TOPOGRAPHY OF THE MOON.
It is scarcely necessary to seek the reasons which prompted astronomers,
soon after the invention of the telescope, to map the surface features
of the moon. They may have considered it desirable to record the
positions of the spots upon her disc, for the purpose of facilitating
observations of the passage of the earth’s shadow over them in lunar
eclipses; or they may have been actuated by a desire to register
appearances then existing, in order that if changes took place in after
years these might be readily detected. Scheiner was one of the earliest
of lunar cartographers; he worked about the middle of the seventeenth
century; but his delineations were very rough and exaggerated. Better
maps—the best of the time, according to an old authority—were engraved
by one Mellan, about the years 1634 or 1635. At about the same epoch,
Langreen and Hevelius were working upon the same subject. Langreen
executed some thirty maps of portions of the moon, and introduced the
practice of naming the spots after philosophers and eminent men.
Hevelius spent several years upon his task, the results of which he
published in a bulky volume containing some 50 maps of the moon in
various phases, and accompanied by 500 pages of letter-press. He
rejected Langreen’s system of nomenclature, and called the spots after
the seas and continents of the earth to which he conceived they bore
resemblance. Riccioli, another selenographer, whose map was compiled
from observations made by Grimaldi, restored Langreen’s nomenclature,
but he confined himself to the names of eminent astronomers, and his
system has gained the adhesion of the map-makers of later times. Cassini
prepared a large map from his own observations, and it was engraved
about the year 1692. It appears to have been regarded as a standard
work, for a reduced copy of it was repeatedly issued with the yearly
volumes of the _Connaissance des Temps_, (the “Nautical Almanac” of
France) some time after its publication. These small copies have no
great merit: the large copper plate of the original was, we are told by
Arago, who received the statement from Bouvard, sold to a brazier by a
director of the French Government Printing-Office, who thought proper to
disembarrass the stores of that establishment, by ridding them of what
he considered lumber! La Hire, Mayer, and Lambert, followed during the
succeeding century, in this branch of astronomical delineation. At the
commencement of the present century, the subject was very earnestly
taken up by the indefatigable Schroeter, who, although he does not
appear to have produced a complete map, produced a topograph of the moon
in a large series of partial maps and drawings of special features.
Schroeter was a fine observer, but his delineations show him to have
been an indifferent draughtsman. Some of his drawings are but the rudest
representations of the objects he intended to depict; many of the bolder
features of conspicuous objects are scarcely recognizable in them. A bad
artist is as likely to mislead posterity as a bad historian, and it
cannot be surprising if observers of this or future generations,
accepting Schroeter’s drawings as faithful representations, should infer
from them remarkable changes in the lunar details. It is much to be
regretted that Schroeter’s work should be thus depreciated. Lohrman of
Dresden, was the next cartographer of the moon; in 1824 he put forth a
small but very excellent map of 15 inches diameter, and published a book
of descriptive text, accompanied by sectional charts of particular
areas. His work, however, was eclipsed by the great one which we owe to
the joint energy of MM. Beer and Maedler, and which represents a
stupendous amount of observing work carried on during several years
prior to 1836, the date of their publication. The long and patient
labour bestowed upon their map and upon the measures on which it
depends, deserve the highest praise which those conversant with the
subject can bestow, and it must be very long before their efforts can be
superseded.
Beer and Maedler’s map has a diameter of 37 inches: it represents the
phase of the moon visible in the condition of mean libration. The
details were charted by a careful process of triangulation. The disc was
first divided into “triangles of the first order,” the points of which
(conspicuous craters) were accurately laid down by reference to the
edges of the disc: one hundred and seventy-six of these triangles,
plotted accurately upon an orthographic projection of the hemisphere,
formed the reliable basis for their charting work. From these a great
number of “points of the second order” were laid down, by measuring
their distance and angle of position with regard to points first
established. The skeleton map thus obtained was filled up by drawings
made at the telescope: the diameters of the measureable craters being
determined by the micrometer.
Beer and Maedler also measured the heights of one thousand and
ninety-five lunar mountains and crater-summits: the resulting measures
are given in a table contained in the comprehensive text-book which
accompanies their map. These heights are found by one of two methods,
either by measuring the length of the shadow which the object casts
under a known elevation of the sun above its horizon, or by measuring
the distance between the illuminated point of the mountain and the
“terminator” in the following manner. In the annexed figure (Fig. 15)
let the circle represent the moon and M a mountain upon it: let S A be
the line of direction of the sun’s rays, passing the normal surface of
the moon at A and just tipping the mountain top. A will be the
terminator, and there will be darkness between it and the star-like
mountain summit M. The distance between A and M is measured: the
distance A B is known, for it is the moon’s radius. And since the line S
M is a tangent to the circle the angle B A M is a right angle. We know
the length of its two sides AB, AM, and we can therefore by the known
properties of the right-angled triangle find the length of the
hypothenuse BM: and since BM is made up of the radius BA plus the
mountain height, we have only to subtract the moon’s radius from the
ascertained whole length of the hypothenuse and we have the height of
the mountain. MM. Beer and Maedler exhibited their measures in French
toises: in the heights we shall have occasion to quote, these have been
turned into English feet, upon the assumption that the toise is equal to
6·39 English feet. The nomenclature of lunar features adopted by Beer
and Maedler is that introduced by Riccioli: mountains and features
hitherto undistinguished were named by them after ancient and modern
philosophers, in continuance of Riccioli’s system, and occasionally
after terrestrial features. Some minute objects in the neighbourhood of
large and named ones were included under the name of the large one and
distinguished by Greek or Roman letters.
[Illustration: Fig. 15.]
[Illustration: PLATE IV.
