meteors and comets has an important bearing on this question. We
know that the most remarkable characteristic of comets is the
enormous diffusion of their substance. Now, in this diffusion there
resides an enormous fund of force. The contraction of a large comet
to dimensions corresponding to a very moderate mean density would
be accompanied by the emission of a vast supply of heat. And the
question is worth inquiring into, whether we can indeed assume that
the meteors which reach our atmosphere are solid bodies, and not
rather of cometic diffusion; since it is difficult otherwise to
account for the light and heat which they emit. Friction through the
rarer upper strata of our atmosphere will certainly not account for
these phenomena; nor, I think, will the compression of the atmosphere
in front of the meteors; on the other hand, the sudden contraction
of a diffused vapor would be accompanied by precisely such results.
But, be this as it may, it is certain that a large portion of the
substance of every comet is in a singularly diffused state. And since
the meteoric systems circling in countless millions round the sun
are, in all probability, associated in the most intimate manner with
comets, we may recognize in this diffusion, as well as in the mere
downfall of meteors, the source of an enormous supply of light and
heat.
And lastly, turning from our sun to the other suns which shine in
uncounted myriads throughout space, we see the same processes at
work upon them all. Each star-sun has its coronal and its zodiacal
disks, formed by meteoric and cometic systems; for otherwise each
would quickly cease to be a sun. Each star-sun emits, no doubt, the
same magnetic influences which give to the Zodiacal Light and to the
solar corona their peculiar characteristics. And thus the worlds
which circle round those orbs may resemble our own in all those
relations which we refer to terrestrial magnetism, as well as in the
circumstance that on them also there must be, as on our own earth, a
continual downfall of minute meteors. In those worlds, perchance, the
magnetic compass directs the traveler over desert wastes or trackless
oceans; in their skies, the aurora displays its brilliant streamers;
while, amid the constellations which deck their heavens, meteors
sweep suddenly into view, and comets extend their vast length athwart
the celestial vault, a terror to millions, but a subject of study and
research to the thoughtful.
FOOTNOTES:
[23] Professor Kirkwood has published a most interesting series of
inquiries, going far to prove that the real secret or the planetary
influences lies in the fact that the sun’s surface is not uniform,
and that on a certain solar longitude the planetary influences are
more effective than elsewhere.
[24] To these may be added the following law:
4. Light reflected from any opaque body gives the same spectrum as it
would have given before reflection.
5. But if the opaque body be surrounded by vapors, the dark lines
corresponding to these vapors make their appearance in the spectrum
with a distinctness proportioned to the extent to which the light has
penetrated those vapors before being reflected to us.
6. If the reflecting body be itself luminous, the spectrum belonging
to it is superadded to the spectrum belonging to the reflected light.
7. Glowing vapors surrounding an incandescent source of light may
cause bright lines or dark lines to appear in the spectrum, according
as they are more or less heated; or, they may emit just so much light
as to make up for what they absorb, in which case there will remain
no trace of their presence.
8. The electric spark presents a bright-line spectrum, compounded
of the spectra belonging to the vapors of those substances between
which, and of those through which, the discharge takes place.
According to the nature of these vapors and of the discharge itself,
the relative intensity of the component parts of the spectrum will be
variable.
Lastly, the appearance of the spectrum belonging to any element will
vary according to the circumstances of pressure and temperature under
which the element may emit light.
[25] It is also shown most conclusively, by a photograph of the
eclipse of August, 1868, taken an instant before the totality. Here
we see the glare trenching upon the moon’s disk (elsewhere black),
as it should theoretically. So soon as totality commenced, the glare
had reached the moon’s limb, whence it must immediately have passed
quickly away.
[26] In fact, if we take the mode of reasoning by which Mr. Lockyer
has endeavored to get over certain physical difficulties presently
to be mentioned, we shall be able to point definitely to the place
where his argument fails. He says, conceive a tiny moon placed so as
to appear coincident with the centre of the sun’s disk. There will
be atmospheric glare as well as direct sunlight. Now, conceive this
small moon to expand until it all but covers the sun. Still there
will be glare and a certain small proportion of direct sunlight. So
far his reasoning is most just. But when he allows his expanding moon
to cover the sun, and to extend beyond the solar disk as in total
eclipse, the atmospheric glare can no longer be assumed to exist all
round the expanding moon: at the moment when the moon just hides the
sun, the glare begins to leave the moon, a gradually expanding black
ring being formed round that body. It is only necessary to consider
where the glare comes from to see that this must be so.
