narrow cone of light whose base apparently rests on the horizon, while
its summit rises among the zodiacal constellations. In general
appearance it somewhat resembles the tail of a large comet whose head is
below the horizon. The most favorable time to observe this phenomenon in
the evening, is immediately after the last trace of twilight has
disappeared; and in the morning, one or two hours before twilight
appears. When observed with attention, it is seen that the light of the
zodiacal cone is not uniform, but gradually increases in brightness
inwardly, especially towards its base, where it sometimes surpasses in
brilliancy the brightest parts of the Milky-Way. In general, its
outlines are vague and very difficult to make out, so gradually do they
blend with the sky. On some favorable occasions, the luminous cone
appears to be composed of several distinct concentric conical layers,
having different degrees of brightness, the inner cone being the most
brilliant of all. There is a remarkable distinction between the evening
and morning zodiacal light. In our climate, the morning light is pale,
and never so bright nor so extended as the evening light.
In general, the zodiacal light is whitish and colorless, but in some
cases it acquires a warm yellowish or reddish tint. These changes of
color may be accidental and due to atmospheric conditions, and not to
actual change in the color of the object. Although the zodiacal light is
quite bright, and produces the impression of having considerable depth,
yet its transparency is great, since all the stars, except the faint
ones, can be seen through its substance.
The zodiacal light is subject to considerable variations in brightness,
and also varies in extent, the apex of its cone varying in distance from
the Sun's place, from 40 to 90 degrees. These variations cannot be
attributed to atmospheric causes alone, some of them being due to real
changes in the zodiacal light itself, whose light and dimensions
increase or decrease under the action of causes at present unknown. From
the discussion of a series of observations on the zodiacal light made at
Paris and Geneva, it appears certain that its light varies from year to
year, and sometimes even from day to day, independently of atmospheric
causes. Some of my own observations agree with these results, and one of
them, at least, seems to indicate changes even more rapid. On December
18th, 1875, I observed the zodiacal light in a clear sky free from any
vapors, at six o'clock in the evening. At that time, the point of its
cone was a little to the north of the ecliptic, at a distance of about
90 degrees from the Sun's place. Ten minutes later, its summit had sunk
down 35 degrees, the cone then being reduced to nearly one-half of its
original dimensions. Ten minutes later, it had risen 25 degrees, and was
then 80 degrees from the Sun's place, where it remained all the evening.
On March 22d, 1878, the sky was very clear and the zodiacal light was
bright when I observed it, at eight o'clock. At that moment the apex of
the cone of light was a little to the south of the Pleiades, but this
cone presented an unusual appearance never noticed by me before, its
northern border appearing much brighter and sharper than usual, while at
the same time its axis of greatest brightness appeared to be much nearer
to this northern border than it was to the southern. After a few minutes
of observation it became evident that the northern border was extending
itself, as stars which were at some distance from it became gradually
involved in its light. At the same time that this border spread
northward, it seemed to diffuse itself, and after a time the cone
presented its usual appearance, having its southern border brighter and
better defined than the other. It would have been impossible to
attribute this sudden change to an atmospheric cause, since only one of
the borders of the cone participated in it, and since some very faint
stars near this northern border were not affected in the least while the
phenomenon occurred. Besides these observations, Cassini, Mairan,
Humboldt, and many other competent observers have seen pulsations,
coruscations and bickerings in the light of the cone, which they thought
could not be attributed to atmospheric causes. It has also been observed
that at certain periods the zodiacal light has shone with unusual
intensity for months together.
When this phenomenon is observed from the tropical regions, it is found
that its axis of symmetry always corresponds with its axis of greatest
brightness, and that both lie in the plane of the ecliptic, which
divides its cone into two equal parts. But when the zodiacal light is
observed in our latitude, the axis of symmetry does not correspond with
the axis of greatest brightness, and both axes are a little to the north
of this plane, the axis of symmetry being the farther removed.