PICTURE MAP OF THE MOON.]
[Illustration: PLATE V.
Skeleton Map of Moon
To Accompany Picture Map, Chap. VII]
The excellent map resulting from the arduous labours of these
astronomers is simply a map: it does not pretend to be a picture. The
asperities and depressions are symbolized by a conventional system of
shading and no attempt is made to exhibit objects as they actually
appear in the telescope. A casual observer comparing details on the map
with the same details on the moon itself would fail to identify or
recognize them except where the features are very conspicuous. Such an
observer would be struck by the shadows by which the lunar objects
reveal themselves: he would get to know them mostly by their shadows,
since it is mainly by those that their forms are revealed to a
terrestrial observer. But such a map as that under notice indicates no
shadows, and objects have to be identified upon it rather by their
positions with regard to one another or to the borders of the moon than
by any notable features they actually present to view. This
inconvenience occurred to us in our early use of Beer and Maedler’s
chart, and we were induced to prepare for ourselves a map in which every
object is shown somewhat, if imperfectly, as it actually appears at some
period of a lunation. This was done by copying Beer and Maedler’s
outlines and filling them up by appropriate shading. To do justice to
our task we enlarged our map to a diameter of six feet. Upon a circle of
this diameter the positions and dimensions of all objects were laid down
from the German original. Then from our own observations we depicted the
general aspect of each object: and we so adjusted the shading that all
objects should be shown under about the same angle of illumination—a
condition which is never fulfilled upon the moon itself, but which we
consider ourselves justified in exhibiting for the purpose of conveying
a fair impression of how the various lunar objects actually appear at
some one or other part of a lunation.
The picture-map thus produced has been photographed to the size
convenient for this work: and in order to make it available for the
identification of such objects as we may have occasion to refer to, we
have placed around it a co-ordinate scale of arbitrary divisions by
which any object can be found as by the latitude and longitude divisions
upon a common geographical map. We have also prepared a skeleton map
which includes the more conspicuous objects, and which faces the picture
map (Plates IV. and V.) The numbers on the skeleton map are those given
in the second column of the accompanying table. The table also gives the
co-ordinate positions of the various craters, the names of which are,
for convenience of reference, printed in alphabetical order.
Name. Number. Map Ordinates.
Abulfeda 107 30·0 120·7
Agrippa 151 31·2 110·0
Airy 93 34·7 123·0
Albategnius 109 35·5 119·7
Aliacensis 61 35·8 131·0
Almanon 94 29·0 122·3
Alpetragius 92 40·8 122·4
Alphonsus 110 39·6 120·9
Apianus 62 33·6 129·3
Apollonius 154 6·5 109·5
Arago 152 24·7 108·7
Archimedes 191 40·3 95·8
Aristarchus 176 62·3 99·2
Aristillus 190 37·0 93·3
Aristotle 209 30·0 84·6
Arzachael 84 39·5 124·0
Atlas 228 20·7 86·6
Autolycus 189 36·8 95·5
Azophi 76 30·7 126·8
Bacon 17 32·5 142·0
Baily 207 26·0 85·4
Barocius 34 31·8 138·5
Bessel 179 27·4 100·1
Bettinus 11 48·8 144·9
Bianchini 215 51·6 86·3
Billy 121 64·3 121·4
Blancanus 12 43·7 144·8
Bonpland 110 48·5 117·6
Borda 56 15·2 131·0
Boscovich 160 31·1 106·8
Bouvard 40 66·6 134·3
Briggs 196 68·0 97·2
Bullialdus 86 50·1 125·5
Burg 206 25·5 87·5
Calippus 199 32·4 90·3
Campanus 71 52·3 129·0
Capella 104 17·8 118·0
Capuanus 43 50·5 132·8
Casatus 7 43·7 147·0
Cassini 200 35·5 89·7
Catherina 95 24·7 124·0
Cavalerius 144 71·2 109·5
Cavendish 88 63·5 127·4
Cichus 44 47·3 132·8
Clavius 13 41·8 143·5
Cleomides 183 10·7 97·0
Colombo 98 12·8 122·7
Condamine 214 48·7 84·2
Condorcet 164 4·5 104·7
Copernicus 147 49·8 107·0
Cyrillus 96 23·5 121·3
Damoiseau 124 69·2 117·2
Davy 113 43·2 119·8
Deambrel 129 26·8 113·5
Delisle 195 55·7 95·2
Descartes 106 28·5 119·3
Diophantus 194 55·5 96·3
Doppelmayer 70 58·6 129·6
Encke 140 59·7 110·6
Endymion 227 20·6 83·8
Epigenes 223 39·0 79·5
Erastothenes 168 44·6 104·0
Eudoxus 208 29·7 88·0
Fabricius 35 20·0 136·8
Fernelius 37 35·1 134·8
