Part 25

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!

In the form of its orbit, in its movements round the sun and on its own axis, in the succession of days and seasons, and in all the phenomena governed by the great law of attraction, the earth becomes the representative of all the other planets; in studying it, we study all the heavenly bodies.

Our planet is a spheroid; that is, a sphere flattened at the two poles and enlarged at the equator, so that all the circles passing through the extremity of the polar axis form ellipses. The presumed depression of each pole is about thirteen miles, nearly a three-hundredth part of the radius of the earth; but it is not altogether certain that the two poles are equally flattened. Perhaps a contrast exists between the two hemispheres, not only in the features of their continents and the distribution of seas, but also in their geometrical shape. Be this as it may, it appears to be proved that the curvature is not exactly the same at all points of the earth at an equal distance from the poles; the meridians appear without exception to be irregular ellipses.

The dimensions of the earth, as we have already seen, are almost as nothing compared with the larger celestial bodies, and especially with the extent of space which can be explored by the telescope. If light, the speed of which has been adopted in astronomy as a term of comparison, could be diffused in a curved line, it would travel seven times round the globe in a second of time; this standard of measurement, therefore, the only one suited to the stellary field, is completely inapplicable to the surface of our globe.

The isolated globule in the immensity of space which we call the earth is not motionless, as the ancients necessarily supposed, looking upon it, as they did, as the immovable base of the firmament of heaven. Hurried on in the vortex of universal vitality, our globe is ever actuated by ceaseless motion, describing in ether a series of elliptic spirals so complicated that astronomers have not yet been able to calculate their various curves. Besides rotating on its own axis, the earth describes an ellipse round the sun, and, under the influence of this body, is drawn along from one heaven to another toward distant constellations. It also oscillates and rocks on its axis, and deviates more or less from its path, to salute, as it were, every heavenly body which meets it. It is probable that it never passes a second time through the same regions of the air; yet, if it has again to traverse the spiral line of ellipses it has already described, it would be after a cycle of so many thousands of millions of years, that the earth itself, completely transformed, would be no longer the same planet.

The motion of the earth, the immediate effects of which are the most obvious to the notice of men, is the daily rotation which takes place round an ideal axis passing through the two poles. The globe turns from right to left, or from west to east—that is, in a contrary direction to the apparent motion of the sun and stars, which seem to rise in the east and to set in the west. As the earth’s axis terminates at each pole, there is least surface-motion at those points, and the motion is the more rapid in any part of the surface of the globe the further it is from the central axis. At St. Petersburg, in 60° latitude, the speed of rotation is about nine miles a minute; in Paris, it exceeds eleven and a half miles during the same brief time; on the equatorial line, which may be looked upon as the ring of an immense wheel, the speed of the earth is twice as great as it is at 60° of latitude—that is, about eighteen miles a minute, or 528 yards a second—a rapidity equal to the flight of a 26-pound cannon-ball impelled by thirteen pounds of powder. By means of this rotatory motion, the earth presents toward the sun each of its faces alternately, and each also in turn toward the comparatively darker regions of space; the succession of day and night is thus constituted. In addition to this, the rotation of the earth is an important fact which must always be taken into account in determining the direction of fluids in motion on the surface of the globe, such as streams and rivers, also marine and atmospheric currents.

The annual revolution which the earth performs round the sun follows the line of an ellipse, one of the _foci_ of which is occupied by the central star; the eccentricity of the ellipse is nearly equal to 17/1000th of the great axis. The distance between the sun and the earth always varies according to the particular point of its orbit which the latter is traveling over. At its _aphelion_, that is, at its greatest remoteness, this distance is about 93¾ millions of miles; at the period of its _perihelion_, when the two heavenly bodies are nearest to each other, it is approximately 90,259,000 miles. The mean distance, as estimated by astronomers since the corrections of Encke, Hansen, Foucault, and Hind, is 91,839,000 miles. This extent of space is traversed by the solar rays in 8 minutes, 16 seconds; sound would take fifteen years in passing through the same distance.

As Kepler has laid down in his celebrated laws, our planet moves with an increased rapidity as it approaches nearer to the sun and travels more slowly in proportion to its distance from that luminary; but its mean speed may be estimated at nearly nineteen miles a second, or sixty times the rapidity of a ball from the cannon’s mouth. This speed, which makes one dizzy to think of, is to be added, as regards each point in the surface of the earth, to the rotatory motion which impels it round the polar axis.

