VENUS, THE TWIN OF THE EARTH
We come now to a planet which seems, at the first glance, to afford a
far more promising outlook than Mercury does for the presence of organic
life forms bearing some resemblance to those of the earth. One of the
strongest arguments for regarding Venus as a world much like ours is
based upon its remarkable similarity to the earth in size and mass,
because thus we are assured that the force of gravity is practically the
same upon the two planets, and the force of gravity governs numberless
physical phenomena of essential importance to both animal and vegetable
life. The mean diameter of the earth is 7,918 miles; that of Venus is
7,700 miles. The difference is so slight that if the two planets were
suspended side by side in the sky, at such a distance that their disks
resembled that of the full moon, the eye would notice no inequality
between them.
The mean density of Venus is about nine tenths of that of the earth, and
the force of gravity upon its surface is in the ratio of about 85 to 100
as compared to its force on the surface of the earth. A man removed to
Venus would, consequently, find himself perceptibly lighter than he was
at home, and would be able to exert his strength with considerably
greater effect than on his own planet. But the difference would amount
only to an agreeable variation from accustomed conditions, and would not
be productive of fundamental changes in the order of nature.
Being, like Mercury, nearer to the sun than the earth is, Venus also is
visible to us only in the morning or the evening sky. But her distance
from the sun, slightly exceeding 67,000,000 miles, is nearly double that
of Mercury, so that, when favorably situated, she becomes a very
conspicuous object, and, instead of being known almost exclusively by
astronomers, she is, perhaps, the most popular and most admired of all
the members of the planetary system, especially when she appears in the
charming rôle of the "evening star." As she emerges periodically from
the blinding glare of the sun's immediate neighborhood and begins to
soar, bright as an electric balloon, in the twilight, she commands all
eyes and calls forth exclamations of astonishment and admiration by her
singular beauty. The intervals between her successive reappearances in
the evening sky, measured by her synodic period of 584 days, are
sufficiently long to give an element of surprise and novelty to every
return of so dazzling a phenomenon.
Even the light of the full moon silvering the tree tops does not
exercise greater enchantment over the mind of the contemplative
observer. In either of her rôles, as morning or as evening star, Venus
has no rival. No fixed star can for an instant bear comparison with her.
What she lacks in vivacity of light--none of the planets twinkles, as do
all of the true stars--is more than compensated by the imposing size of
her gleaming disk and the striking beauty of her clear lamplike rays.
Her color is silvery or golden, according to the state of the
atmosphere, while the distinction of her appearance in a dark sky is so
great that no eye can resist its attraction, and I have known an
unexpected glimpse of Venus to put an end to an animated conversation
and distract, for a long time, the attention of a party of ladies and
gentlemen from the social occupation that had brought them together.
As a telescopic object Venus is exceedingly attractive, even when
considered merely from the point of view of simple beauty. Both Mercury
and Venus, as they travel about the sun, exhibit phases like those of
the moon, but Venus, being much larger and much nearer to the earth than
Mercury, shows her successive phases more effectively, and when she
shines as a thin crescent in the morning or evening twilight, only a
very slight magnifying power is required to show the sickle form of her
disk.
A remarkable difference between Venus and Mercury comes out as soon as
we examine the shape of the former's orbit. Venus's mean distance from
the sun is 67,200,000 miles, and her orbit is so nearly a circle, much
more nearly than that of any other planet, that in the course of a
revolution her distance from the sun varies less than a million miles.
The distance of the earth varies 3,000,000 miles, and that of Mercury
14,000,000. Her period of revolution, or the length of her year, is 225
of our days. When she comes between the sun and the earth she approaches
us nearer than any other planet ever gets, except the asteroid Eros, her
distance at such times being 26,000,000 miles, or about one hundred and
ten times the distance of the moon.