I have taken no account of diffraction here, because it has been
abundantly proved that no corona of appreciable width could be formed
around the moon during total eclipse by the diffraction of the rays
of light as they pass near the moon’s limb.
MERCURY.—WILLIAM F. DENNING
Mercury is the nearest known planet to the sun. It is true that a
body, provisionally named Vulcan, has been presumed to exist in
the space inferior to the orbit of Mercury; but absolute proof is
lacking, and every year the idea is losing strength in the absence
of any confirmation of a reliable kind. Not one of the regular and
best observers of the sun has recently detected any such body during
its transits (which would be likely to occur pretty frequently), and
there is other evidence of a negative character; so that the ghost of
Vulcan may be said to have been laid, and we may regard it as proven
that no major planet revolves in the interval of 36,000,000 miles
separating Mercury from the sun.
Copernicus, amid the fogs of the Vistula, looked for Mercury in
vain, and complained in his last hours that he had never seen it.
Tycho Brahe, in the Island of Hueen, appears to have been far more
successful. The planet is extremely fugitive in his appearances, but
is not nearly so difficult to find as many suppose. Whenever the
horizon is very clear, and the planet well placed, a small sparkling
object, looking more like a scintillating star than a planetary body,
will be detected at a low altitude and may be followed to the horizon.
Mercury revolves round the sun in 87 days, 23 hours, 15 minutes,
and 44 seconds in an eccentric orbit, so that his distance from
that luminary varies from 43,350,000 to 28,570,000 miles. When in
superior conjunction the apparent diameter of the planet is 4″.5;
at inferior conjunction it is 12″.9, and at elongation 7″. His real
diameter is 3,000 miles.
Being situated so near to the sun, it is obvious that to an observer
on the earth he must always remain in the same general region of
the firmament as that body. His orbital motion enables him to
successively assume positions to the east and west of the sun, and
these are known as his elongations, which vary in distance from 18°
to 28°. He becomes visible at these periods either in the morning or
evening twilight, and under the best circumstances may remain above
the horizon two hours in the absence of the sun. The best times to
observe the planet are at his E. elongations during the first half of
the year, or at his W. elongations in the last half; for his position
at such times being N. of the sun’s place, he remains a long while in
view.
Occasionally he presents quite a conspicuous aspect on the horizon,
as in February, 1868, when I thought his lustre vied with that of
Jupiter, and in November, 1882, when he shone brighter than Sirius.
The planet is generally most conspicuous _a few mornings after his W.
elongations and a few evenings before his E. elongations_.
In the course of his orbital round, Mercury exhibits all the phases
of the moon. Near his elongations the disk is about half illuminated,
and similar in form to that of our satellite when in the first or
third quarter. But the phase is not to be distinctly made out unless
circumstances are propitious. Galileo’s telescope failed to reveal
it, and Hevelius, many years afterward, found it difficult. This
is explained by the small diameter of the planet and the rarity
with which his disk appears sharply defined. The phase is sometimes
noted to be less than theory indicates; for the planet has been seen
crescented when he should have presented the form of a semicircle.
Several observers have also remarked that his surface displays a rosy
tint, and that the terminator is more deeply shaded and indefinite
than that of Venus.
The atmosphere of Mercury is probably far less dense than that of
Venus. The latter being furthest from the sun might be expected to
shine relatively more faintly than the former, but the reverse is the
case. Mercury has a dingy aspect in comparison with the bright white
lustre of Venus. On May 12, 1890, when the two planets were visible
as evening stars, and separated from each other by a distance of
only 2°, I examined them in a 10-inch reflector, power 145. The disk
of Venus looked like newly polished silver, while that of Mercury
appeared of a dull leaden hue. A similar observation was made by Mr.
Nasmyth on September 28, 1878. The explanation appears to be that the
atmosphere of Mercury is of great rarity, and incapable of reflection
in the same high degree as the dense atmosphere of Venus.
