THE MOON

BY

OLIVER C. FARRINGTON
Curator of Geology

Geology
Leaflet 6

FIELD MUSEUM OF NATURAL HISTORY
CHICAGO

1925

LIST OF GEOLOGICAL LEAFLETS ISSUED TO DATE

Model of an Arizona Gold Mine $ .10

Models of Blast Furnaces for Smelting Iron . .10
Amber — Its Physical Properties and

Geological Occurrence 10

Meteorites 10

Soils 10

The Moon 10

D. C. DA VIES, Director

FIELD MUSEUM OF NATURAL HISTORY
CHICAGO. U. S. A.

No.

1.

No.

2.

No.

3.

No.

4.

No.

5.

No.

6.

PHOTOGRAPH OF MODEL IN RELIEF OF THE VISIBLE HEMISPHERE OF THE MOON. HALL 35.
The model is 19 feet in diameter.

I

Field Museum of Natural History

DEPARTMENT OF GEOLOGY
Chicago, 1925

Leaflet Number 6

THE MOON

Except for occasional comets and meteors, the
Moon is the celestial body nearest the Earth. Its mean
distance from the Earth is 237,640 miles, but as it
moves in an elliptical orbit, it has at one point a
remoteness of 253,263 miles and opposite to this
one of 221,436 miles. The diameter of the Moon is
about one-fourth that of the Earth, or 2,160 miles,
and its volume is 1/49 that of the Earth. The mass of
the Moon (volume multiplied by density) is 1/81 and
the density 3/5 that of the Earth. The period of the
Moon's revolution about the Earth is 27 days, 7 hours,
43 minutes and III/2 seconds. As its period of rota-
tion on its axis is the same, only one side of the Moon
is ever seen from the Earth. Since, however, the
Moon's axis is inclined about 83° to the plane of its
orbit, we sometimes see a little distance beyond each
of its poles, and, since the rate of motion of the Moon
in its orbit varies slightly, we sometimes see a little
beyond the eastern and western edges of the hemi-
sphere. The total result of these librations, as they
are called, is to make four-sevenths of the Moon's sur-
face visible to us. Of the remaining three-sevenths,
nothing is known. So far as is known, the Moon is
not flattened at the poles.

Owing to its slow rotation on its axis, the Moon's
day has a length of 291/2 of our days. Each portion
of its surface is therefore exposed to or shielded from
the light of the Sun for a fortnight continuously.

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2 Field Museum of Natural History

The Moon has no atmosphere. Hence, it can have
no diffused Hght, and nothing can be seen on it except
where the Sun's rays shine directly. "If a man stepped
into the shadow of a lunar crag," says Todd, "he would
instantly become invisible. For a similar reason, no
sound, however loud, can be heard on the Moon. The
rolhng of a rock down the wall of a lunar crater, will
be known only by the tremor it produces." Moreover,
changes of temperature on the Moon are rapid and
violent. Where the Sun's rays strike, a temperature
about that of boiling water is believed to be reached,
while in unilluminated portions it is thought to go as
low as 100° below zero.

The force of gravity upon the surface of the Moon
is only 1/6 of that on the Earth. Therefore, a man
weighing 150 pounds on the Earth, would weigh only
25 pounds on the Moon, and the same muscular energy
by which he could jump 6 feet on the Earth would
carry him a distance of 36 feet on the Moon. On the
Earth a body falls 16 feet in one second; on the Moon
only 2.6 feet in the same time.

The surface of the Moon is made up of mountains,
valleys and plains, resembling in general appearance
those of the Earth. As a whole, however, the surface
of the Moon is much more uneven than that of the
Earth. Some of the mountains of the Moon have a
height of over 20,000 feet. As there is no sea-level to
measure from, this figure expresses height above the
surrounding surface, it being determined by the length
of the shadows cast by the mountains.

In order to represent in a vivid and accurate
manner the character and appearance of the Moon's
surface, the construction of a large model of the Moon
was undertaken a number of years ago by Th. Dickert,
Curator of the Natural History Museum of Bonn, Ger-
many and Dr. J. F. Julius Schmidt, Director of the
Observatory of Athens, Greece and an eminent seleno-

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The Moon 3

grapher. The model was presented to the Museum
by the late Lewis Reese of Chicago, and is installed at
the west end of Hall 35 of the Museum. The model is
19.2 feet in diameter, and is by far the largest and
most elaborate representation of the Moon's surface
ever made. Its horizontal scale is 1 : 600,000, one inch
on the model equaling 9^^miles on the surface of
the Moon, and its vertical scale is 1:200,000, one inch
equaling 3-L^-miles on the Moon.