Being nearer to the sun in the ratio of 67 to 93, Venus receives almost
twice as much solar light and heat as we get, but less than one third as
much as Mercury gets. There is reason to believe that her axis, instead
of being considerably inclined, like that of the earth, is perpendicular
to the plane of her orbit. Thus Venus introduces to us another novelty
in the economy of worlds, for with a perpendicular axis of rotation she
can have no succession of seasons, no winter and summer flitting, one
upon the other's heels, to and fro between the northern and southern
hemispheres; but, on the contrary, her climatic conditions must be
unchangeable, and, on any particular part of her surface, except for
local causes of variation, the weather remains the same the year around.
So, as far as temperature is concerned, Venus may have two regions of
perpetual winter, one around each pole; two belts of perpetual spring in
the upper middle latitudes, one on each side of the equator; and one
zone of perpetual summer occupying the equatorial portion of the planet.
But, of course, these seasonal terms do not strictly apply to Venus, in
the sense in which we employ them on the earth, for with us spring is
characterized rather by the change in the quantity of heat and other
atmospheric conditions that it witnesses than by a certain fixed and
invariable temperature.
To some minds it may appear very undesirable, from the point of view of
animate existences, that there should be no alternation of seasons on
the surface of a planet, but, instead, fixed conditions of climate; yet
it is not clear that such a state of affairs might not be preferable to
that with which we are familiar. Even on the earth, we find that
tropical regions, where the seasonal changes are comparatively moderate,
present many attractions and advantages in contrast with the violent and
often destructive vicissitudes of the temperate zones, and nature has
shown us, within the pale of our own planet, that she is capable of
bringing forth harvests of fruit and grain without the stimulus of
alternate frost and sunshine.
Even under the reign of perpetual summer the fields and trees find time
and opportunity to rest and restore their productive forces.
The circularity of Venus's orbit, and the consequently insignificant
change in the sun's distance and heating effect, are other elements to
be considered in estimating the singular constancy in the operation of
natural agencies upon that interesting planet, which, twin of the earth
though it be in stature, is evidently not its twin in temperament.
And next as to the all-important question of atmosphere. In what
precedes, the presence of an atmosphere has been assumed, and,
fortunately, there is very convincing evidence, both visual and
spectroscopic, that Venus is well and abundantly supplied with air, by
which it is not meant that Venus's air is precisely like the mixture of
oxygen and nitrogen, with a few other gases, which we breathe and call
by that name. In fact, there are excellent reasons for thinking that the
atmosphere of Venus differs from the earth's quite as much as some of
her other characteristics differ from those of our planet. But, however
it may vary from ours in constitution, the atmosphere of Venus contains
water vapor, and is exceedingly abundant. Listen to Professor Young:
"Its [Venus's] atmosphere is probably from one and a half to two times
as extensive and as dense as our own, and the spectroscope shows
evidence of the presence of water vapor in it."
And Prof. William C. Pickering, basing his statement on the result of
observations at the mountain observatory of Arequipa, says: "We may feel
reasonably certain that at the planet's [Venus's] surface the density of
its atmosphere is many times that of our own."
We do not have to depend upon the spectroscope for evidence that Venus
has a dense atmosphere, for we can, in a manner, _see_ her atmosphere,
in consequence of its refractive action upon the sunlight that strikes
into it near the edge of the planet's globe. This illumination of
Venus's atmosphere is witnessed both when she is nearly between the sun
and the earth, and when, being exactly between them, she appears in
silhouette against the solar disk. During a transit of this kind, in
1882, many observers, and the present writer was one, saw a bright
atmospheric bow edging a part of the circumference of Venus when the
planet was moving upon the face of the sun--a most beautiful and
impressive spectacle.
Even more curious is an observation made in 1866 by Prof. C.S. Lyman, of
Yale College, who, when Venus was very near the sun, saw her atmosphere
_in the form of a luminous ring_. A little fuller explanation of this
appearance may be of interest.