As a naked-eye object, Mercury must necessarily be looked for when
near the horizon; but there is no such need in regard to telescopic
observation, which ought to be only attempted when the planet
surmounts the dense lower vapors and is placed at a sufficient
elevation to give the instrument a fair chance of producing a steady
image. The presence of sunshine need not seriously impair the
definition, or make the disk too faint for detail.
I have occasionally seen Mercury, about two or three hours after
his rising, with outlines of extreme sharpness and quite comparable
with the excellent views obtained of Venus at the time of sunrise
or sunset. Those who possess equatorials should pick up the planet
in the afternoon and follow him until after sunset, when the
horizontal vapors will interfere. Others who work with ordinary
altazimuth stands will find it best to examine the planet at his
western elongations during the last half of the year, when he may
be found soon after rising by the naked eye or with an opera-glass,
and retained in the telescope for several hours after sunrise if
necessary.
Mercury was displayed under several advantages in the morning
twilight of November, 1882, and I made a series of observations with
a 10-inch reflector, power 212. Several dark markings were perceived,
and a conspicuous white spot. The general appearance of the disk was
similar to that of Mars, and I forwarded a summary of my results to
Professor Schiaparelli of Milan, who favored me with the following
interesting reply:
“I have myself been occupied with this planet during the past year
(1882). You are right in saying that Mercury is much easier to
observe than Venus, and that his aspect resembles Mars more than any
other of the planets of the Solar System. It has some spots which
become partially obscured and sometimes completely so; it has also
some brilliant white spots in a variable position.”
Professor Schiaparelli used an 8½-inch refractor in this work, and
was able under some favorable conditions to apply a power of 400.
The outcome of his researches, encouraged since 1882 by the addition
of an 18-inch refractor to the appliances of his observatory, was
announced in the curious fact that the rotation of Mercury is
performed in the same time that the planet revolves round the sun! If
this conclusion is just, Mercury constantly presents one and the same
hemisphere to the sun, and the behavior of the moon relatively to the
earth has found an analogy.
Spots or markings of any kind have rarely been distinguished on
Mercury. On June 11, 1867, Prince recorded a bright spot, with
faint lines diverging from it northeast and south. The spot was a
little south of the centre. Birmingham on March 13, 1870, glimpsed
a large white spot near the planet’s east limb, and Vögel, at
Bothkamp, observed spots on April 14 and 22, 1871. These instances
are quoted by Webb, and they, in combination with the markings seen
by Schiaparelli at Milan and by the author at Bristol in 1882,
sufficiently attest that this object deserves more attentive study.
One of the most interesting phenomena, albeit a somewhat rare event,
in connection with Mercury, is that of a transit across the sun.
The planet then appears as a black circular spot. Observers have
noticed one or two very small luminous points on the black disk, and
an annulus has been visible round it. These features are probably
optical effects.
THE PLANET VENUS.—CAMILLE FLAMMARION
Revolving round the sun in 224 days, Venus has its motion combined
with ours in such a manner that it passes its inferior conjunction,
between the sun and us, every 584 days; but the plane in which it
revolves is inclined 3° 23′ to that in which the earth itself moves.
When Venus attains its greatest elongations from the sun it shines
in the west in the evening, then in the morning in the east, with
a splendid brightness which eclipses that of all the stars. It is,
without comparison, the most magnificent star of our sky. Its light
is so vivid that it casts a shadow. Sometimes, even, it pierces the
azure of the sky, in spite of the presence of the sun above the
horizon, and _shines in full daylight_.
The maximum visibility of Venus is produced by its greatest phase,
by its greatest elongation from the sun, and by the clearness of our
atmosphere.
The brilliant Venus was certainly the first planet noticed by the
ancients, as much on account of its brightness as its rapid motion.
Hardly is the sun set than it sparkles in the twilight; from
evening to evening it removes further from the west and increases
in brightness; during several months it reigns sovereign of the
skies, then plunges into the solar fires and disappears. It was
pre-eminently the star of the evening, the shepherd’s star, the star
of sweet confidences. It was the first of celestial beauties, and
the names conferred upon it correspond to the direct impression which
it produced on contemplative minds. Homer called it “Callistos,”
the _Beautiful_; Cicero named it _Vesper_, the evening star, and
_Lucifer_, the morning star, a name likewise given in the Bible and
the ancient mythologies to the chief of the celestial army.