Some characteristic features of the Moon's sur-
face which are especially well illustrated on the model
are the following:

1. GRAY PLAINS or "SEAS." These are the
darker portions of the Moon's surface as it is seen
with the naked eye. They were thought by earlier
observers to be seas and were so named. We now
know, however, that there is no water on the Moon's
surface and that the so-called "seas" are really low-
land plains, some of them of vast extent. The Oceanus
Procellarum, for instance, covers an area of 90,000
square miles. As seen from the Earth, the plains
show a gray-green color, often of varying intensity
and sometimes a little bluish in portions. The bright-
est green color is shown by the area known as Mare
Serenitatis. Though appearing perfectly level, a close
study shows that these plains have undulating sur-
faces. They occupy about one-third of the visible sur-
face of the Moon.

2. MOUNTAINS and HIGHLANDS. These con-
stitute the bright portions of the Moon's surface as
it is seen with the naked eye.

Although these elevated areas are conveniently
called mountains, Shaler has drawn attention to the
fact that they are unlike those on the Earth since

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4 Field Museum of Natural History

they lack features due to erosion and there is absence
of order in their association. The average declivity of
their slopes is also much greater than that of the
mountains on the Earth. It has been estimated that
the average angle of the lunar surface to its horizon
is 52°, while on the Earth it does not amount to more
than one-tenth of that figure. This difference is prob-
ably due to the lack of water on the Moon, the work
of which on the Earth tends continually to reduce
slopes to a level. Using the term mountains for con-
venience, however, those on the Moon may be divided
into the following classes:

a. Mountain Chains. These may have a length
of 80 to 100 miles and heights of from 5,000 to 17,000
feet. As in the case with the mountains of the Earth,
they are usually steeper on one side than on the other.
The range called the Appenines, seen near the north
pole of the Moon, is a good illustration of such moun-
tain chains. Other ranges are the so-called Alps and
Caucasus. These names were applied by Hevelius, an
astronomer of Danzig, who made the first map of the
Moon in 1647. He gave to the features of the Moon's
surface names of localities similar to those on the
Earth which they most resembled. His system was
largely abandoned by later astronomers, however, the
later method being to name the different features of
the Moon after celebrated astronomers and philosoph-
ers.

b. Highlands Surrounded By Mountains. These are
partly with and partly without well-determined direc-
tions. •

c. Isolated Mountains. These usually occur on
the gray plains. They vary from 4,000 to 7,000 feet
in height.

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The Moon 5

d. Vein Mountains. These occur only on the gray
plains. They are long, narrow, contorted ridges, usu-
ally from 700 to 1,000 feet in height.

e. Circular Mountains. These are the most char-
acteristic and peculiar features of the Moon's sur-
face. They vary in size from the so-called "Walled
Plains," 150 to 15 miles in diameter, to crater mount-
ains whose diameters range from 15 miles down to a
few hundred feet. Thirty-three thousand of these
crater mountains have been counted by one astron-
omer, the number increasing as the size diminishes.

The form of these craters is that of pits, which
generally have ring-like walls about them. These
wails slope very steeply to a central cavity and more
gently toward the surrounding country. In all these
pits, as pointed out by Shaler, except those of the
smallest size, and possibly in these, also, there is, with-
in the ring wall and at a considerable though variable
depth below its summit, a nearly flat floor, which often
has a central pit of small size or, in its place, a
steep cone. When this floor is more than 20 miles
in diameter, and in increasing numbers as it is wider,
there are generally other pits and cones irregularly
scattered upon it. Thus, within the ring called Plato,
which is about 60 miles in diameter, there are some
scores of these lesser pits. On the interior of the
ring walls of the pits over 10 miles in diameter, there
are usually more or less distinct terraces, which sug-
gest that the material now forming the solid floors
they inclose was once fluid and stood at greater heights
in the pit than that at which it became permanently
frozen. It is, indeed, tolerably certain that the last
movement of this material of the floors was one of
interrupted subsidence from an originally greater
elevation on the outside of the ring wall. The ring
wall is commonly of irregular height, with many peaks.

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6 Field Museum of Natural History

In some places there may be seen tongues or protru-
sions of the substance which forms the ring, as if it
had flowed a short distance and then had cooled with
steep slopes. It may also be noted: (a) that the pits
or craters in many instances intersect each other,
showing that they were not all formed at the same
time, but in succession; (b) that the larger of them
are not found on the plains (seas) but on the upland
and apparently the older parts of the surface; and
(c) that the evidence from the intersections clearly
indicates that the larger of these structures are pre-
vailingly the older and that in general the smallest
were the latest formed. In other words, says Shaler,
whatever was the nature of the action involved in
the production of the craters, its energy diminished
with time, until in the end it could no longer break the
crust. These features indicate that the surface of
the Moon has been subject to forces similar to those
which produce volcanoes on the Earth, and it is there-
fore customary to refer to the crater-like mountains
of the Moon as volcanoes. As the parallel cannot be
drawn too closely, however, Shaler has urged that the
term vulcanoids, meaning volcano-like, be applied to
these mountains.