When approaching inferior conjunction--i.e., passing between the earth
and sun--Venus appears, with a telescope, in the shape of a very thin
crescent. Professor Lyman watched this crescent, becoming narrower day
after day as it approached the sun, and noticed that its extremities
gradually extended themselves beyond the limits of a semicircle, bending
to meet one another on the opposite side of the invisible disk of the
planet, until, at length, they did meet, and he beheld a complete ring
of silvery light, all that remained visible of the planet Venus! The
ring was, of course, the illuminated atmosphere of the planet refracting
the sunlight on all sides around the opaque globe.
In 1874 M. Flammarion witnessed the same phenomenon in similar
circumstances. One may well envy those who have had the good fortune to
behold this spectacle--to actually see, as it were, the air that the
inhabitants of another world are breathing and making resonant with all
the multitudinous sounds and voices that accompany intelligent life. But
perhaps some readers will prefer to think that even though an atmosphere
is there, there is no one to breathe it.
[Illustration: VENUS'S ATMOSPHERE SEEN AS A RING OF LIGHT.]
As the visibility of Venus's atmosphere is unparalleled elsewhere in the
solar system, it may be worth while to give a graphic illustration of
it. In the accompanying figure the planet is represented at three
successive points in its advance toward inferior conjunction. As it
approaches conjunction it slowly draws nearer the earth, and its
apparent diameter consequently increases. At _A_ a large part of the
luminous crescent is composed of the planet's surface reflecting the
sunshine; at _B_ the ratio of the reflecting surface to the illuminated
atmosphere has diminished, and the latter has extended, like the curved
arms of a pair of calipers, far around the unilluminated side of the
disk; at _C_ the atmosphere is illuminated all around by the sunlight
coming through it from behind, while the surface of the planet has
passed entirely out of the light--that is to say, Venus has become an
invisible globe embraced by a circle of refracted sunshine.
We return to the question of life. With almost twice as much solar heat
and light as we have, and with a deeper and denser atmosphere than ours,
it is evident, without seeking other causes of variation, that the
conditions of life upon Venus are notably different from those with
which we are acquainted. At first sight it would seem that a dense
atmosphere, together with a more copious supply of heat, might render
the surface temperature of Venus unsuitable for organic life as we
understand it. But so much depends upon the precise composition of the
atmosphere and upon the relative quantities of its constituents, that it
will not do to pronounce a positive judgment in such a case, because we
lack information on too many essential points.
Experiment has shown that the temperature of the air varies with changes
in the amount of carbonic acid and of water vapor that it contains. It
has been suggested that in past geologic ages the earth's atmosphere was
denser and more heavily charged with vapors than it is at present; yet
even then forms of life suited to their environment existed, and from
those forms the present inhabitants of our globe have been developed.
There are several lines of reasoning which may be followed to the
conclusion that Venus, as a life-bearing world, is younger than the
earth, and, according to that view, we are at liberty to imagine our
beautiful sister planet as now passing through some such period in its
history as that at which the earth had arrived in the age of the
carboniferous forests, or the age of the gigantic reptiles who ruled
both land and sea.
But, without making any assumptions as to the phase of evolution which
life may have attained on Venus, it is also possible to think that the
planet's thick shell of air, with its abundant vapors, may serve as a
shield against the excessive solar radiation. Venus is extraordinarily
brilliant, its reflective power being greatly in excess of Mercury's,
and it has often been suggested that this may be due to the fact that a
large share of the sunlight falling upon it is turned back before
reaching the planet's surface, being reflected both from the atmosphere
itself and from vast layers of clouds.
Even when viewed with the most powerful telescopes and in the most
favoring circumstances, the features of Venus's surface are difficult
to see, and generally extremely difficult. They consist of faint shadowy
markings, indefinite in outline, and so close to the limit of visibility
that great uncertainty exists not only as to their shape and their
precise location upon the planet, but even as to their actual existence.