Furthermore, as already stated, the southern border of the cone always
appears better defined and brighter than the corresponding northern
margin. It is very probable, if not absolutely certain, that these
phenomena are exactly reversed when the zodiacal light is observed from
corresponding latitudes in the southern hemisphere, and that there, its
axes, both of symmetry and of greatest brightness, appear south of the
ecliptic, while the northern margin is the brightest. This seems to be
established by the valuable observations of Rev. George Jones, made on
board the U. S. steam frigate Mississippi, in California, Japan, and the
Southern Ocean. "When I was north of the ecliptic," says this observer,
"the greatest part of the light of the cone appeared to the north of
this line; when I was to the south of the ecliptic, it appeared to be
south of it; while when my position was on the ecliptic, or in its
vicinity, the zodiacal cone was equally divided by this line."
Besides the zodiacal light observed in the East and West, some observers
have recognized an exceedingly faint, luminous, gauzy band, about 10 or
12 degrees wide, stretching along the ecliptic from the summit of the
western to that of the eastern zodiacal cone. This faint narrow belt has
been called the Zodiacal Band. It has been recognized by Mr. H. C.
Lewis, who has made a study of this phenomenon, that the zodiacal band
has its southern margin a little brighter and a little sharper than the
northern border. This observation is in accordance with similar
phenomena observed in the zodiacal light, and may have considerable
importance.
In 1854, Brorsen recognized a faint, roundish, luminous spot in a point
of the heavens exactly opposite to the place occupied by the Sun, which
he has called "Gegenschein," or counter-glow. This luminous spot has
sometimes a small nucleus, which is a little brighter than the rest.
Night after night this very faint object shifts its position among the
constellations, keeping always at 180 degrees from the Sun. The position
of the counter-glow, like that of the zodiacal light and zodiacal band,
is not precisely on the plane of the ecliptic, but a little to the north
of this line. It is very probable that near the equator the phenomenon
would appear different and there would correspond with this plane.
There seems to be some confusion among observers in regard to the
spectrum of the zodiacal light. Some have seen a bright green line in
its spectrum, corresponding to that of the aurora borealis; while others
could only see a faint grayish continuous spectrum, which differs,
however, from that of a faint solar light, by the fact that it presents
a well-defined bright zone, gradually blending on each side with the
fainter light of the continuous spectrum. I have, myself, frequently
observed the faint continuous spectrum of the zodiacal light, and on one
occasion recognized the green line of the aurora; but it might have been
produced by the aurora itself, as yet invisible to the eye, and not by
the zodiacal light, since, later in the same evening, there was a
brilliant auroral display. If it were demonstrated that this green line
exists in the spectrum of the zodiacal light, the fact would have
importance, as tending to show that the aurora and the zodiacal light
have a common origin.
Rev. Geo. Jones describes a very curious phenomenon which he observed
several times a little before the moon rose above the horizon. The
phenomenon consisted in a short, oblique, luminous cone rising from the
Moon's place in the direction of the ecliptic. This phenomenon he has
called the Moon Zodiacal Light. In 1874, I had an opportunity to observe
a similar phenomenon when the Moon was quite high in the sky. By taking
the precaution to screen the Moon's disk by the interposition of some
buildings between it and my eye, I saw two long and narrow cones of
light parallel to the ecliptic issuing from opposite sides of our
satellite. The phenomenon could not possibly be attributed to vapors in
our atmosphere, since the sky was very clear at the moment of the
observation. Later on, these appendages disappeared with the formation
of vapors near the Moon, but they reappeared an hour later, when the sky
had cleared off, and continued visible for twenty minutes longer, and
then disappeared in a clear sky.
Although the zodiacal light has been studied for over two centuries, no
wholly satisfactory explanation of the phenomenon has yet been given.
Now, as in Cassini's time, it is generally considered by astronomers to
be due to a kind of lens-shaped ring surrounding the Sun, and extending
a little beyond the Earth's orbit. This ring is supposed to lie in the
plane of the ecliptic, and to be composed of a multitude of independent
meteoric particles circulating in closed parallel orbits around the Sun.