3. RILLS or CLEFTS. These are small, deep,
ditch-like furrows to be found over various parts of
the Moon's surface. Their course seems quite in-
dependent of the surface topography, for they traverse
mountains and plains with equal facility. They are
without doubt the latest formation on the Moon and
some of them may have had their origin in modern
times.

4. BRIGHT STREAKS. These radiate prom-
inently from many of the great craters of the Moon.
They are streaks of narrow width but sometimes
nearly a hundred miles in length. They are perhaps

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The Moon 7

the most puzzling of all the Moon's features. They
have been supposed by some observers to represent
lava flows whose surface reflected light more brilliantly
than other portions of the Moon. It is more generally
believed, however, that the streaks do not represent
any independent elevations, since they run over the
highest mountains as well as through the deepest
craters without variation.

GENERAL OBSERVATIONS.

The condition of the Moon's surface as a whole
indicates that it has been a theater of extraordinary
volcanic activity. In size and number its vulcanoids
far exceed the volcanoes of the Earth. The largest
terrestrial crater known is that of Kilauea in the
Hawaiian Islands which is 2I/2 miles in diameter.
Several craters of the Moon, however, exceed 50 miles
in diameter and one measures II414 miles. While the
absolute heights of the mountains of the Moon do not
greatly exceed those of the Earth, proportionally they
are much higher, since the Moon's diameter is only
one-fourth that of the Earth. The vulcanoids of the
Moon differ in other respects from the volcanoes of
our globe. "On the Earth they are usually openings
on the summits or sides of mountains — on the Moon,
depressions below the adjacent surface even when it
is a plain or valley; on the Earth the mass of the
cone usually far exceeds the capacity of the crater —
on the Moon they are much nearer equality; on the
Earth they are commonly the sources of long lava
streams — on the Moon, traces of such outpourings are
rare." (Webb.)

DESCRIPTION OF INDIVIDUAL FEATURES.

(Abridged from Nasmyth and Carpenter.)
The numbers refer to those on the accompanying
chart.

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8 Field Museum of Natural History

Copernicus. 147. This may deservedly be consid-
ered one of the grandest and most instructive of
lunar craters. Though its diameter (46 miles) is ex-
ceeded by that of other craters, its situation near
the center of the lunar disc renders it so conspicuous
as to make it a favorite object for observation. Its
vast rampart rises to upwards of 12,000 feet above
the level of the plateau, nearly in the center of which
stands a magnificent group of cones attaining the
height of upwards of 2400 feet. The rampart is di-
vided by concentric segmented terraced ridges, which
present every appearance of being enormous landslips,
resulting from the crushing of their overloaded sum-
mits which have slid down in vast segments and
scattered their debris on the plateau. Corresponding
vacancies in the rampart may be observed from whence
these prodigious masses have broken away. The same
may be noticed, to a somewhat modified degree, around
the exterior of the rampart. For upwards of 70 miles
around Copernicus myriads of comparatively minute
but perfectly formed craters can be seen. The district
on the southeast side is especially rich in them. Many
somewhat radial ridges or spurs may be observed lead-
ing away from the exterior banks of the great rampart.
They appear to be due to the freer egress which the
extruded matter found near the focus of disruption.

Triesnecker. 150. A fine example of a normal
lunar volcanic crater having all the usual charac-
teristic 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, is the remarkable display
of cracks or chasms which may be seen to the west
side of it. Several of these cracks obviously diverge
from near the west external bank of the great crater
and they sub-divide or branch out as they extend

[66]

The Moon 9

from the apparent point of divergence, while they are
crossed or intersected by others. These cracks or
chasms are nearly one mile wide at their widest part
and after extending for fully 100 miles taper away
till they become invisible.