The most ancient astronomical _observation_ we have of Venus is a
Babylonian record of the year 685 B. C. It is written on a brick and
preserved in the British Museum.
The best hours for examining Venus in a telescope are those of
daylight. In the night the irradiation produced by the brilliant
light of this beautiful planet prevents us from distinguishing
clearly the outlines of its phases.
When Venus occupies the region of its orbit behind the sun, with
reference to us—which is called the point of superior conjunction—it
is at its greatest distance, and is reduced to a disk of 9½ seconds
in diameter. It comes imperceptibly toward us, and when it passes
its quadrature, at its mean distance, it presents the aspect of a
half-moon. It soon attains its most brilliant light, at the epoch
when it shines at a distance of 39° from the sun, and shows the
third phase 69 days before its inferior conjunction. Its apparent
diameter is then 40 seconds, and the width of its illuminated part
is scarcely 10 seconds. In this position we see the fourth of the
disk illuminated; but this quarter emits more light than the more
complete phases. Finally, when it reaches the region of its orbit
nearest to the earth, it shows us nothing more than an excessively
thin crescent, since it is then between the sun and us, and presents
to us, so to say, its dark hemisphere. This is the position where
its apparent size is greatest, and it then measures 62 seconds in
diameter. After passing its inferior conjunction the phases are
reproduced, in inverse order, as a morning star.
Venus is constantly visible in full daylight in astronomical
instruments, even at the moment of its superior conjunction. It is
then round and quite small. At the epochs of its inferior conjunction
it presents itself under the form of a very thin crescent.
We sometimes notice that the interior of the crescent of Venus, the
remainder of the disk, is less black than the background of the sky.
This has been called the ashy light (_lumière cendrée_) of Venus,
although it has no satellite to produce it. It seems to me that this
visibility, rather subjective than objective, arises from clouds on
the planet, which whiten its disk and vaguely reflect the stellar
light scattered through space. The eye instinctively continues the
outline of the crescent, and imagines, rather than sees, the rest.
The revolution of Venus round the sun is performed in an orbit almost
exactly circular, and without perceptible eccentricity (0.0068), in a
period of 224 days, 16 hours, 49 minutes, 8 seconds.
The days of Venus, also, are a little more rapid than ours, but not
much. Since the year 1666 attentive observation of the planet led
Cassini to conclude that it turns on itself in 23 hours, 15 minutes.
This observation is extremely difficult, on account of the brightness
of the planet and the faintness of the irregularities visible on its
disk.
The year of Venus, composed of 224 terrestrial days, consequently
contains 231 of its own, since the day is a little shorter there than
here.
These same observations show that the axis of rotation of this planet
is much more inclined than ours, and that this inclination is 55
degrees. It follows that the seasons, although each lasting but 56
terrestrial days, or 58 Venusian days, are much more intense on this
world than on ours. They pass, without transition, from summer to
winter.
The inclination of the world of Venus being more than twice as great
as ours, we have only to take a terrestrial globe and incline it by
the same quantity to understand the climates and seasons which will
result. We may easily see that the torrid zone extends, in this case,
up to the frigid zone, and even beyond it; and, reciprocally, the
frigid zone extends to the torrid zone, and even encroaches on it; so
that no place remains for a temperate zone. There is not, then, on
Venus any temperate climate, but all latitudes are both tropical and
arctic.
It follows, then, from all these circumstances, that the seasons
and climates are much more violent and more varied than ours. This
neighboring world shows nearly the same dimensions as ours. Thus this
planet is truly the twin sister of ours.
The resemblance will be still more complete if we add that this world
is certainly surrounded by an atmosphere.
When we examine with the spectroscope the light reflected by this
planet we first find the lines of the solar spectrum (and this is
natural, since the planets have no light of their own, and merely
reflect that of the sun); but we notice besides several absorption
lines similar to those which the terrestrial atmosphere gives, and
particularly those of clouds and water vapor.