But many difficulties lie in the way of this theory. It seems as
incompetent to explain the slow and rapid changes in the light of this
object as it is to explain the contractions and extensions of its cone.
It fails, moreover, to explain the flickering motions, the coruscations
observed in its light, or the displacement of its cone and of its axes
of brightness and symmetry by a mere change in the position of the
observer. Rev. Geo. Jones, unable to explain by this theory the
phenomena which came under his observation, has proposed another, which
supposes the zodiacal light to be produced by a luminous ring
surrounding the Earth, this ring not extending as far as the orbit of
the Moon. But this theory also fails in many important points, so that
at present no satisfactory explanation of the phenomenon can be given.
As the phenomenon is connected in some way with the Sun, and as we have
many reasons to believe this body to be always more or less electrified,
it might be supposed that the Sun, acting by induction on our globe,
develops feeble electric currents in the rarefied gases of the superior
regions of our atmosphere, and there forms a kind of luminous ridge
moving with the Sun in a direction contrary to the diurnal motion, and
so producing the zodiacal light. On this hypothesis, the counter-glow
would be the result of a smaller cone of light generated by the solar
induction on the opposite point of the Earth.
Plate 5, which sufficiently explains itself, represents the zodiacal
light as it appeared in the West on the evening of February 20th, 1876.
All the stars are placed in their proper position, and their relative
brightness is approximately shown by corresponding variations in
size--the usual and almost the only available means of representation.
Of course, it must be remembered that a star does not, in fact, show any
disk even in the largest telescopes, where it appears as a mere point of
light, having more or less brilliancy. The cone of light rises obliquely
along the ecliptic, and the point forming its summit is found in the
vicinity of the well-known group of stars, called the Pleiades, in the
constellation of Taurus, or the Bull.
THE MOON
PLATE VI
In its endless journey through space, our globe is not solitary, like
some of the planets, but is attended by the Moon, our nearest celestial
neighbor. Although the Moon does not attain to the dignity of a planet,
and remains a secondary body in the solar system, yet, owing to its
proximity to our globe, and to the great influence it exerts upon it by
its powerful attraction, it is to us one of the most important celestial
bodies.
While the Moon accompanies the Earth around the Sun, it also revolves
around the Earth at a mean distance of 238,800 miles. For a celestial
distance this is only a trifling one; the Earth in advancing on its
orbit travels over such a distance in less than four hours. A cannon
ball would reach our satellite in nine days; and a telegraphic dispatch
would be transmitted there in 1½ seconds of time, if a wire could be
stretched between us and the Moon.
Owing to the ellipticity of the Moon's orbit, its distance from the
Earth varies considerably, our satellite being sometimes 38,000 miles
nearer to us than it is at other times. These changes in the distance of
the Moon occasion corresponding changes from 29' to 33' in its apparent
diameter. The real diameter of the Moon is 2,160 miles, or a little over
one-quarter the diameter of our globe; our satellite being 49 times
smaller than the Earth.
The mean density of the materials composing the Moon is only
⁶⁄₁₀ that of the materials composing the Earth, and the force of
gravitation at the surface of our satellite is six times less than it is
at the surface of our globe. If a person weighing 150 lbs. on our Earth
could be transported to the Moon, his weight there would be only 25 lbs.
The Moon revolves around the Earth in about 27⅓ days, with a mean
velocity of one mile per second, the revolution constituting its
sidereal period. If the Earth were motionless, the lunar month would be
equal to the sidereal period; but owing to its motion in space, the Sun
appears to move with the Moon, though more slowly, so that after having
accomplished one complete revolution, our satellite has yet to advance
2¼ days before reaching the same apparent position in regard to the
Earth and the Sun that it had at first. The interval of time comprised
between two successive New Moons, which is a little over 29½ days,
constitutes the synodical period of the Moon, or the lunar month.