Firmicus 156 5·8 107·7
Flamsteed 126 62·8 114·5
Fontana 122 65·9 123·0
Fontenelle 221 43·0 81·3
Fourier 67 62·5 130·7
Fracastorius 78 20·5 127·0
Furnerius 52 11·7 133·0
Gambart 138 47·2 112·2
Gartner 224 26·5 82·3
Gassendi 90 59·7 123·3
Gauricus 46 43·5 132·5
Gauss 201 10·3 90·3
Gay Lussac 169 50·1 103·8
Geber 83 29·6 124·8
Geminus 187 13·0 93·0
Gérard 218 63·7 88·8
Goclenius 101 11·8 118·5
Godin 135 31·3 111·7
Grimaldi 125 70·8 116·3
Gruemberger 6 41·4 145·8
Gueriké 114 46·5 119·6
Guttemberg 102 13·9 118·3
Inghirami 27 61·3 138·9
Isidorus 103 16·7 118·0
Kant 105 25·8 118·5
Kepler 146 60·0 108·0
Kies 72 49·7 128·8
Kircher 10 47·5 145·8
Klaproth 8 43·5 146·7
La Caille 74 37·5 126·8
Lagrange 68 67·0 131·3
La Hire 177 54·3 99·3
Lalande 117 43·4 115·3
Lambert 193 49·6 97·8
Landsberg 127 54·0 113·0
Langreen 100 6·3 117·7
Letronne 120 62·0 119·0
Licetus 21 34·1 139·6
Lichtenberg 197 66·5 94·9
Linnæus 188 31·7 95·7
Littrow 185 20·5 99·4
Lohrman 143 71·3 112·8
Longomontanus 23 45·7 140·6
Lubiniezky 91 51·3 123·5
Macrobius 182 13·7 100·2
Maginus 22 40·0 140·4
Mairan 217 56·7 89·5
Manilius 167 32·2 103·9
Manzinus 4 31·3 146·0
Maraldi 181 18·6 100·8
Marius 171 65·0 105·5
Maskelyne 132 19·5 111·0
Mason 204 23·7 88·8
Maupertius 213 48·7 85·8
Maurolycus 33 31·8 137·0
Menelaus 165 28·3 103·0
Mercator 65 51·4 130·2
Mersenius 89 61·7 125·7
Messala 202 14·0 90·5
Messier 131 10·8 114·0
Metius 36 18·8 105·9
Moretus 5 39·5 146·5
Moesting 128 41·6 113·2
Neander 57 18·7 131·0
Nearchus 18 26·8 142·0
Newton 1 41·0 147·7
Nonius 49 36·5 133·2
Olbers 172 73·0 107·7
Pallas 149 38·6 109·5
Parrot 108 35·8 121·6
Petavius 80 9·5 127·5
Phocylides 25 55·5 141·6
Piazzi 41 65·0 133·5
Picard 163 8·3 104·7
Piccolomini 58 21·7 131·0
Pico 211 41·9 87·3
Pitatus 63 44·1 130·2
Plana 205 24·8 88·8
Plato 210 41·8 84·8
Playfair 75 33·5 127·5
Pliny 165 24·2 103·4
Poisson 60 32·8 131·0
Polybius 82 24·5 125·6
Pontanus 59 29·0 130·2
Posidonius 186 22·2 94·3
Proclus 162 11·4 104·5
Ptolemy 111 39·5 118·2
Purbach 73 38·7 128·4
Pythagoras 220 53·0 81·2
Pytheas 178 49·7 100·4
Ramsden 42 52·9 132·5
Reamur 118 37·3 114·6
Reiner 145 67·3 108·5
Reinhold 139 51·5 111·2
Repsold 219 60·2 85·7
Rheita 51 16·1 134·2
Riccioli 142 72·7 113·8
Riccius 50 23·7 133·5
Ritter 134 26·0 111·6
Roemer 184 18·3 97·6
Ross 161 25·0 105·3
Sabine 133 25·0 112·0
Sacrobosco 77 27·5 127·7
Santbech 79 15·7 126·8
Saussure 31 39·6 137·7
Scheiner 14 45·5 143·5
Schickard 28 59·0 137·5
Schiller 24 51·3 141·0
Schroeter 137 42·3 110·7
Schubert 155 2·3 110·8
Segner 16 51·3 143·5
Seleucus 174 69·0 99·8
Sharp 216 54·2 87·7
Short 2 39·7 147·4
Silberschlag 157 32·0 108·1
Simpelius 3 35·8 147·7
Snell 55 11·3 129·6
Soemmering 136 42·8 112·2
Stadius 148 45·6 107·0
Stevinus 53 11·9 130·7
Stoefler 32 35·6 136·8
Strabo 226 23·2 81·6
Struve 203 18·3 88·7
Taruntius 153 11·7 109·0
Taylor 130 27·6 116·2
Thales 225 24·3 81·8
Thebit 85 40·8 126·8
Theophilus 97 22·3 120·0
Timæus 222 38·3 80·8
Timocharis 192 45·1 97·0
Tobias Mayer 170 54·5 103·0
Triesnecker 150 35·5 109·8
Tycho 30 43·0 142·3
Ukert 159 37·1 107·5
Vasco de Gama 173 72·8 104·9
Vendelinus 99 6·8 121·6
Vieta 69 64·3 129·7
Vitello 66 55·8 130·7
Vitruvius 180 20·1 102·0
Vlacq 19 25·0 140·1
Zuchius 15 50·7 144·2
The strong family likeness pervading the craters of the moon renders it
unnecessary that we should attempt a description of each one of them or
even of one in twenty. We have, however, thought that a few remarks upon
the salient features of a few of the most important may be acceptable in
explanation of our illustrative plates; and what we have to say of the
few may be taken as representative of the many.
COPERNICUS, 147—(49·8—107·0). Plate VIII.
This may deservedly be considered as one of the grandest and most
instructive of lunar craters. Although its vast diameter (46 miles) is
exceeded by others, yet, taken as a whole, it forms one of the most
impressive and interesting objects of its class. Its situation, near the
centre of the lunar disc, renders all its wonderful details, as well as
those of its immediately surrounding objects, so conspicuous as to
establish it as a very favourite object. Its vast rampart rises to
upwards of 12,000 feet above the level of the plateau, nearly in the
centre of which stands a magnificent group of cones, three of them
attaining the height of upwards of 2400 feet.