Theophilus. 97. Cyrillus. 96. Catharina. 95.
These three magnificent craters form a conspicuous
group. Their diameters and depths are as follows :
Theophilus, diameter, 64 miles; depth of interior
plateau from summit of crater wall, 16,000 feet;
central cone, 5200 feet high; Cyrillus, diameter, 60
miles; depth of interior plateau from summit of
crater wall, 15,000 feet; central cone, 5800 feet high;
Catharina, diameter, 65 miles; depth of interior pla-
teau from summit of crater wall, 13,000 feet; center
of plateau occupied by a confused group of minor crat-
ers and debris. Each of these craters is full of inter-
esting details presenting in every variety the char-
acteristic features of the lunar volcanoes and giving
unmistakable evidence of the tremendous energy
which at some remote period piled up such gigantic
formations. The intrusion of Theophilus within Cyril-
lus shows that it is of more recent formation than
the latter. The flanks of Theophilus, especially on the
west side, are studded with apparently minute craters.
These would be considered of great size but for the
enormous crater so near.

Ptolemy. 111. Alphons. 110. Arzachael. 84. The
portion of the moon's surface which includes these
features, being near the center of the lunar disc, is
exceptionally well placed for observation. Within this
area may be seen every variety of volcanic craters
and a number of other interesting forms. Ptolemy
belongs to the class of walled plains, its ramparts en-
closing a plain 86 miles in diameter. Alphons and

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10 Field Museum of Natural History

Arzachael are respectively 60 and 55 miles in diameter.
They have all the distinctive features of lunar craters,
viz : — central cones, lofty, ragged ramparts, manifesta-
tions of landslip formations in the great segmental
terraces within their ramparts and minor craters in-
terpolated within their plateaus. A notable object
near Alphons is an enormous straight cliff traversing
the diameter of a low, ridged, circular formation. This
great cliff is 60 miles long and from 1000 to 2000
feet high. It is a well known object to lunar observers
and has been termed "The Railway" on account of
its straightness. The existence of this remarkable
cliff appears to be due either to an upheaval or a down-
sinking of a portion of the surface of the circular area
across whose diameter it extends.

Tycho. 80. This magnificent crater 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. It is one of the most conspicuous of 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. The interior of the crater presents striking
examples of the concentric, terrace-like formations
that are regarded as formed by landslips.

Wargentin. 26. ScHicKARD. 28. Wargentin is an
object quite unique of its kind — a crater about 52
miles across, that to all appearance has been filled
to the brim with lava that has been left to con-
solidate. There are evidences of the remains of a
rampart, especially on the southwest portion of the
rim. The general aspect of Wargentin has been com-
pared to that of a "thin cheese." The terraced and
rutted exterior of the rampart has all the details of a

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The Moon

11

true crater. The surface of the high plateau is marked
by a few ridges branching from a point nearly in the
center.

Schickard is one of the finest examples of a walled
plain. It is 153 miles in diameter. Within its ram-
part are 16 smaller craters and without, numberless
others.

The following are the names of topographic fea-
tures of the Moon which can be located by the cor-
responding numbers on the accompanying chart.

1. Newton.

2. Short.

3. Simpelius.

4. Manzinus.

5. Moretus.

6. Gruemberger.

7. Casatus.

8. Klaproth.

9. Wilson.

10. Kircher.

11. Bettinus.

12. Blancanus.

13. Clavius.

14. Scheiner.

15. Zuchius.

16. Segner.

17. Bacon.

18. Nearchus.

19. Vlacq.

20. Hommel.

21. Licetus.

22. Maginus.

23. Longomon-

tanus.

24. Schiller.

25. Phocylides.

26. Wargentin.

27. Inghirami.

28. Schickard.

29. Wilhelm I.

30. Tycho.

31. Saussure.

32. Stoefler.

33. Maurolycus.

34. Barocius.

35. Fabricius.

36. Metius.

37. Fernelius.

38. Heinsius.

39. Hainzel.

40. Bouvard.

41. Piazzi.

42. Ramsden.

43. Capuanus.

44. Cichus.

45. Wurzelbauer.

46. Gauricus.

47. Hell.

48. Walter.

49. Nonius.

50. Riccius.

51. Rheita.

52. Furnerius.

53. Stevinus.

54. Hase.

55. Snell.

56. Borda.

57. Neander.

58. Piccolomini.

59. Pontanus.

60. Poisson.

61. Aliacensis.

62. Werner.

63. Pitatus.

64. Hesiodus.

65. Mercator.

66. Vitello.

67. Fourier.

68. Lagrange.

69. Vieta.

70. Doppelmayer.

71. Campanus.

72. Kies.

73. Purbach.

74. La Caille.

75. Playfair.

76. Azophi.

77. Sacrobosco.

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78. Fracastorius.

79. Santbech.

80. Petavius.

81. Wilhelm Hum-

boldt.