The Moon is not a self-luminous body, but, like the Earth and the
planets, it reflects the light which it receives from the Sun, and so
appears luminous. That such is the case is sufficiently demonstrated by
the phases exhibited by our satellite in the course of the lunar month.
Every one is familiar with these phases, which are a consequence of the
motion of the Moon around the Earth. When our satellite is situated
between us and the Sun, it is New Moon; since we cannot see its
illuminated side, which is then turned away from us towards the Sun.
When, on the contrary, it reaches that point of its orbit which, in
regard to us, is opposite to the Sun's place, it is Full Moon; since
from the Earth we can only see the fully illuminated side of our
satellite. Again, when the Moon arrives at either of the two opposite
points of its orbit, the direction of which from the Earth is at right
angles with that of the Sun, it is either the First or the Last Quarter;
since in these positions we can only see one-half of its illuminated
disk.
The curve described by the Moon around the Earth lies approximately in a
plane, this plane being inclined about 5° to the ecliptic. Since our
satellite, in its motion around us and the Sun, closely follows the
ecliptic, which is inclined 23½° to the equator, it results that when
this plane is respectively high or low in the sky, the moon is also high
or low when crossing the meridian of the observer. In winter that part
of the ecliptic occupied by the Sun is below the equator, and,
consequently, the New Moons occurring in that season are low in the sky,
since at New Moon our satellite must be on the same side of the ecliptic
with the Sun. But the Full Moons in the same season are necessarily high
in the sky, since a Full Moon can only occur when our satellite is on
the opposite side of the ecliptic from the Sun, in which position it is,
of course, as many degrees above the equator as the Sun is below. The
Full Moon which happens nearest to the autumnal equinox is commonly
called the Harvest Moon, from the fact that, after full, its delays in
rising on successive evenings are very brief and therefore favorable for
the harvest work in the evening. The same phenomenon occurs in every
other lunar month, but not sufficiently near the time of Full Moon to be
noticeable. When, in spring, a day or two after New Moon, our satellite
begins to show its thin crescent, its position on the ecliptic is north
as well as east of that occupied by the Sun; hence, its horns are nearly
upright in direction, and give it a crude resemblance to a tipping bowl,
from which many people who are unaware of its cause, and that this
happens every year, draw conclusions as to the amount of rain to be
expected.
One of the most remarkable features of the Moon's motions is that our
satellite rotates on its axis in exactly the same period of time
occupied by its revolution around the Earth, from which it results that
the Moon always presents to us the same face. To explain this
peculiarity, astronomers have supposed that the figure of our satellite
is not perfectly spherical, but elongated, so that the attraction of the
Earth, acting more powerfully upon its nearest portions, always keeps
them turned toward us, as if the Moon were united to our globe by a
string. It is not exactly true, however, that the Moon always presents
its same side to us, although its period of rotation exactly equals that
of its revolution; since in consequence of the inclination of its axis
of rotation to its orbit, combined with the irregularities of its
orbital motion about us, apparent oscillations in latitude and in
longitude, called librations, are created, from which it results that
nearly ⁶⁄₁₀ of the Moon's surface is visible from the Earth at
one time or another.
The Moon is a familiar object, and every one is aware that our
satellite, especially when it is fully illuminated, presents a variety
of bright and dark markings, which, from their distant resemblance to a
human face, are popularly known as "the man in the moon." A day or two
after New Moon, when the thin crescent of our satellite is visible above
the western horizon after sunset, the dark portion of its disk is
plainly visible, and appears of a pale, ashy gray color, although not
directly illuminated by the Sun. This phenomenon is due to the
Earth-shine, or to that portion of solar light which the illuminated
surface of our globe reflects to the dark side of the Moon, exactly in
the same manner that the Moon-shine, on our Earth, is due to the solar
light reflected to our globe by the illuminated Moon.