The rampart is divided by concentric segmental terraced ridges, which
present every appearance of being enormous landslips, resulting from the
crushing of their over-loaded summits, which have slid down in vast
segments and scattered their débris on to the plateau. Corresponding
vacancies in the rampart may be observed from whence these prodigious
masses have broken away. The same may be noticed, although in a somewhat
modified degree, around the exterior of the rampart. In order to
approach a realization of the sublimity and grandeur of this magnificent
example of a lunar volcanic crater, our reader would do well to
endeavour to fix his attention on its enormous magnitude and attempt to
establish in his mind’s eye a correct conception of the scale of its
details as well as its general dimensions, which, as they so
prodigiously transcend those of the largest terrestrial volcanic
craters, require that our ideas as to magnitude of such objects should
be, so to speak, educated upon a special standard. It is for this reason
we are anxious our reader, when examining our illustrations, should
constantly refer the objects represented in them to the scale of miles
appended to each plate, otherwise a just and true conception of the
grandeur of the objects will escape him.
Copernicus is specially interesting, as being evidently the result of a
vast discharge of molten matter which has been ejected at the focus or
centre of disruption of an extensively upheaved portion of the lunar
crust. A careful examination of the crater and the district around it,
even to the distance of more than 100 miles on every side, will supply
unmistakable evidence of the vast extent and force of the original
disruption, manifested by a wonderfully complex reticulation of bright
streaks which diverge in every direction from the crater as their common
centre. These streaks do not appear on our plate, nor are they seen upon
the moon except at and near the full phase. They show conspicuously,
however, by their united lustre on the full moon, Plate III. Every one
of those bright streaks, we conceive, is a record of what was originally
a crack or chasm in the solid crust of the moon, resulting from some
vastly powerful upheaving agency over the site of whose focus of energy
Copernicus stands. The cracking of the crust must have been followed by
the ejection of subjacent molten matter up through the reticulated
cracks; this, spreading somewhat on either side of them, has left these
bright streaks as a visible record of the force and extent of the
upheaval; while at the focus of disruption from whence the cracks
diverge, the grand outburst appears to have taken place, leaving
Copernicus as its record and result.
Many somewhat radial ridges or spurs may be observed leading away from
the exterior banks of the great rampart. These appear to be due to the
more free egress which the extruded matter would find near the focus of
disruption. The spur-ridges may be traced fining away for fully 100
miles on all sides, until they become such delicate objects as to
approach invisibility. Several vast open chasms or cracks may be
observed around the exterior of the rampart. They appear to be due to
some action subsequent to the formation of the great crater—probably the
result of contraction on the cooling of the crust, or of a deep-seated
upheaval long subsequent to that which resulted in the formation of
Copernicus itself, as they intersect objects of evidently prior
formation.
Under circumstances specially favourable for “fine vision,” for upwards
of 70 miles on all sides around Copernicus, myriads of comparatively
minute but perfectly-formed craters may be observed. The district on the
south-east side is specially rich in these wonderfully thickly-scattered
craters, which we have reason to suppose stand over or upon the
reticulated bright streaks; but, as the circumstances of illumination
which are requisite to enable us to detect the minute craters are widely
adverse to those which render the bright streaks visible, namely, nearly
full moon for the one and gibbous for the other, it is next to
impossible to establish the fact of coincidence of the sites of the two
by actual simultaneous observation.
At the east side of the rampart, multitudes of these comparatively
minute craters may also be detected, although not so closely crowded
together as those on the west side; but among those on the east may be
seen myriads of minute prominences roughening the surface; on close
scrutiny these are seen to be small mounds of extruded matter which, not
having been ejected with sufficient energy to cause the erupted material
to assume the crater form around the vent of ejection, have simply
assumed the mound form so well known to be the result of volcanic
ejection of moderate force.
Were we to select a comparatively limited portion of the lunar surface
abounding in the most unmistakable evidence of volcanic action in every
variety that can characterize its several phases, we could not choose
one yielding in all respects such instructive examples as Copernicus and
its immediate surroundings.
GASSENDI, 90—(59·7—123·3). Frontispiece.
An interesting crater about 54 miles diameter; the height of the most
elevated portion of the surrounding wall from the plateau being about
9600 feet. The centre is occupied by a group of conical mountains, three
of which are most conspicuous objects and rise to nearly 7000 feet above
the level of the plateau. As in other similar cases, these central
mountains are doubtless the result of the expiring effort of the
eruption which had formed the great circular wall of the crater. The
plateau is traversed by several deep cracks or chasms nearly one mile
wide.
Both the interior and exterior of the wall of the crater are terraced
with the usual segmental ridges or landslips. A remarkable detached
portion of the interior bank is to be seen on the east side, while on
the west exterior of the wall may be seen an equally remarkable example
of an outburst of lava subsequent to the formation of the wall or bank
of the crater; it is of conical form and cannot fail to secure the
attention of a careful observer.
Interpolated on the north wall of the crater may be seen a crater of
about 18 miles diameter which has burst its bank in towards the great
crater, upon whose plateau the lava appears to have discharged itself.
The neighbourhood of Gassendi is diversified by a vast number of mounds
and long ridges of exudated matter, and also traversed by enormous
chasms and cracks, several of which exceed one mile wide and are fully
100 miles in length, and, as is usual with such cracks, traverse plain
and mountain alike, disregarding all surface inequalities.
Numbers of small craters are scattered around; the whole forming an
interesting and instructive portion of the lunar surface.
EUDOXUS, 208 (29·7—88·0), and ARISTOTLE, 209 (30·0—84·6). Plate X.
