. I would refer
the reader to Neison’s and Webb’s books for many references in detail
to lunar features, and must be content here with a brief description of
a few leading objects:—

_Plato_ is an extensive walled plain, 60 miles in diameter, and
situated on the N.E. boundary of the Mare Imbrium. Nasmyth and
Carpenter describe the wall as “serrated with noble peaks, which cast
their black shadows across the plateau in a most picturesque manner,
like the towers and spires of a great cathedral.” It has received
a large amount of attention, with a view to trace whether changes
are occurring in the numerous white spots and streaks lying in its
interior. In 1869-71 Mr. Birt collected many observations, and on
discussing them was led to believe that “there is strong probability
that activity, of a character sufficient to render its effects visible,
had been manifested.” The inquiry was renewed by Stanley Williams in
1882-84, and he concluded that the results were strongly confirmatory
of actual change having occurred since 1869-71. The relative visibility
of several of the bright spots had altered in the interim, and
the curious intermingling bright streaks also exhibited traces of
variation. At sunrise the interior of Plato is pure grey; but with
the sun at a considerable height above it, the plain becomes a dark
steel-grey. The change is an abnormal one, and difficult to explain.
South of Plato there is a fine example of an isolated peak, named Pico,
which is about 8000 feet high.

[Illustration: Fig. 25.

Light-spots and streaks on Plato, 1879-82. (A. Stanley Williams.)]

_Great Alpine Valley._ This object, supposed to have been discovered
by Bianchini in 1727, and having a length, according to Mädler, of 83
miles and a breadth varying from 3½ to 6 miles, is a very conspicuous
depression situated near Plato, and running from the Mare Frigoris
to the Mare Imbrium. It exhibits at its southern extremity an oval
formation, and a narrow gorge issues from it to the northward, opening
out further on, and imparting to the whole appearance a shape which
Webb likened to a Florence oil-flask. Elger has fully described this
singular structure. “It is only when far removed from the terminator
that its V-shaped outlet to the Mare Imbrium flanked on either side
by the lofty Alps can be traced to advantage, or the flask-like
expansion with the constricted gorge leading up to it from the N.W.
satisfactorily observed. At other times these features are always more
or less concealed by the shadows of neighbouring heights. The details
of the upper or more attenuated end of the valley are, however, best
seen under a setting sun, when many striking objects come to light, of
which few traces appear at other times.”

_Archimedes._ One of the most definite and regular of the walled
plains. It is 60 miles in diameter, with a wall rising about 4200 feet
above the surface. Some small craters and various streaks diversify its
centre.

_Tycho._ A grand ring-plain, 54 miles in diameter and about 17,000
feet (= nearly 3 miles) deep, and forming the centre of the chief
ray-system of the Moon. The light-radiations stretch over one fourth of
the visible hemisphere at the full, but they are imperceptible with the
Sun’s altitude below 20°. These remarkable radiations from Tycho form a
striking aspect of lunar scenery, and any small telescope reveals them.
Webb has termed Tycho “the metropolitan formation of the Moon;” and the
idea embodied in the expression must strike observers as very apposite.
This object is visible to the naked eye at the time of full. A fine
hill rises from its centre to a height of 5500 feet.

_Copernicus._ A magnificent ring-plain, 56 miles in diameter, and
surrounded by a wall (in which there are terraces and lofty peaks,
separated by ravines) attaining an elevation of 11,000 feet. There
is a central hill of nearly 2500 feet. From Copernicus light-streaks
are plentifully extended on all sides, and apparently connect this
object with the many others of similar character which are situated in
this region. Neison says that near Copernicus the light-streaks unite
and form a kind of nimbus or light-cloud about it. The streaks are
most conspicuous towards the N., where they are from 5 to 14 miles in
width. To the N.W. of Copernicus, about halfway in the direction of
the neighbouring ring-plain _Eratosthenes_ (and N. of Stadius), there
is a considerable number of crater-pits. Mädler figured sixty-one of
these, and regarded that number as certainly less than half the total
number visible. They appear to be ranged in rows or streams, and are
so close together in places as to nearly form crater-rills. Schmidt
saw the ground hereabout pierced like a honeycomb, and managed to
count about 300 little craters; but they are so thickly strewn in this
district that exact numbers or places cannot be assigned. They are best
observable when the Sun is rising on the E. wall of Copernicus. The
interior of this fine object shows six or seven peaks, which are often
capped with sunshine, and very brilliant amid the black shadow thrown
from the surrounding wall.

_Theophilus._ Another ring-plain, and one of the deepest visible. Its
terraced lofty wall, 64 miles in diameter, rises in a series of peaks
to heights varying between 14,000 and 18,000 feet. There is a central
mountain, broken by ravines; but from one of the masses a peak ascends
to a height of about 6000 feet.

_Petavius._ A large walled plain, surrounded by a double wall or
rampart, which rises to 11,000 feet on its E. side. There are hills
and ridges in the interior, and a central peak, A, reaching to 5500
feet above the E. part of the floor, which is convex in form. A smaller
peak, of nearly 4000 feet, lies W. of A. Several small craterlets have
been seen in the interior.

_Newton._ The deepest walled plain known upon the Moon’s surface. In
form it is elliptical; its length is 143 miles, while its breadth
is only 69 miles. The walls show the terracing so common in these
objects, and one lofty peak reaches the unusual height of 24,000 feet
above the floor. The interior includes some small craters, mountain
protuberances, and other irregularities. Neison says that, owing to
“the immense height of the wall, a great part of the floor is entirely
lost in shadow, neither Earth nor Sun being ever visible from it.”

[Illustration: Fig. 26.

Petavius and Wrottesley at sunset. 1885, Dec. 23, 9^h to 10^h 30^m.

(T. Gwyn Elger.)]

_Grimaldi._ An immense walled plain, extending over 148 miles from N.
to S. and about 130 miles from E. to W. Its interior is very dark.
_Clavius_ is another grand example of this class of object, and is
rather larger than Grimaldi, but unfavourably placed near the S. pole.
_Schickard_ may also be mentioned as a large formation of similar type,
and situated near the S.E. limb of the Moon.



Online LibraryWilliam F. DenningTelescopic Work for Starlight Evenings → online text (page 11 of 32)