The Moon
The total area of the moon's surface is about equal to that of North and
South America. The actual surface visible at any one time is about equal
to North America.
The famous lunar observer, Schröter, thought that the moon had an
atmosphere, but estimated its height at only a little over a mile. Its
density he supposed to be less than that of the vacuum in an air-pump.
Recent investigations, however, seem to show that owing to its small mass
and attractive force the moon could not retain an atmosphere like that of
the earth.
Prof. N. S. Shaler, of Harvard (U.S.A.), finds from a study of the moon
(from a geological point of view) with the 15-inch refractor of the
Harvard Observatory, that our satellite has no atmosphere nor any form of
organic life, and he believes that its surface "was brought to its present
condition before the earth had even a solid crust."[80]
There is a curious illusion with reference to the moon's apparent
diameter referred to by Proctor.[81] If, when the moon is absent in the
winter months, we ask a person whether the moon's diameter is greater or
less than the distance between the stars [Greek: d] and [Greek: e], and
[Greek: e] and [Greek: z] Orionis, the three well-known stars in the "belt
of Orion," the answer will probably be that the moon's apparent diameter
is about equal to each of these distances. But in reality the apparent
distance between [Greek: d] and [Greek: e] Orionis (or between [Greek: e]
and [Greek: z], which is about the same) is more than double the moon's
apparent diameter. This seems at first sight a startling statement; but
its truth is, of course, beyond all doubt and is not open to argument.
Proctor points out that if a person estimates the moon as a foot in
diameter, as its apparent diameter is about half a degree, this would
imply that the observer estimates the circumference of the star sphere as
about 720 feet (360° × 2), and hence the radius (or the moon's distance
from the earth) about 115 feet. But in reality all such estimates have no
scientific (that is, accurate) meaning. Some of the ancients, such as
Aristotle, Cicero, and Heraclitus, seem to have estimated the moon's
apparent diameter at about a foot.[82] This shows that even great minds
may make serious mistakes.
It has been stated by some writer that the moon as seen with the highest
powers of the great Yerkes telescope (40 inches aperture) appears "just
as it would be seen with the naked eye if it were suspended 60 miles over
our heads." But this statement is quite erroneous. The moon as seen with
the naked eye or with a telescope shows us nearly a whole hemisphere of
its surface. But if the eye were placed only 60 miles from the moon's
surface, we should see only a small portion of its surface. In fact, it is
a curious paradox that the nearer the eye is to a sphere the less we see
of its surface! The truth of this will be evident from the fact that on a
level plain an eye placed at a height, say 5 feet, sees a very small
portion indeed of the earth's surface, and the higher we ascend the more
of the surface we see. I find that at a distance of 60 miles from the
moon's surface we should only see a small portion of its visible
hemisphere (about 1/90th). The lunar features would also appear under a
different aspect. The view would be more of a landscape than that seen in
any telescope. This view of the matter is not new. It has been previously
pointed out, especially by M. Flammarion and Mr. Whitmell, but its truth
is not, I think, generally recognized. Prof. Newcomb doubts whether with
any telescope the moon has ever been seen so well as it would be if
brought within 500 miles of the earth.
A relief map of the moon 19 feet in diameter was added, in 1898, to the
Field Columbian Museum (U.S.A.). It was prepared with great care from the
lunar charts of Beer and Mädler, and Dr. Schmidt of the Athens
Observatory, and it shows the lunar features very accurately. Its
construction took five years.
On a photograph of a part of the moon's surface near the crater
Eratosthenes, Prof. William H. Pickering finds markings which very much
resemble the so-called "canals" of Mars. The photograph was taken in
Jamaica, and a copy of it is given in Prof. Pickering's book on the Moon,
and in _Popular Astronomy_, February, 1904.
