VENUS, THE TWIN OF THE EARTH

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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,[Pg 48] 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[Pg 49] 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[Pg 50] 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[Pg 51] 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.[Pg 52]

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[Pg 53] 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[Pg 54] 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[Pg 55] 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.[Pg 56]

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.

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 suc[Pg 57]cessive 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 ac[Pg 58]quainted. 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[Pg 59] 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[Pg 60] 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.[Pg 61]

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.[Pg 62]

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, grow[Pg 63]ing, 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 nebu[Pg 64]la, 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[Pg 65] 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[Pg 66] 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[Pg 67] 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[Pg 68] 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[Pg 69] 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,[Pg 70] 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[Pg 71] 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 inter[Pg 72]est 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[Pg 73] 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[Pg 74] 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.[Pg 75]

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[Pg 76] 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[Pg 77] 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, ex[Pg 78]tremely 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 mo[Pg 79]tion 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[Pg 80] 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 effect[Pg 81]iveness 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[Pg 82] 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[Pg 83] 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,[Pg 84] 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.

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This book is part of the public domain. Garrett Putman Serviss (2006). Other Worlds. Urbana, Illinois: Project Gutenberg. Retrieved October 2022 https://www.gutenberg.org/cache/epub/18431/pg18431-images.html

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