Two gigantic craters, Eudoxus being nearly 35 miles in diameter and
upwards of 11,000 feet deep, while Aristotle is about 48 miles in
diameter, and about 10,000 feet deep (measuring from the summit of the
rampart to the plateau). These two magnificent craters present all the
true volcanic characteristics in a remarkable degree. The outsides as
well as the insides of their vast surrounding walls or banks display on
the grandest scale the landslip feature, the result of the over-piling
of the ejected material, and the consequent crushing down and crumbling
of the substructure. The true eruptive character of the action which
formed the craters is well evinced by the existence of the groups of
conical mountains which occupy the centres of their circular plateaux,
since these conical mountains, there can be little doubt, stand over
what were once the vents from whence the ejected matter of the craters
was discharged.
On the west side of these grand craters may be seen myriads of
comparatively minute ones (we use the expression “comparatively minute,”
although most of them are fully a mile in diameter). So thickly are
these small craters crowded together, that counting them is totally out
of the question; in our original notes we have termed them “Froth
craters” as the most characteristic description of their aspect.
The exterior banks of Aristotle are characterized by radial ridges or
spurs: these are most probably the result of the flowing down of great
currents of very fluid lava. To the east of the craters some very lofty
mountains of exudation may be seen, and immediately beyond them an
extensive district of smaller mountains of the same class, so thickly
crowded together as under favourable illumination to present a multitude
of brilliant points of light contrasted by intervening deep shade. On
the west bank of Aristotle a very perfect crater may be seen, 27 miles
in diameter, having all the usual characteristic features.
About 40 miles to the east of Eudoxus there is a fine example of a crack
or fissure extending fully 50 miles—30 miles through a plain, and the
remaining 20 miles cutting through a group of very lofty mountains. This
great crack is worthy of attention, as giving evidence of the
deep-seated nature of the force which occasioned it, inasmuch as it
disregards all surface impediments, traversing plain and group of
mountains alike.
There are several other features in and around these two magnificent
craters well worthy of careful observation and scrutiny, all of them
excellent types of their respective classes.
TRIESNEKER, 150 (35·5—109·8). Plate XI.
A fine example of a normal lunar volcanic crater, having all the usual
characteristic features in great perfection. Its diameter is about 20
miles, and it possesses a good example of the central cone and also of
interior terracing.
The most notable feature, however, in connection with this crater, and
on account of which we have chosen it as a subject for one of our
illustrations, is the very remarkable display of chasms or cracks which
may be seen to the west side of it. Several of these great cracks
obviously diverge from a small crater near the west external bank of the
great one, and they subdivide or branch out, as they extend from the
apparent point of divergence, while they are crossed or intersected by
others. These cracks or chasms (for their width merits the latter
appellation) are nearly one mile broad at the widest part, and after
extending to fully 100 miles, taper away till they become invisible.
Although they are not test objects of the highest order of difficulty,
yet to see them with perfect distinctness requires an instrument of some
perfection and all the conditions of good vision. When such are present,
a keen and practised eye will find many details to rivet its attention,
among which are certain portions of the edges of these cracks or chasms
which have fallen in and caused interruptions to their continuity.
THEOPHILUS, 97 (22·3—120·0). CYRILLUS, 96 (23·5—121·3). CATHARINA, 95
(24·7—124·0). Plate XII.
These three magnificent craters form a very conspicuous group near the
middle of the south-east quarter of the lunar disc.
Their respective diameters and depths are as follows:—
Theophilus, 64 miles diameter; depth of plateau from summit of crater
wall, 16,000 feet; central cone, 5200 feet high.
Cyrillus, 60 miles diameter; depth of plateau from summit of crater
wall, 15,000 feet; central cone, 5800 feet high.
Catharina, 65 miles diameter; depth of plateau from summit of crater
wall, 13,000 feet; centre of plateau occupied by a confused group of
minor craters and débris.
Each of these three grand craters is full of interesting details,
presenting in every variety the characteristic features which so
fascinate the attention of the careful observer of the moon’s wonderful
surface, and affording unmistakable evidence of the tremendous energy of
the volcanic forces which at some inconceivably remote period piled up
such gigantic formations.
Theophilus by its intrusion within the area of Cyrillus shows in a very
striking manner that it is of comparatively more recent formation than
the latter crater. There are many such examples in other parts of the
lunar disc, but few of so very distinct and marked a character.
The flanks or exterior banks of Theophilus, especially those on the west
side, are studded with apparently minute craters, all of which when
carefully scrutinized are found to be of the true volcanic type of
structure; and minute as they are, by comparison, they would to a
beholder close to them appear as very imposing objects; but so gigantic
are the more notable craters in the neighbourhood, that we are apt to
overlook what are in themselves really large objects. It is only by duly
training the mind, as we have previously urged, so as ever to keep
before us the vast scale on which the volcanic formations of the lunar
surface are displayed, that we can do them the justice which their
intrinsic grandeur demands. We trust that our illustrations may in some
measure tend to educate the mind’s eye, so as to derive to the full the
tranquil enjoyment which results from the study of the manifestation of
one of the Creator’s most potent agencies in dealing with the materials
of his worlds, namely, volcanic force. So rich in wonderful features and
characteristic details is this magnificent group and its neighbourhood,
that a volume might be filled in the attempt to do justice, by
description, to objects so full of suggestive subject for study.
THEBIT, 85—(40·8—126·8).
A crater about 32 miles in diameter and about 9700 feet deep, devoid of
a central cone. It appears on the upper part and near the middle of
Plate XIII. The plateau has five minute craters upon it. On the east
outside are two small craters, the lesser of which, about 2·75 miles
diameter, has a central cone. We specially note this fact, because it is
the smallest crater but one in which we have detected a central cone: no
doubt, however, many smaller craters possess this unmistakable stamp of
true volcanic origin, but so minute are the specks of light which the
central cones of such very small craters reflect, that they fail to be
visible to us.