No two observers have represented them exactly alike in drawings of the
planet, and, unfortunately, photography is as yet utterly unable to deal
with them. Mr. Percival Lowell, in his special studies of Venus in 1896,
using a 24-inch telescope of great excellence, in the clear and steady
air of Arizona, found delicate spokelike streaks radiating from a
rounded spot like a hub, and all of which, in his opinion, were genuine
and definite markings on the planet's surface. But others, using larger
telescopes, have failed to perceive the shapes and details depicted by
Mr. Lowell, and some are disposed to ascribe their appearances to
Venus's atmosphere. Mr. Lowell himself noticed that the markings seemed
to have a kind of obscuring veil over them.
In short, all observers of Venus agree in thinking that her atmosphere,
to a greater or less extent, serves as a mask to conceal her real
features, and the possibilities of so extensive an atmosphere with
reference to an adjustment of the peculiar conditions of the planet to
the requirements of life upon it, are almost unlimited. If we could
accurately analyze that atmosphere we would have a basis for more exact
conclusions concerning Venus's habitability.
But the mere existence of the atmosphere is, in itself, a strong
argument for the habitability of the planet, and as to the temperature,
we are really not compelled to imagine special adaptations by means of
which it may be brought into accord with that prevailing upon the earth.
As long as the temperature does not rise to the _destructive_ point,
beyond which our experience teaches that no organic life can exist, it
may very well attain an elevation that would mean extreme discomfort
from our point of view, without precluding the existence of life even in
its terrestrial sense.
And would it not be unreasonable to assume that vital phenomena on other
planets must be subject to exactly the same limitations that we find
circumscribing them in our world? That kind of assumption has more than
once led us far astray even in dealing with terrestrial conditions.
It is not so long ago, for instance, since life in the depths of the sea
was deemed to be demonstrably impossible. The bottom of the ocean, we
were assured, was a region of eternal darkness and of frightful
pressure, wherein no living creatures could exist. Yet the first dip of
the deep-sea trawl brought up animals of marvelous delicacy of
organization, which, although curiously and wonderfully adapted to live
in a compressed liquid, collapsed when lifted into a lighter medium, and
which, despite the assumed perpetual darkness of their profound abode,
were adorned with variegated colors and furnished with organs of
phosphorescence whereby they could create for themselves all the light
they needed.
Even the fixed animals of the sea, growing, like plants, fast to the
rocks, are frequently vivid with living light, and there is a splendid
suggestion of nature's powers of adaptation, which may not be entirely
inapplicable to the problems of life on strange planets, in Alexander
Agassiz's statement that species of sea animals, living below the depths
to which sunlight penetrates, "may dwell in total darkness and be
illuminated at times merely by the movements of abyssal fishes through
the forests of phosphorescent alcyonarians."
In attempting to judge the habitability of a planet such as Venus we
must first, as far as possible, generalize the conditions that govern
life and restrict its boundaries.
On the earth we find animated existence confined to the surface of the
crust of the globe, to the lower and denser strata of the atmosphere,
and to the film of water that constitutes the oceans. It does not exist
in the heart of the rocks forming the body of the planet nor in the void
of space surrounding it outside the atmosphere. As the earth condensed
from the original nebula, and cooled and solidified, a certain quantity
of matter remained at its surface in the form of free gases and unstable
compounds, and, within the narrow precincts where these things were,
lying like a thin shell between the huge inert globe of permanently
combined elements below, and the equally unchanging realm of the ether
above, life, a phenomenon depending upon ceaseless changes, combinations
and recombinations of chemical elements in unstable and temporary union,
made its appearance, and there only we find it at the present time.
It is because air and water furnish the means for the continual
transformations by which the bodies of animals and plants are built up
and afterward disintegrated and dispersed, that we are compelled to
regard their presence as prerequisites to the existence, on any planet,
of life in any of the forms in which we are acquainted with it. But if
we perceive that another world has an atmosphere, and that there is
water vapor in its atmosphere--both of which conditions are fulfilled by
Venus--and if we find that that world is bathed in the same sunshine
that stimulates the living forces of our planet, even though its
quantity or intensity may be different, then it would seem that we are
justified in averring that the burden of proof rests upon those who
would deny the capability of such a world to support inhabitants.