Experiments made in America by Messrs. Stebbins and F. C. Brown, by means
of selenium cells, show that the light of the full moon is about nine
times that of the half moon;[83] and that "the moon is brighter between
the first quarter and full than in the corresponding phase after full
moon." They also find that the light of the full moon is equal to "0·23
candle power,"[83] that is, according to the method of measurement used in
America, its light is equal to 0·23 of a standard candle placed at a
distance of one metre (39·37 inches) from the eye.[84]
Mr. H. H. Kimball finds that no less than 52 per cent. of the observed
changes in intensity of the "earth-shine" visible on the moon when at or
near the crescent phase is due to the eccentricity of the lunar orbit,
and "this is probably much greater than could be expected from any
increase or diminution in the average cloudiness over the hemisphere of
the earth reflecting light to the moon."[85]
The "moon maiden" is a term applied to a fancied resemblance of a portion
of the Sinus Iridum to a female head. It forms the "promontory" known as
Cape Heraclides, and may be looked for when the moon's "age" is about 11
days. Mr. C. J. Caswell, who observed it on September 29, 1895, describes
it as resembling "a beautiful silver statuette of a graceful female figure
with flowing hair."
M. Landerer finds that the angle of polarization of the moon's
surface--about 33°--agrees well with the polarizing angle for many
specimens of igneous rocks (30° 51' to 33° 46'). The polarizing angle for
ice is more than 37°, and this fact is opposed to the theories of lunar
glaciation advanced by some observers.[86]
Kepler states in his _Somnium_ that he saw the moon in the crescent phase
on the morning and evening of the _same_ day (that is, before and after
conjunction with the sun). Kepler could see 14 stars in the Pleiades with
the naked eye, so his eyesight must have been exceptionally keen.
Investigations on ancient eclipses of the moon show that the eclipse
mentioned by Josephus as having occurred before the death of Herod is
probably that which took place on September 15, B.C. 5. This occurred
about 9.45 p.m.; and probably about six months before the death of Herod
(St. Matthew ii. 15).
The total lunar eclipse which occurred on October 4, 1884, was remarkable
for the almost total disappearance of the moon during totality. One
observer says that "in the open air, if one had not known exactly where to
look for it, one might have searched for some time without discovering it.
I speak of course of the naked eye appearance."[87] On the other hand the
same observer, speaking of the total eclipse of the moon on August 23,
1877, which was a bright one, says--
"The moon even in the middle of the total phase was a conspicuous
object in the sky, and the ruddy colour was well marked. In the very
middle of the eclipse the degree of illumination was as nearly as
possible equal all round the edge of the moon, the central parts being
darker than those near the edge."
In Roger de Hovedin's _Chronicle_ (A.D. 756) an account is given of the
occultation of "a bright star," by the moon during a total eclipse. This
is confirmed by Simeon of Durham, who also dates the eclipse A.D. 756.
This is, however, a mistake, the eclipse having occurred on the evening of
November 23, A.D. 755. Calvisius supposed that the occulted "star" might
have been Aldebaran. Pingré, however, showed that this was impossible, and
Struyck, in 1740, showed that the planet Jupiter was the "star" referred
to by the early observer. Further calculations by Hind (1885) show
conclusively that Struyck was quite correct, and that the phenomenon
described in the old chronicles was the occultation of Jupiter by a
totally eclipsed moon--a rather unique phenomenon.[88]
An occultation of Mars by the moon is recorded by the Chinese, on February
14, B.C. 69, and one of Venus, on March 30, A.D. 361. These have also been
verified by Hind, and his calculations show the accuracy of these old
Chinese records.
It has been suggested that the moon may possibly have a satellite
revolving round it, as the moon itself revolves round the earth. This
would, of course, form an object of great interest. During the total lunar
eclipses of March 10 and September 3, 1895, a careful photographic search
was made by Prof. Barnard for a possible lunar satellite. The eclipse of
March 10 was not very suitable for the purpose owing to a hazy sky, but
that of September 3 was "entirely satisfactory," as the sky was very
clear, and the duration of totality was very long. On the latter occasion
"six splendid" photographs were obtained of the total phase with a 6-inch
Willard lens. The result was that none of these photographs "show
anything which might be taken for a lunar satellite," at least any
satellite as bright as the 10th or 12th magnitude. It is, of course, just
possible that the supposed satellite might have been behind the moon
during the totality.