East of Thebit is a very remarkable straight cliff 60 miles long by
about 1000 feet high, called by some observers the “Railway,” and
apparently the result either of an upheaval or of a down-sinking of the
surface of the circular area across whose diameter it stretches.
Under moderate magnifying power, this cliff appears straight, but with
higher power and under favourable conditions, its face is seen to be
serrated, and along the upper edge may be detected several very minute
craters. A more conspicuous small crater is seen at the north end of the
cliff. To the east of the cliff nearly opposite the centre are two
craters, from the east side of the larger of which proceeds a fine crack
parallel to the cliff and passing through a dome-shaped hill of low
eminence.
PLATO, 210 (41·8—81·8). Plate XIV.
This crater, besides being a conspicuous object on account of its great
diameter, has many interesting details in and around it requiring a fine
instrument and favourable circumstances to render them distinctly
visible. The diameter of the crater is 70 miles; the surrounding wall or
rampart varies in height from 4000 to upwards of 8000 feet, and is
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. Reference to our illustration will convey a very fair
idea of this interesting appearance. On the north-east inside of the
circular wall or rampart may be observed a fine example of landslip, or
sliding down of a considerable mass of the interior side of the crater’s
wall. The landslip nature of this remarkable detail is clearly
established by the fact of the bottom edge of the downslipped mass
projecting in towards the centre of the plateau to a considerable
extent. Other smaller landslip features may be seen, but none on so
grand and striking a scale as the one referred to. A number of
exceedingly minute craters may be detected on the surface of the
plateau. The plateau itself is remarkable for its low reflective power,
which causes it to look like a dingy spot when Plato is viewed with a
small magnifying power. The exterior of the crater wall is remarkable
for the rugged character of its formation, and forms a great contrast in
that respect to the comparatively smooth unbroken surface of the
plateau, which by the way is devoid of a central cone. The surrounding
features and objects indicated in our illustration are of the highest
interest, and a few of them demand special description.
THE VALLEY OF THE ALPS (37·0—86·0). Plate XIV.
This remarkable object lays somewhat diagonally to the west of Plato;
when seen with a low magnifying power (80 or 100), it appears as a rut
or groove tapering towards each extremity. It measures upwards of 75
miles long by about six miles wide at the broadest part. When examined
under favourable circumstances, with a magnifying power of from 200 to
300, it is seen to be a vast flat-bottomed valley bordered by gigantic
mountains, some of which attain heights upwards of 10,000 feet; towards
the south-east of this remarkable valley, and on both sides of it, are
groups of isolated mountains, several of which are fully 8000 feet high.
This flat-bottomed valley, which has retained the integrity of its form
amid such disturbing forces as its immediate surroundings indicate, is
one of the many structural enigmas with which the lunar surface abounds.
To the north-west of the valley a vast number of isolated mounds or
small mountains of exudation may be seen; so numerous are they as to
defy all attempts to count them with anything like exactness; and among
them, a power of 200 to 300 will enable an observer, under favourable
circumstances, to detect vast numbers of small but perfectly-formed
craters.
PICO, 211 (41·9—87·3). Plate XIV.
This is one of the most interesting examples of an isolated volcanic
“mountain of exudation,” and it forms a very striking object when seen
under favourable circumstances. Its height is upwards of 8000 feet, and
it is about three times as long at the base as it is broad. The summit
is cleft into three peaks, as may be ascertained by the three-peaked
shadow it casts on the plain. Five or six minute craters of very perfect
form may be detected close to the base of this magnificent mountain.
There are several other isolated peaks or mountains of the same class
within 30 or 40 miles of it which are well worthy of careful scrutiny,
but Pico is the master of the situation, and offers a glorious subject
for realizing a lunar day-dream in the mind’s eye, if we can only by an
effort of imagination conceive its aspect under the fiercely brilliant
sunshine by which it is illuminated, contrasted with the intensely black
lunar heavens studded with stars shining with a steady brightness of
which, by reason of _our_ atmosphere intervening, we can have no
adequate conception save by the aid of a well-directed imagination.
TYCHO, 30 (43·0—142·3). Plate XVI.
This magnificent crater, which occupies the centre of the crowded group
in our Plate, is 54 miles in diameter, and upwards of 16,000 feet deep,
from the highest ridge of the rampart to the surface of the plateau,
whence rises a grand central cone 5000 feet high. It is one of the most
conspicuous of all the lunar craters, not so much on account of its
dimensions as from its occupying the great focus of disruption from
whence diverge those remarkable bright streaks, many of which may be
traced over 1000 miles of the moon’s surface, disregarding in their
course all interposing obstacles. There is every reason to conclude that
Tycho is an instance of a vast disruptive action which rent the solid
crust of the moon into radiating fissures, which were subsequently
occupied by extruded molten matter, whose superior luminosity marks the
course of the cracks in all directions from the crater as their common
centre of divergence. So numerous are these bright streaks when examined
by the aid of the telescope, and they give to this region of the moon’s
surface such an extra degree of luminosity, that, when viewed as a
whole, their locality can be distinctly seen at full moon by the
unassisted eye as a bright patch of light on the southern portion of the
disc. (See Plate III.) The causative origin of the streaks is discussed
and illustrated in Chapter XI.
The interior of this fine crater presents striking examples of the
concentric terrace-like formations that we have elsewhere assigned to
vast landslip actions. Somewhat similar concentric terraces may be
observed in other lunar craters; some of these, however, appear to be
the results of some temporary modification of the ejective force, which
has caused the formation of more or less perfect inner ramparts: what we
conceive to be true landslip terraces are always distinguished from
these by their more or less fragmentary character.