The generally accepted hypothesis of the origin of the solar system
leads us to believe that Venus has experienced the same process of
evolution as that which brought the earth into its present condition,
and we may fairly argue that upon the rocky shell of Venus exists a
region where chemical combinations and recombinations like those on the
surface of the earth are taking place. It is surely not essential that
the life-forming elements should exist in exactly the same states and
proportions as upon the earth; it is enough if some of them are
manifestly present. Even on the earth these things have undergone much
variation in the course of geological history, coincidently with the
development of various species of life. Just at present the earth
appears to have reached a stage where everything contributes to the
maintenance of a very high organization in both the animal and vegetable
kingdoms.
So each planet that has attained the habitable stage may have a typical
adjustment of temperature and atmospheric constitution, rendering life
possible within certain limits peculiar to that planet, and to the
special conditions prevailing there. Admitting, as there is reason for
doing, that different planets may be at different stages of development
in the geological and biological sense, we should, of course, not expect
to find them inhabited by the same living species. And, since there is
also reason to believe that no two planets upon arriving at the same
stage of evolution as globes would possess identical gaseous
surroundings, there would naturally be differences between their organic
life forms notwithstanding the similarity of their common phase of
development in other respects. Thus a departure from the terrestrial
type in the envelope of gases covering a planet, instead of precluding
life, would only tend to vary its manifestations.
After all, why should the intensity of the solar radiation upon Venus be
regarded as inimical to life? The sunbeams awaken life.
It is not impossible that relative nearness to the sun may be an
advantage to Venus from the biologic point of view. She gets less than
one third as much heat as Mercury receives on the average, and she gets
it with almost absolute uniformity. At aphelion Mercury is about two and
four tenths times hotter than Venus; then it rushes sunward, and within
forty-four days becomes six times hotter than Venus. In the meantime the
temperature of the latter, while high as compared with the earth's,
remains practically unchanged. Not only may Mercury's temperature reach
the destructive point, and thus be too high for organic life, but
Mercury gets nothing with either moderation or constancy. It is a world
both of excessive heat and of violent contrasts of temperature. Venus,
on the other hand, presents an unparalleled instance of invariableness
and uniformity. She may well be called the favorite of the sun, and,
through the advantages of her situation, may be stimulated by him to
more intense vitality than falls to the lot of the earth.
It is open, at least to the writers of the interplanetary romances now
so popular, to imagine that on Venus, life, while encompassed with the
serenity that results from the circular form of her orbit, and the
unchangeableness of her climates, is richer, warmer, more passionate,
more exquisite in its forms and more fascinating in its experiences,
keener of sense, capable of more delicious joys, than is possible to it
amid the manifold inclemencies of the colder earth.
We have seen that there is excellent authority for saying that Venus's
atmosphere is from one and a half to two times as dense and as extensive
as ours. Here is an interesting suggestion of aerial possibilities for
her inhabitants. If man could but fly, how would he take to himself
wings and widen his horizons along with the birds! Give him an
atmosphere the double in density of that which now envelopes him, take
off a little of his weight, thereby increasing the ratio of his strength
and activity, put into his nervous system a more puissant stimulus from
the life-giving sun, and perchance he _would_ fly.
Well, on Venus, apparently, these very conditions actually exist. How,
then, do intellectual creatures in the world of Venus take wing when
they choose? Upon what spectacle of fluttering pinions afloat in
iridescent air, like a Raphael dream of heaven and its angels, might we
not look down if we could get near enough to our brilliant evening star
to behold the intimate splendors of its life?