82. Polybius.

83. Geber.

84. Arzachael.

85. Thebit.

86. Bullialdus.

87. Hippalus.

88. Cavendish.

89. Mersenius.

90. Gassendi.

91. Lubiniezky.

92. Alpetragius.

93. Airy,

94. Almanon.

95. Catharina.

96. Cyrillus.

97. Theophilus.

98. Colombo.

99. Vendelinus.

100. Langreen,

101. Goclenius,

102. Guttemberg.

103. Isidorus.

104. Capella.

105. Kant.

106. Descartes.

107. Abulfeda.

108. Parrot.

109. Albategnius,

110. Alphons.

111. Ptolemy,

112. Herschel.

113. Davy.

114. Guerike.
116, Bonpland.

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Field Museum of Natural History

117. Lalande.

118. Reaumur.

120. Letronne.

121. Billy.

122. Fontana.

123. Hansteen.

124. Danioiseau.

125. Grimaldi.

126. Flamsteed.

127. Landsberg.

128. Moesting.

129. Deambrel.

130. Taylor.

131. Messier.

132. Maskelyne.

133. Sabine.

134. Ritter.

135. Godin.

136. Soemmering.

137. Schroeter.

138. Gambart.

139. Reinhold.

140. Encke.

141. Hevelius.

142. Riccioli.

143. Lohrman.

144. Cavalerius.

145. Reiner.

146. Kepler.

147. Copernicus.

148. Stadius.

149. Pallas.

150. Triesnecker.

151. Agrippa.

152. Arago.

153. Taruntius.

154. Apollonius.

155. Schubert.

156. Firmicus.

157. Silberschlag.

158. Hyginus.

159. Ukert.

160. Boscovich.

161. Ross.

162. Proclus.

163. Picard.

164. Condorcet.

165. Pliny or

Menelaus.

167. Manilius.

168. Erastothenes.

169. Gay Lussac.

170. Tobias Mayer.

171. Marius.

172. Olbers.

173. Vasco de Gama.

174. Seleucus.

175. Herodotus.

176. Aristarchus.

177. La Hire.

178. Pytheas.

179. Bessel.

180. Vitruvius.

181. Maraldi.

182. Macrobius.

183. Cleomides.

184. Roemer.

185. Littrow.

186. Posidonius.

187. Geminus.

188. Linnaeus.

189. Autolycus.

190. Aristillus.

191. Archimedes.

192. Timocharis.

193. Lambert.

194. Diophantus.

195. Delisle.

196. Briggs.

197. Lichtenberg.

199. Calippus.

200. Cassini.

201. Gauss.

202. Messala.

203. Struve.

204. Mason.

205. Plana.

206. Burg.

207. Baily.

208. Eudoxus.

209. Aristotle.

210. Plato.

211. Pico.

212. Helicon.

213. Maupertuis.

214. Condamine.

215. Bianchini.

216. Sharp.

217. Mairan.

218. Gerard.

219. Repsold.

220. Pythagoras.

221. Fontenelle.

222. Timaeus.

223. Epigenes.

224. Gartner.

225. Thales.

226. Strabo.

227. Endymion.

228. Atlas.

229. Hercules.

Oliver C. Farrington.

[TO]

The Moon 13

SOURCES OF ADDITIONAL INFORMATION ABOUT
THE MOON

A number of textbooks and popular works on astronomy
deal more or less fully with the Moon. Among them the
following may be mentioned.

MouLTON, Forest Ray — Introduction to Astronomy. Macmil-
lan & Co., New York. 1916. 577 pp.

Young, Charles A. — A Textbook of General Astronomy. Ginn
& Co., Boston. 1898. 630 pp.

Todd, David P. — Stars and Telescopes. Little, Brown & Co.,
Boston. 1899. 419 pp.

The following are some works which treat exclusively of
the Moon.

Nasmyth, James and Carpenter, James — The Moon. John
Murray, London. 1885. 213 pp. 25 "Woodburytype"
plates and several text figures.

Pickering, William H. — The Moon. Doubleday, Page & Co.,
New York. 1903. Quarto. 103 pp. and many full-sized
plates.

Proctor, Richard A. — The Moon. Longmans, Green & Co.,
London. 1898. 314 pp.

Gilbert, Grove K. — The Moon's Face. Bulletin of the Philos-
ophical Society of Washington, 1892-93. Vol. 12, pp.
241-292.

Shaler, Nathaniel S. — A Comparison of the Features of the
Earth and the Moon. Smithsonian Contributions to Knowl-
edge. 1907. Vol. 34, pp. 1-79. 25 plates.

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N

The figures

The figures refer to the

CHART OF THE MOON'S SURFACE. AFTER NaSMYTH.

vith the model will enable the readei

2 the different features

PRINTED BY FIELD MUSEUM PRESS