On reference to Plate III., showing the full moon, a very remarkable and
special appearance will be observed in a dingy district or zone
immediately surrounding the exterior of the rampart of Tycho, and of
which we venture to hazard what appears to us a rational explanation:
namely, that as Tycho may be considered to have acted as a sort of
safety-valve to the rending and ejective force which caused, in the
first instance, the cracking of this vast portion of the moon’s
crust—the molten matter that appears to have been forced up through
these cracks, on finding a comparatively free exit by the vent of Tycho,
so relieved the district immediately around him as to have thereby
reduced, in amount, the exit of the molten matter, and so left a zone
comparatively free from the extruded lava which, according to our view
of the subject, came up simultaneously through the innumerable fissures,
and, spreading sideways along their courses, left everlasting records of
the original positions of the radiating cracks in the form of the bright
streaks which we now behold.
“WARGENTIN,” 26 (57·5—140·2). Plate XVII.
This object is quite unique of its kind—a crater about 53 miles across
that to all appearance has been filled to the brim with lava that has
been left to consolidate. There are evidences of the remains of a
rampart, especially on the south-west portion of the rim. The general
aspect of this extraordinary object has been not unaptly compared to a
“thin cheese.” The terraced and rutted exterior of the rampart has all
the usual characteristic details of the true crater. The surface of the
high plateau is marked by a few ridges branching from a point nearly in
its centre, together with some other slight elevations and depressions;
these, however, can only be detected when the sun’s rays fall nearly
parallel to the surface of the plateau.
To the north of this interesting object is the magnificent ring
formation Schickard, whose vast diameter of 123 miles contrasts
strikingly with that of the sixteen small craters within his rampart,
and equally so with a multitude of small craters scattered around. There
are many objects of interest on the portion of the lunar surface
included within our illustration, but as they are all of the usual type,
we shall not fatigue the attention of our readers by special
descriptions of them.
ARISTARCHUS, 176 (6·3—99·2), and HERODOTUS, 175 (63·2—99·6). Plate XVIII.
These two fine examples of lunar volcanic craters are conspicuously
situated in the north-east quarter of the moon’s disc. Aristarchus has a
circular rampart nearly 28 miles diameter, the summit of which is about
7500 feet above the surface of the plateau, while its height above the
general surface of the moon is 2600 feet. A central cone having several
subordinate peaks completes the true volcanic character of this crater:
its rampart banks, both outside and inside, have fine examples of the
segmental crescent-shaped ridges or landslips, which form so constant
and characteristic a feature in the structure of lunar craters. Several
very notable cracks or chasms may be seen to the north of these two
craters. They are contorted in a very unusual and remarkable manner, the
result probably of the force which formed them having to encounter very
varying resistance near the surface.
Some parts of these chasms gape to the width of two to three miles, and
when closely scrutinized are seen to be here and there partly filled by
masses which have fallen inward from their sides. Several smaller
craters are scattered around, which, together with the great chasms and
neighbouring ridges, give evidence of varied volcanic activity in this
locality. We must not omit to draw attention to the parallelism or
general similarity of “strike” in the ridges of extruded matter; this
appearance has special interest in the eyes of geologists, and is well
represented in our illustration.
Aristarchus is specially remarkable for the extraordinary capability
which the material forming its interior and rampart banks has of
reflecting light. Although there are many portions of the lunar surface
which possess the same property, yet few so remarkably as in the case of
Aristarchus, which shines with such brightness, as compared with its
immediate surroundings, as to attract the attention of the most
unpractized observer. Some have supposed this appearance to be due to
active volcanic discharge still lingering on the lunar surface, an idea
in which, for reasons to be duly adduced, we have no faith. Copernicus,
in the remarkable bright streaks which radiate from it, and Tycho also,
as well as several other spots, are apparently composed of material very
nearly as highly reflective as that of Aristarchus. But the comparative
isolation of Aristarchus, as well as the extraordinary light-reflecting
property of its material, renders it especially noticeable, so much so
as to make it quite a conspicuous object when illuminated only by
earth-light, when but a slender crescent of the lunar disc is
illuminated, or when, as during a lunar eclipse, the disc of the moon is
within the shadow of the earth, and is lighted only by the rays
refracted through the earth’s atmosphere.
There are no features about Herodotus of any such speciality as to call
for remark, except it be the breach of the north side of its rampart by
the southern extremity of a very remarkable contorted crack or chasm,
which to all appearance owes its existence to some great disruptive
action subsequent to the formation of the crater.
WALTER, 48 (37·8—131·9), and adjacent Intrusive Craters. Plate XX.
This Plate represents a southern portion of the moon’s surface measuring
170 by 230 miles. It includes upwards of 200 craters of all dimensions,
from Walter, whose rampart measures nearly 70 miles across, down to
those of such small apparent diameter as to require a well practized eye
to detect them. In the interior of the great crater Walter a remarkable
group of small craters may be observed surrounding his central cone,
which in this instance is not so perfectly in the centre of the rampart
as is usually the case. The number of small craters which we have
observed within the rampart is 20, exclusive of those on the rampart
itself. The entire group represented in the Plate suggests in a striking
manner the wild scenery which must characterize many portions of the
lunar surface; the more so if we keep in mind the vast proportions of
the objects which they comprise, upon which point we may remark that the
smallest crater represented in this Plate is considerably larger than
that of Vesuvius.
ARCHIMEDES, 191 (40·3—95·8), AUTOLYCUS, 189 (36·8—95·5), ARISTILLUS,
190 (37·0—93·3), and the APENNINES. Plate IX.