As Venus herself would be the most brilliant member of the celestial
host to an observer stationed on the night side of Mercury, so the earth
takes precedence in the midnight sky of Venus. For the inhabitants of
Venus Mercury is a splendid evening and morning star only, while the
earth, being an outer planet, is visible at times in that part of the
sky which is directly opposite to the place of the sun. The light
reflected from our planet is probably less dazzling than that which
Venus sends to us, both because, at our greater distance, the sunlight
is less intense, and because our rarer atmosphere reflects a smaller
proportion of the rays incident upon it. But the earth is, after all, a
more brilliant phenomenon seen from Venus than the latter is seen from
the earth, for the reason that the entire illuminated disk of the earth
is presented toward our sister planet when the two are at their nearest
point of approach, whereas, at that time, the larger part of the surface
of Venus that is turned earthward has no illumination, while the
illuminated portion is a mere crescent.
Owing, again, to the comparative rarity of the terrestrial atmosphere,
it is probable that the inhabitants of Venus--assuming their
existence--enjoy a superb view of the continents, oceans, polar snows,
and passing clouds that color and variegate the face of the earth. Our
astronomers can study the full disk of Venus only when she is at her
greatest distance, and on the opposite side of the sun from us, where
she is half concealed in the glare. The astronomers of Venus, on the
other hand, can study the earth under the most favorable conditions of
observation--that is to say, when it is nearest to them and when, being
in opposition to the sun, its whole disk is fully illuminated. In fact,
there is no planet in the entire system which enjoys an outlook toward a
sister world comparable with that which Venus enjoys with regard to the
earth. If there be astronomers upon Venus, armed with telescopes, it is
safe to guess that they possess a knowledge of the surface of the earth
far exceeding in minuteness and accuracy the knowledge that we possess
of the features of any heavenly body except the moon. They must long ago
have been able to form definite conclusions concerning the meteorology
and the probable habitability of our planet.
It certainly tends to increase our interest in Venus when, granting
that she is inhabited, we reflect upon the penetrating scrutiny of which
the earth may be the object whenever Venus--as happens once every 584
days--passes between us and the sun. The spectacle of our great planet,
glowing in its fullest splendor in the midnight sky, pied and streaked
with water, land, cloud, and snow, is one that might well excite among
the astronomers of another world, so fortunately placed to observe it,
an interest even greater than that which the recurrence of total solar
eclipses occasions upon the earth. For the inhabitants of Venus the
study of the earth must be the most absorbing branch of observational
astronomy, and the subject, we may imagine, of numberless volumes of
learned memoirs, far exceeding in the definiteness of their conclusions
the books that we have written about the physical characteristics of
other members of the solar system. And, if we are to look for attempts
on the part of the inhabitants of other worlds to communicate with us by
signals across the ether, it would certainly seem that Venus is the
most likely source of such efforts, for from no other planet can those
features of the earth that give evidence of its habitability be so
clearly discerned. Of one thing it would seem we may be certain: if
Venus has intellectual inhabitants they possess far more convincing
evidence of our existence than we are likely ever to have of theirs.
In referring to the view of the earth from Mercury it was remarked that
the moon is probably visible to the naked eye. From Venus the moon is
not only visible, but conspicuous, to the naked eye, circling about the
earth, and appearing at times to recede from it to a distance of about
half a degree--equal to the diameter of the full moon as we see it. The
disk of the earth is not quite four times greater in diameter than that
of the moon, and nowhere else in the solar system is there an instance
in which two bodies, no more widely different in size than are the moon
and the earth, are closely linked together. The moons of the other
planets that possess satellites are relatively so small that they
appear in the telescope as mere specks beside their primaries, but the
moon is so large as compared with the earth that the two must appear, as
viewed from Venus, like a double planet. To the naked eye they may look
like a very wide and brilliant double star, probably of contrasted
colors, the moon being silvery white and the earth, perhaps, now of a
golden or reddish tinge and now green or blue, according to the part of
its surface turned toward Venus, and according, also, to the season that
chances to be reigning over that part.
Such a spectacle could not fail to be of absorbing interest, and we can
not admit the possibility of intelligent inhabitants on Venus without
supposing them to watch the motions of the moon and the earth with the
utmost intentness. The passage of the moon behind and in front of the
earth, and its eclipses when it goes into the earth's shadow, could be
seen without the aid of telescopes, while, with such instruments, these
phenomena would possess the highest scientific interest and importance.