With reference to the attraction between the earth and moon, Sir Oliver
Lodge says--
"The force with which the moon is held in its orbit would be great
enough to tear asunder a steel rod 400 miles thick, with a tenacity of
30 tons to the square inch, so that if the moon and earth were
connected by steel instead of gravity, a forest of pillars would be
necessary to whirl the system once a month round their common centre
of gravity. Such a force necessarily implies enormous tensure or
pressure in the medium. Maxwell calculates that the gravitational
stress near the earth, which we must suppose to exist in the invisible
medium, is 3000 times greater than what the strongest steel can stand,
and near the sun it should be 2500 times as great as that."[89]
With reference to the names given to "craters" on the moon, Prof. W. H.
Pickering says,[90] "The system of nomenclature is, I think, unfortunate.
The names of the chief craters are generally those of men who have done
little or nothing for selenography, or even for astronomy, while the men
who should be really commemorated are represented in general by small and
unimportant craters," and again--
"A serious objection to the whole system of nomenclature lies in the
fact that it has apparently been used by some selenographers, from the
earliest times up to the present, as a means of satisfying their spite
against some of their contemporaries. Under the guise of pretending to
honour them by placing their names in perpetuity upon the moon, they
have used their names merely to designate the smallest objects that
their telescopes were capable of showing. An interesting illustration
of this point is found in the craters of Galileo and Riccioli, which
lie close together on the moon. It will be remembered that Galileo was
the discoverer of the craters on the moon. Both names were given by
Riccioli, and the relative size and importance of the craters
[Riccioli large, and Galileo very small] probably indicates to us the
relative importance that he assigned to the two men themselves. Other
examples might be quoted of craters named in the same spirit after men
still living.... With the exception of Maedler, one might almost say,
the more prominent the selenographer the more insignificant the
crater."
The mathematical treatment of the lunar theory is a problem of great
difficulty. The famous mathematician, Euler, described it as _incredibile
stadium atque indefessus labor_.[91]
With reference to the "earth-shine" on the moon when in the crescent
phase, Humboldt says, "Lambert made the remarkable observation (14th of
February, 1774) of a change of the ash-coloured moonlight into an
olive-green colour, bordering upon yellow. The moon, which then stood
vertically over the Atlantic Ocean, received upon its night side the green
terrestrial light, which is reflected towards her when the sky is clear by
the forest districts of South America."[92] Arago said, "Il n'est donc pas
impossible, malgré tout ce qu'un pareil résultat exciterait de surprise au
premier coup d'oeil qu'un jour les météorologistes aillent puiser dans
l'aspect de la Lune des notions précieuses sur _l'etat moyen_ de
diaphanité de l'atmosphère terrestre, dans les hemisphères qui
successivement concurrent à la production de la lumière cendrée."[93]
The "earth-shine" on the new moon was successfully photographed in
February, 1895, by Prof. Barnard at the Lick Observatory, with a 6-inch
Willard portrait lens. He says--
"The earth-lit globe stands out beautifully round, encircled by the
slender crescent. All the 'seas' are conspicuously visible, as are
also the other prominent features, especially the region about
_Tycho_. _Aristarchus_ and _Copernicus_ appear as bright specks, and
the light streams from _Tycho_ are very distinct."[94]
Kepler found that the moon completely disappeared during the total eclipse
of December 9, 1601, and Hevelius observed the same phenomenon during the
eclipse of April 25, 1642, when "not a vestige of the moon could be
seen."[95] In the total lunar eclipse of June 10, 1816, the moon during
totality was not visible in London, even with a telescope![95]
The lunar mountains are _relatively_ much higher than those on the earth.
Beer and Mädler found the following heights: Dörfel, 23,174 feet; Newton,
22,141; Casatus, 21,102; Curtius, 20,632; Callippus, 18,946; and Tycho,
18,748 feet.[96]
Taking the earth's diameter at 7912 miles, the moon's diameter, 2163
miles, and the height of Mount Everest as 29,000 feet, I find that
Everest 1 Dörfel 1
---------------- = ----, and --------------- = ---
Earth's diameter 1440 moon's diameter 492
From which it follows that the lunar mountains are _proportionately_ about
three times higher than those on the earth.
According to an hypothesis recently advanced by Dr. See, all the
satellites of the solar system, including our moon, were "captured" by
their primaries. He thinks, therefore, that the "moon came to earth from
heavenly space."[97]