This group of three magnificent craters, together with their remarkable
surroundings, especially including the noble range of mountains termed
the Apennines, forms on the whole one of the most striking and
interesting portions of the lunar surface. If the reader is not
acquainted with what the telescope can reveal as to the grandeur of the
effect of sunrise on this very remarkable portion of the moon’s surface,
he should carefully inspect and study our illustration of it; and if he
will pay due regard to our previously repeated suggestion concerning the
attached scale of miles, he will, should he have the good fortune to
study the actual objects by the aid of a telescope, be well prepared to
realize and duly appreciate the magnificence of the scene which will be
presented to his sight.
Were we to attempt an adequate detail description of all the interesting
features comprised within our illustration, it would, of itself, fill a
goodly volume; as there is included within the space represented every
variety of feature which so interestingly characterizes the lunar
surface. All the more prominent details are types of their class; and
are so favourably situated in respect to almost direct vision, as to
render their nature, forms, and altitudes above and depths below the
average surface of the moon most distinctly and impressively cognizable.
Archimedes is the largest crater in the group; it has a diameter of
upwards of 52 miles, measuring from summit to summit of its vast
circular rampart or crater wall, the average height of which, above the
plateau, is about 4300 feet; but some parts of it rise considerably
higher, and, in consequence, cast steeple-like shadows across the
plateau when the sun’s rays are intercepted by them at a low angle. The
plateau of this grand crater is devoid of the usual central cone. Two
comparatively minute but beautifully-formed craters may be detected
close to the north-east interior side of the surrounding wall of the
great crater. Both outside and inside of the crater wall may be seen
magnificent examples of the landslip subsidence of its overloaded banks;
these landslips form vast concentric segments of the outer and inner
circumference of the great circular rampart, and doubtless belong to its
era of formation. Two very fine examples of cracks, or chasms, may be
observed proceeding from the opposite external sides of the crater, and
extending upwards of 100 miles in each direction; these cracks, or
chasms, are fully a mile wide at their commencement next the crater, and
narrow away to invisibility at their further extremity. Their course is
considerably crooked, and in some parts they are partially filled by
masses of the material of their sides, which have fallen inward and
partially choked them. The depths of these enormous chasms must be very
great, as they probably owe their existence to some mighty upheaving
action, which there is every reason to suppose originated at a profound
depth, since the general surface on each side of the crater does not
appear to be disturbed as to altitude, which would have been the case
had the upheaving action been at a moderate depth beneath. We would
venture to ascribe a depth of not less than ten miles as the most
moderate estimate of the profundity of these terrible chasms. If the
reader would realize the scale of them, let him for a moment imagine
himself a traveller on the surface of the moon coming upon one of them,
and finding his onward progress arrested by the sudden appearance of its
vast black yawning depths; for by reason of the angle of his vision
being almost parallel to the surface, no appearance of so profound a
chasm would break upon his sight until he came comparatively close to
its fearful edge. Our imaginary lunar traveller would have to make a
very long détour, ere he circumvented this terrible interruption to his
progress. If the reader will only endeavour to realize in his mind’s eye
the terrific grandeur of a chasm a mile wide and of such dark profundity
as to be, to all appearance, fathomless—portions of its rugged sides
fallen in wild confusion into the jaws of the tortuous abyss, and
catching here and there a ray of the sun sufficient only to render the
darkness of the chasm more impressive as to its profundity—he will, by
so doing, learn to appreciate the romantic grandeur of this, one of the
many features which the study of the lunar surface presents to the
careful observer, and which exceed in sublimity the wildest efforts of
poetic and romantic imagination. The contemplation of these views of the
lunar world are, moreover, vastly enhanced by special circumstances
which add greatly to the impressiveness of lunar scenery, such as the
unchanging pitchy-black aspect of the heavens and the death-like silence
which reigns unbroken there.
These digressions are, in some respects, a forestallment of what we have
to say by-and-by, and so far they are out of place; but with the
illustration to which the above remarks refer placed before the reader,
they may, in some respects, enhance the interest of its examination.
The upper portion of our illustration is occupied by the magnificent
range of volcanic mountains named after our Apennines, extending to a
length of upwards of 450 miles. This mountain group rises gradually from
a comparatively level surface towards the south-west, in the form of
innumerable comparatively small mountains of exudation, which increase
in number and altitude towards the north-east, where they culminate and
suddenly terminate in a sublime range of peaks, whose altitude and
rugged aspect must form one of the most terribly grand and romantic
scenes which imagination can conceive. The north-east face of the range
terminates abruptly in an almost vertical precipitous face, and over the
plain beneath intense black steeple or spire-like shadows are cast, some
of which at sunrise extend fully 90 miles, till they lose themselves in
the general shading due to the curvature of the lunar surface. Nothing
can exceed the sublimity of such a range of mountains, many of which
rise to heights of 18,000 to 20,000 feet at one bound from the plane at
their north-east base. The most favourable time to examine the details
of this magnificent range is from about a day before first quarter to a
day after, as it is then that the general structure of the range as well
as the character of the contour of each member of the group can, from
the circumstances of illumination then obtaining, be most distinctly
inferred.
Several comparatively small perfectly-formed craters are seen
interspersed among the mountains, giving evidence of the truly volcanic
character of the surrounding region, which, as before said, comprises in
a comparatively limited space the most perfect and striking examples of
nearly every class of lunar volcanic phenomena.
We have endeavoured on Plate XXIII. to give some idea of a landscape
view of a small portion of this mountain range.
[Illustration: PLATE VI.
TERRESTRIAL AND LUNAR VOLCANIC AREAS COMPARED.
PORTION OF THE MOON’S SURFACE.]
[Illustration: VESUVIUS AND NEIGHBOURHOOD OF NAPLES.]