Because the earth has a satellite so easily observable, the astronomers
of Venus could not remain ignorant of the exact mass of our planet, and
in that respect they would outstrip us in the race for knowledge, since,
on account of the lack of a satellite attending Venus, we have been able
to do no more than make an approximate estimate of her mass.
With telescopes, too, in the case of a solar eclipse occurring at the
time of the earth's opposition, they could see the black spot formed by
the shadow of the moon, where the end of its cone moved across the earth
like the point of an invisible pencil, and could watch it traversing
continents and oceans, or thrown out in bold contrast upon the white
background of a great area of clouds. Indeed, the phenomena which our
globe and its satellite present to Venus must be so varied and wonderful
that one might well wish to visit that planet merely for the sake of
beholding them.
Thus far we have found so much of brilliant promise in the earth's twin
sister that I almost hesitate to approach another phase of the subject
which may tend to weaken the faith of some readers in the habitability
of Venus. It may have been observed that heretofore nothing has been
said as to the planet's rotation period, but, without specifically
mentioning it, I have tacitly assumed the correctness of the generally
accepted period of about twenty-four hours, determined by De Vico and
other observers. This period, closely accordant with the earth's, is, as
far as it goes, another argument for the habitability of Venus.
But now it must be stated that no less eminent an authority than
Schiaparelli holds that Venus, as well as Mercury, makes but a single
turn on its axis in the course of a revolution about the sun, and,
consequently, is a two-faced world, one side staring eternally at the
sun and the other side wearing the black mask of endless night.
Schiaparelli made this announcement concerning Venus but a few weeks
after publishing his discovery of Mercury's peculiar rotation. He
himself appears to be equally confident in both cases of the
correctness of his conclusions and the certainty of his observation. As
with Mercury, several other observers have corroborated him, and
particularly Percival Lowell in this country. Mr. Lowell, indeed, seems
unwilling to admit that any doubt can be entertained. Nevertheless, very
grave doubt is entertained, and that by many, and probably by the
majority, of the leading professional astronomers and observers. In
fact, some observers of great ability, equipped with powerful
instruments, have directly contradicted the results of Schiaparelli and
his supporters.
The reader may ask: "Why so readily accept Schiaparelli's conclusions
with regard to Mercury while rejecting them in the case of Venus?"
The reply is twofold. In the first place the markings on Venus, although
Mr. Lowell sketched them with perfect confidence in 1896, are, by the
almost unanimous testimony of those who have searched for them with
telescopes, both large and small, extremely difficult to see,
indistinct in outline, and perhaps evanescent in character. The sketches
of no two observers agree, and often they are remarkably unlike. The
fact has already been mentioned that Mr. Lowell noticed a kind of veil
partially obscuring the markings, and which he ascribed, no doubt
correctly, to the planet's atmosphere. But he thinks that,
notwithstanding the atmospheric veil, the markings noted by him were
unquestionably permanent features of the planet's real surface.
Inasmuch, however, as his drawings represent things entirely different
from what others have seen, there seems to be weight in the suggestion
that the radiating bands and shadings noticed by him were in some manner
illusory, and perhaps of atmospheric origin.
If the markings were evidently of a permanent nature and attached to the
solid shell of the planet, and if they were of sufficient distinctness
to be seen in substantially the same form by all observers armed with
competent instruments, then whatever conclusion was drawn from their
apparent motion as to the period of the planet's rotation would have to
be accepted. In the case of Mercury the markings, while not easily seen,
appear to be sufficiently distinct to afford confidence in the result of
observations based upon them, but Venus's markings have been represented
in so many different ways that it seems advisable to await more light
before accepting any extraordinary, and in itself improbable, conclusion
based upon them.
It should also be added that in 1900 spectroscopic observations by
Belopolski at Pulkova gave evidence that Venus really rotates rapidly on
her axis, in a period probably approximating to the twenty-four hours of
the earth's rotation, thus corroborating the older conclusions.
