Other Worlds<br />Their Nature, Possibilities and Habitability in the Light of the Latest Discoveries by Garrett Putman Serviss (best ebook pdf reader android .TXT) 📗
- Author: Garrett Putman Serviss
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The Regions of Perpetual Day, Perpetual Night, and Alternate Day and Night on Mercury. In the Left-Hand View the Observer looks at the Planet in the Plane of its Equator; in the Right-Hand View he looks down on its North Pole..
Another effect of the libratory motion of the sun as seen from Mercury is represented in the next figure, where we have a view of the planet showing both the day and the night hemisphere, and where we see that between the two there is a region upon which the sun rises and sets once every eighty-eight days. There are, in reality, two of these lune-shaped regions, one at the east and the other at the west, each between 1,200 and 1,300 miles broad at the equator. At the sunward edge of these regions, once in eighty-eight days, or once in a Mercurial year, the sun rises to an elevation of forty-seven degrees, and then descends again straight to the horizon from which it rose;[Pg 39] at the nightward edge, once in eighty-eight days, the sun peeps above the horizon and quickly sinks from sight again. The result is that, neglecting the effects of atmospheric refraction, which would tend to expand the borders of the domain of sunlight, about one quarter of the entire surface of Mercury is, with regard to day and night, in a condition resembling that of our polar regions, where there is but one day and one night in the course of a year—and on Mercury a year is[Pg 40] eighty-eight days. One half of the remaining three quarters of the planet's surface is bathed in perpetual sunshine and the other half is a region of eternal night.
And now again, what of life in such a world as that? On the night side, where no sunshine ever penetrates, the temperature must be extremely low, hardly greater than the fearful cold of open space, unless modifying influences beyond our ken exist. It is certain that if life flourishes there, it must be in such forms as can endure continual darkness and excessive cold. Some heat would be carried around by atmospheric circulation from the sunward side, but not enough, it would seem, to keep water from being perpetually frozen, or the ground from being baked with unrelaxing frost. It is for the imagination to picture underground dwellings, artificial sources of heat, and living forms suited to unearthlike environment.
What would be the mental effects of perpetual night upon a race of intelligent creatures doomed to that condition? Perhaps[Pg 41] not quite so grievous as we are apt to think. The constellations in all their splendor would circle before their eyes with the revolution of their planet about the sun, and with the exception of the sun itself—which they could see by making a journey to the opposite hemisphere—all the members of the solar system would pass in succession through their mid-heaven, and two of them would present themselves with a magnificence of planetary display unknown on the earth. Venus, when in opposition under the most favorable circumstances, is scarcely more than 24,000,000 miles from Mercury, and, showing herself at such times with a fully illuminated disk—as, owing to her position within the orbit of the earth, she never can do when at her least distance from us—she must be a phenomenon of unparalleled beauty, at least four times brighter than we ever see her, and capable, of course, of casting a strong shadow.
The earth, also, is a splendid star in the midnight sky of Mercury, and the moon may be visible to the naked eye as a little at[Pg 42]tendant circling about its brilliant master. The outer planets are slightly less conspicuous than they are to us, owing to increase of distance.
The revolution of the heavens as seen from the night side of Mercury is quite different in period from that which we are accustomed to, although the apparent motion is in the same direction, viz., from east to west. The same constellations remain above the horizon for weeks at a time, slowly moving westward, with the planets drifting yet more slowly, but at different rates, among them; the nearer planets, Venus and the earth, showing the most decided tendency to loiter behind the stars.
On the side where eternal sunlight shines the sky of Mercury contains no stars. Forever the pitiless blaze of day; forever,
The bloody sun at noon."
As it is difficult to understand how water can exist on the night hemisphere, except in the shape of perpetual snow and[Pg 43] ice, so it is hard to imagine that on the day hemisphere water can ever be precipitated from the vaporous form. In truth, there can be very little water on Mercury even in the form of vapor, else the spectroscope would have given unquestionable evidence of its presence. Those who think that Mercury is entirely waterless and almost, if not quite, airless may be right. In these respects it would then resemble the moon, and, according to some observers, it possesses another characteristic lunar feature in the roughening of its surface by what seem to be innumerable volcanic craters.
But if we suppose Mercury to possess an atmosphere much rarer than that of the earth, we may perceive therein a possible provision against the excessive solar heat to which it is subjected, since, as we see on high mountains, a light air permits a ready radiation of heat, which does not become stored up as in a denser atmosphere.
As the sun pours its heat without cessation upon the day hemisphere the warmed[Pg 44] air must rise and flow off on all sides into the night hemisphere, while cold air rushes in below, to take its place, from the region of frost and darkness. The intermediate areas, which see the sun part of the time, as explained above, are perhaps the scene of contending winds and tempests, where the moisture, if there be any, is precipitated, through the rapid cooling of the air, in whelming floods and wild snow-storms driven by hurrying blasts from the realm of endless night.
Enough seems now to have been said to indicate clearly the hopelessness of looking for any analogies between Mercury and the earth which would warrant the conclusion that the former planet is capable of supporting inhabitants or forms of life resembling those that swarm upon the latter. If we would still believe that Mercury is a habitable globe we must depend entirely upon the imagination for pictures of creatures able to endure its extremes of heat and cold, of light and darkness, of instability, swift vicissitude, and violent contrast.[Pg 45]
In the next chapter we shall study a more peaceful and even-going world, yet one of great brilliancy, which possesses some remarkable resemblances to the earth, as well as some surprising divergences from it.[Pg 46]
CHAPTER III VENUS, THE TWIN OF THE EARTHWe 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[Pg 47] 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
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