We may also add that attentive observation of the indentations
visible on the crescent of Venus has shown that the surface of this
planet is quite as uneven as that of the earth, and even more so;
that there are there Andes, Cordilleras, Alps, and Pyrenees, and
that the most elevated summits attain a height of 44,000 metres (27
miles). It has even been ascertained that the Northern Hemisphere is
more mountainous than the Southern.
Even the study of the geography of Venus has already been commenced.
But it is extremely difficult to draw, and the hours of sufficiently
pure atmosphere and possible observation are very rare. This
difficulty will be easily understood if we reflect that it is exactly
when Venus arrives at its nearest to us that it is least visible,
since, its illuminated hemisphere being always turned toward the
sun, it is its dark hemisphere which is presented to us. The nearer
it approaches us, the narrower the crescent becomes. Add to this
its vivid light and its clouds, and you may imagine what difficulty
astronomers have in dealing with it.
[Illustration: Twelve Views of Jupiter
Taken at Intervals within Six Consecutive Weeks]
However, by observing it in the daytime to avoid the glare, and
not waiting till the crescent becomes too thin, by choosing the
quadratures, and making use of moments of great atmospherical purity,
observers succeed, from time to time, in perceiving grayish spots,
which may indicate the place of its seas.
Of what nature are the inhabitants of Venus? Do they resemble us in
physical form? Are they endowed with an intelligence analogous to
ours? Do they pass their life in pleasure, as Bernardin de St. Pierre
said, or, rather, are they so tormented by the inclemency of their
seasons that they have no delicate perception, and are incapable
of any scientific or artistic attention? These are interesting
questions, to which we have no reply. All that we can say is, that
organized life on Venus must be little different from terrestrial
life, and that this world is one of those which resembles ours most.
The imaginary travelers to these worlds of the sky have always
carried with them their terrestrial ideas. The only scientific
conclusion which we can draw from astronomical observation is that
this world differs little from ours in volume, in weight, in density,
and in the duration of its days and nights; that it differs a little
more in the rapidity of its years, the intensity of its climates and
seasons, the extent of its atmosphere, and its greater proximity to
the sun. It should, then, be inhabited by vegetable, animal, and
human races but little different from those which people our planet.
As to imagining it desert or sterile, this is a hypothesis which
could not arise in the brain of any naturalist. The action of the
divine sun must be there, as in Mercury, still more fertile than his
terrestrial work, already so wonderful. We may add that Venus and
Mercury, having been formed after the earth, are relatively younger
than our planet.
The inhabitants of Venus see us shining in their sky like a
magnificent star of the first magnitude, soaring in the zodiac, and
showing motions similar to those which the planet Mars presents to
us; but instead of showing a reddish brightness, the earth shines
in the sky as a bluish light. It is from Venus that we are most
luminous. The inhabitants of Venus with the naked eye see our moon
shining beside the earth and revolving round it in twenty-seven days.
They form a magnificent couple. Our planet seen from there measures
65″, and the moon nearly 18″; the moon seen from Venus shows the same
diameter as the earth seen from the sun. Mercury is brilliant, and
comes immediately after the earth in brightness. Mars, Jupiter, and
Saturn are also visible as from here, but a little less luminous. The
constellations of the whole sky show exactly the same aspect as seen
from the earth.
THE EARTH AS A PLANET.—ÉLISÉE RECLUS
The earth on which we dwell is one of the lowest in rank among the
heavenly bodies. If an astronomer in some other planet were exploring
the immensity of space, our earth, owing to its small size, might
readily elude his intelligent view. A mere satellite of the sun,
the volume of which is 1,255,000 times greater, the earth is but a
point as compared with the immense tract of ether traversed by the
planets in their courses round their central globe. The sun itself is
only a spark, which seems lost amid the eighteen millions of stars
which Herschel’s telescope discerned in the Milky Way; the latter, an
immense agglomeration of suns and planets, which looks to us like a
broad streak of light round the whole universe, is in reality nothing
but a nebula. Beyond our own sky, other skies stretch far away into
infinity, and others beyond these, so that light notwithstanding its
prodigious rapidity, takes eternities to cross them. How small the
earth seems in this fathomless abyss of stars!