Belopolski's observation, it may be remarked, was based upon what is
known as the Doppler principle, which is employed in measuring the
motion of stars in the line of sight, and in other cases of rapidly
moving sources of light. According to this principle, when a source of
light, either original or reflected, is approaching the observer, the
characteristic lines in its spectrum are shifted toward the blue end,
and when it is retreating from the observer the lines are shifted toward
the red end. Now, in the case of a planet rotating rapidly on its axis,
it is clear that if the observer is situated in, or nearly in, the plane
of the planet's equator, one edge of its disk will be approaching his
eye while the opposite edge is retreating, and the lines in the spectrum
of a beam of light from the advancing edge will be shifted toward the
blue, while those in the spectrum of the light coming from the
retreating edge will be shifted toward the red. And, by carefully noting
the amount of the shifting, the velocity of the planet's rotation can be
computed. This is what was done by Belopolski in the case of Venus, with
the result above noted.
Secondly, the theory that Venus rotates but once in the course of a
revolution finds but slight support from the doctrine of tidal friction,
as compared with that which it receives when applied to Mercury. The
effectiveness of the sun's attraction in slowing down the rotation of a
planet through the braking action of the tides raised in the body of the
planet while it is yet molten or plastic, varies inversely as the sixth
power of the planet's distance. For Mercury this effectiveness is nearly
three hundred times as great as it is for the earth, while for Venus it
is only seven times as great. While we may admit, then, that Mercury,
being relatively close to the sun and subject to an enormous braking
action, lost rotation until--as occurred for a similar reason to the
moon under the tidal attraction of the earth--it ended by keeping one
face always toward its master, we are not prepared to make the same
admission in the case of Venus, where the effective force concerned is
comparatively so slight.
It should be added, however, that no certain evidence of polar
compression in the outline of Venus's disk has ever been obtained, and
this fact would favor the theory of a very slow rotation because a
plastic globe in swift rotation has its equatorial diameter increased
and its polar diameter diminished. If Venus were as much flattened at
the poles as the earth is, it would seem that the fact could not escape
detection, yet the necessary observations are very difficult, and Venus
is so brilliant that her light increases the difficulty, while her
transits across the sun, when she can be seen as a round black disk, are
very rare phenomena, the latest having occurred in 1874 and 1882, and
the next not being due until 2004.
Upon the whole, probably the best method of settling the question of
Venus's rotation is the spectroscopic method, and that, as we saw, has
already given evidence for the short period.
Even if it were established that Venus keeps always the same face to the
sun, it might not be necessary to abandon altogether the belief that she
is habitable, although, of course, the obstacles to that belief would be
increased. Venus's orbit being so nearly circular, and her orbital
motion so nearly invariable, she has but a very slight libration with
reference to the sun, and the east and west lunes on her surface, where
day and night would alternate once in her year of 225 days, would be so
narrow as to be practically negligible.
But, owing to her extensive atmosphere, there would be a very broad band
of twilight on Venus, running entirely around the planet at the inner
edge of the light hemisphere. What the meteorological conditions within
this zone would be is purely a matter of conjecture. As in the case of
Mercury, we should expect an interchange of atmospheric currents between
the light and dark sides of the planet, the heated air rising under the
influence of the unsetting sun in one hemisphere, and being replaced by
an indraught of cold air from the other. The twilight band would
probably be the scene of atmospheric conflicts and storms, and of
immense precipitation, if there were oceans on the light hemisphere to
charge the air with moisture.
It has been suggested that ice and snow might be piled in a vast circle
of glaciers, belting the planet along the line between perpetual day
and night, and that where the sunbeams touched these icy deposits near
the edge of the light hemisphere a marvelous spectacle of prismatic
hills of crystal would be presented!
It may be remarked that it would be the inhabitants of the dark
hemisphere who would enjoy the beautiful scene of the earth and the moon
in opposition.