After having turned round 366 times on its axis, our planet has terminated its orbicular course, and is in the same position relatively to the sun as at its starting-point; it has then accomplished its _year_.

This daily rotation of the earth round its axis produces the succession of days and nights, and, in the same way, its annual revolution round the sun causes the alternations of the seasons. If the axis of the earth, that is the ideal line which passes through its two poles, were perpendicular to the plane of its annual orbit, it is evident that the portion of the globe lighted by the sun would invariably extend from one pole to the other, and that in both hemispheres the days and nights would always consist of twelve hours each. But this is not the case. The earth performs its revolutionary movements in an inclined position; its ideal polar axis is sloped about 23° 28′ from a perpendicular to its plane, and this position is so far maintained that as regards the comparatively rapid succession of days and seasons it may be looked upon as invariable. This obliquity of axis causes continued changes in the phase presented to the sun. The portion of the earth illumined by the rays of the sun varies every day; for, although the planetary axis may appear to maintain its extremity in a fixed position as regards some point in infinite space, in respect to the sun it presents a constantly varying degree of inclination, in consequence of the continual motion of the earth. Twice during the course of the year it so happens that the solar rays fall perpendicularly upon the equator of the earth; at every other period in the annual revolution, sometimes the Northern and sometimes the Southern Hemisphere receives the greatest amount of light.

The astronomical year commences on the 20th of March, at the exact moment when the sun illumines the equator in a vertical direction, and the line of separation between light and shade passes through the two poles. The period of darkness is then equal to that of light, and admits of exactly twelve hours at all points of the earth. Hence the name of “equinox” (equality of nights). But after this day, which in the Northern Hemisphere serves as the starting-point of spring, the earth continues its translatory movement. In consequence of the inclination of its axis, the Northern Hemisphere, being turned toward the sun, receives a greater quantity of light, while the southern half of the globe is less vividly lighted. The vertical rays of the sun now fall more and more to the north of the equator, and the circle of light, far from arresting its progress at the poles, where the day of six months’ duration is commencing to dawn, extends far beyond it over the regions of the north. On the 21st of June, the day of the first solstice, the axis of the earth being deeply inclined toward the sun, this luminary shines on the zenith of the tropic of Cancer at 23½° north of the equator, and its light illumines the whole of the arctic zone, that is, the portion of the earth’s surface extending to 23½° round the North Pole. Then spring ceases and summer begins as regards the Northern Hemisphere. In the Southern Hemisphere, on the contrary, autumn is giving place to winter. Above the equator long days are prevailing, interrupted by short nights; while in the south it is the nights which last the longest. In the arctic zone the sun performs its apparent course of diurnal rotation entirely above the horizon. The six months’ day, which spring inaugurated at the North Pole, attains its high noon on the first day of summer. At the same moment midnight arrives in the darkness which is oppressing its antipodes.

Immediately after the 21st of June all the phenomena which took place during the preceding season are directly reversed. The sun appears to retrograde toward the southern horizon; its vertical rays cease to fall on the line of the northern tropic, and constantly approach the equator. The zone of light in the northern pole and of shade in the southern equally diminish, and the days shorten in the Northern Hemisphere in the same proportion as they lengthen in the Southern; an equilibrium is gradually being re-established between the two halves of the earth. On the 22d of September the position of the sun is again exactly above the equator, and its light just reaches both poles. The equinox, or the absolute equality of day and night in every part of the globe, occurs for the second time in the year; but this moment of equilibrium is, so to speak, but a mathematical point between the two seasons. The axis of the earth which, during the six months past, turned the North Pole toward the sun, now presents to him the South Pole; the vertical rays of the central luminary fall to the south of the earth’s equator, and the Southern Hemisphere, in its turn, is the best endowed of the two halves of the globe in the amount of light it receives and in the length of its days. In the Southern Hemisphere spring is commencing; in the Northern, autumn. Three months afterward, on the 21st of December, the sun comes directly over the southern tropic, or the tropic of Capricorn, 23½° south of the equator, and the whole of the antarctic zone is presented to the solar rays. Summer has begun in the Southern Hemisphere, and at the same time winter commences in that of the north. Then, as the globe moves on, these two seasons follow each other in their course, until at length the earth attains a position similar to that from which it started; the March equinox, the first day of spring in Europe, and the first day of autumn in Australia, commences anew the astronomical year.