Seen with a telescope of moderate power, or even with a good
opera-glass, the Moon presents a peculiar mottled appearance, and has a
strong resemblance to a globe made of plaster of Paris, on the surface
of which numerous roundish, saucer-shaped cavities of various sizes are
scattered at random. This mottled structure is better seen along the
boundary line called the _terminator_, which divides the illuminated
from the dark side of the Moon. The line of the terminator always
appears jagged, and it is very easy to recognize that this irregularity
is due to the uneven and rugged structure of the surface of our
satellite.
A glance at the Moon through a larger telescope shows that the bright
spots recognized with the naked eye belong to very uneven and
mountainous regions of our satellite, while the dark ones belong to
comparatively smooth, low surfaces, comparable to those forming the
great steppes and plains of the Earth. When examined with sufficient
magnifying power, the white, rugged districts of the Moon appear covered
over by numerous elevated craggy plateaus, mountain-chains, and deep
ravines; by steep cliffs and ridges; by peaks of great height and
cavities of great depth. This rugged formation, which is undoubtedly of
volcanic origin, gives our satellite a desolate and barren appearance.
The rugged tract occupies more than one-half of the visible surface of
the Moon, forming several distinct masses, the principal of which occupy
the south and south-western part of the disk. That this formation is
elevated above the general level is proved by the fact that the
mountains, peaks, and other objects which compose it, all cast a shadow
opposite to the Sun; and further, that the length of these shadows
diminishes with the elevation of the Sun above the lunar horizon.
Since Galileo's time the surface of the Moon has been studied by a host
of astronomers, and accurate maps of its topographical configuration
have been made, and names given to all features of any prominence. It
may even be said that in its general features, the visible surface of
our satellite is now better known to us than is the surface of our own
Earth.
One of the most striking and common features of the mountainous
districts of the Moon, is the circular, ring-like disposition of their
elevated parts, which form numerous crater-like objects of different
sizes and depths. Many thousands of crater-like objects are visible on
the Moon through a good telescope, and, considering how numerous the
small ones are, there is, perhaps, no great exaggeration in fixing their
number at 50,000, as has been done by some astronomers. These volcanic
regions of the Moon cannot be compared to anything we know, and far
surpass in extent those of our globe. The number and size of the craters
of our most important volcanic regions in Europe, in Asia, in North and
South America, in Java, in Sumatra, and Borneo, are insignificant when
compared with those of the Moon. The largest known craters on the Earth
give only a faint idea of the magnitude of some of the lunar craters.
The great crater Haleakala, in the Sandwich Islands, probably the
largest of the terrestrial volcanoes, has a circumference of thirty
miles, or a diameter of a little less than ten miles. Some of the great
lunar craters, called walled plains, such as Hipparchus, Ptolemæus,
etc., have a diameter more than ten times larger than that of Haleakala,
that of the first being 115 miles and that of the last 100 miles. These
are, of course, among the largest of the craters of the Moon, although
there are on our satellite a great number of craters above ten miles in
diameter.
The crater-forms of the Moon have evidently appeared at different
periods of time, since small craters are frequently found on the walls
of larger ones; and, indeed, still smaller craters are not rarely seen
on the walls of these last. The walls of the lunar craters are usually
quite elevated above the surrounding surface, some of them attaining
considerable elevations, especially at some points, which form peaks of
great height. Newton, the loftiest of all, rises at one point to the
height of 23,000 feet, while many others range from ten to twenty
thousand feet in height. Several craters have their floor above the
general surface--Plato, for instance. Wargentin has its floor nearly on
a level with the summit of its walls, showing that at some period of its
history liquid lavas, ejected from within, have filled it to the brim
and then solidified. The floors of some of the craters are smooth and
flat, but in general they are occupied by peaks and abrupt mountainous
masses, which usually form the centre. Many of their outside walls are
partly or wholly covered by numerous ravines and gullies, winding down
their steep declivities, branching out and sometimes extending to great
distances from their base. It would seem that these great volcanic
mouths have at some time poured out torrents of lavas, which, in their
descent, carved their passage by the deep gullies now visible.
Sometimes, also, the crater slopes are strewn with debris, giving them a
peculiar volcanic appearance.