It is to distances such as these that the mind unconsciously refers the celestial3 bodies. We know that the moon is about 2,000 miles in diameter, but the mind refuses to present her to us as other than a round disc much smaller than those other objects in sight which occupy a much larger portion of the field of vision. The sun cannot be conceived to exceed the moon enormously in size, seeing that he appears no larger; and all the multitude of stars are judged by the sight to be mere2 bright points of light in reality as they appear to be.
How, then, can we hope to appreciate the vastness of space whereof astronomy tells us? To the student of science attempting to conceive the immensities of whose existence he is assured, the same lesson might be taught in parable4 which the child of St. Augustine's vision taught the Numidian theologian. As reasonably might an infant hope to pour the waters of ocean into a hollow, scooped5 with his tiny fingers in the sand, as man to picture in his narrow mind the length and breadth and depth of the abysses of space in which our earth is lost.
Yet, as a picture of a great mansion6 may be so drawn7 on a small scrap8 of paper as to convey just ideas of its proportions, so may the great truths which astronomy has taught us about the depths of space be so presented that just conceptions may be formed of the proportions of at least those parts of the universe which lie within the range of scientific vision, though it would be hopeless to attempt to conceive their real dimensions.
Thus, when we learn that a globe as large as our earth, suspended beside the moon, would seem to have a diameter exceeding hers nearly four times, so that the globe would cover a space in the heavens about thirteen times as large as the moon covers, we form a just conception of the size of the moon as compared with the earth, though the mind cannot conceive such a body as the moon or the earth really is. When, in turn, we are told that if a globe as large as the earth, but glowing as brightly as the sun, were set beside the sun, it would look a mere point of light, we not only learn to picture rightly to ourselves how largely the sun exceeds the earth, but also how enormous must be the real distance of the sun.
Another step leads us to a standpoint whence we can form a correct estimate of the vast distance of the fixed9 stars; for we learn that so enormous is the distance of even the nearest fixed star, that the tremendous space separating the earth from that star sinks in turn into the merest point, insomuch that if a globe as bright as the sun had the earth's orbit as a close fitting girdle, then this glorious orb10 (with a diameter of some 184,000,000 of miles) would look very much smaller than such a globe as our earth would look at the sun's distance—would, in fact, occupy but about one-fortieth part of the space in the sky which she, though she would then look a mere point, would occupy if viewed from that distance.
But there is a way of viewing the immensities of space which, though not aiding us indeed to conceive them, enables the mind to picture their proportions better than any other. The dimensions of the earth's path around the sun sink into insignificance11 beside those of the outermost12 planets; but these in their turn dwindle13 into nothingness beside those of some among the comets. From the paths of these comets, if only sentient14 and reasoning beings could trace out in a comet's company those mighty15 orbits, and could have for the duration of their existence not the brief span of time which measures the longest human life, but many circuits of their comet home around the same ruling orb (as we live during many circuits of our globe around the sun), the dimensions of the star-depths, which even to scientific insight are all but immeasurable, would be directly discernible. Not only would the proportions of that mighty system be perceived, whose fruits and blossoms are suns and worlds, but even the gradually changing arrangement of its parts could be discerned.
Some comets, indeed, as I pointed16 out in an essay on comets several months ago (see Expanse of Heaven, p. 149), do not travel around the sun, but flit from sun to sun on journeys lasting17 millions of years, paying each sun but a single visit. A being inhabiting such a comet, and having these interstellar journeys as the years of his existence, so that he could live through many of them, would have a wonderful insight into the economy of the stellar system. If his powers of conception as far exceeded ours as the range of his travels and the duration of his existence, he would be able to recognise the proportions of a large part of the stellar universe as clearly as we recognise the proportions of the solar system.
But leaving these wonderful wanderers, whose journeys are as far beyond our powers of conception as the immensity of the regions of star-strewn space, we may find, among the comets belonging to the sun's domain18, bodies whose range of travel would give their inhabitants far clearer views of the architecture of the heavens than even the profoundest terrestrial astronomer19 can possibly obtain.
Such a comet as Halley's (fig20. 3) for instance, though one of comparatively limited range in space, yet travels so far from the sun that, from the extreme part of its path, it sees the stars displaced nearly twenty times as much (owing to its own change of position) as they are from the earth on opposite sides of her comparatively narrow orbit. And the length of this comet's year, if it indicated the length of the lives of all creatures travelling along with it, would suggest a power of patiently watching the progress of changes lasting not a few of our years only, but for centuries. Seventy-five or seventy-six years elapse between each return of this comet to the sun's neighbourhood, and one who should have lived during sixty or seventy circuits of this body around its mighty orbit would have been able to watch the rush of stars, with their velocities21 of many miles per second, until visible displacements22 had taken place in their positions.
Fig. 3.—Halley's Comet of 1835.
This, however, is as nothing compared with the mighty range in space and the enormous period of the orbit of the great comet of the year 1811 (fig. 4). This comet is, on the whole, the most remarkable23 ever known. It was visible for nearly seventeen months, and though it did not approach the sun within 100,000,000 miles, and was therefore not subject to that violence of action which has caused enormous tails to be thrown out from comets which have come within a few million miles of him, or even within less than a quarter of his own diameter, it flourished forth24 a tail 120,000,000 of miles in length. Its orbit has, according to the calculations of the astronomer Argelander, a space exceeding the earth's distance from the sun 211 times, and thus surpassing even the mighty distance of Neptune25 fully26 seven times. It occupies in circuiting this mighty path no less than 3065 of our years (with a possible error either way of about forty-three years). So that, according to Bible chronology, this comet's last appearance probably occurred during the rule of the judge Tola, son of Puah, son of Dodo, over the children of Israel, though it may have occurred during the rule of his predecessor27 Abimelech, or during that of his successor Jair.[1] During one half of the enormous interval28 between that time and 1811 the comet was rushing outwards29 into space, reaching the remotest part of its path somewhere about the year 278 (A.D.), and from that time to 1811 it was on its return journey. It is strange to think, however, that though the remotest part of its path lay 211 times farther from the sun than the earth's orbit, yet even this mighty path, requiring more than 3000 years for a single circuit, cannot be said to have carried the comet into the star-depths. If the earth were to shift its position by the same enormous amount the nearest fixed star would have its apparent position changed only by about an eighth part of the apparent diameter of the sun or moon, or by about one-quarter of the distance separating the middle star of the Bear's tail from its close companion.
Fig. 4.—Comet of 1811.
But this fact of itself is most strikingly suggestive of the vast distance of the stars. For consider what it means. Imagine the middle star of the Bear's tail to be the really nearest of all the stars instead of lying probably twenty or thirty times farther away. Conceive a comet belonging to that sun after making its nearest approach to it to travel away upon an orbit requiring 3000 years for each circuit. Then (supposing that star equal to our sun in mass), the comet, though rushing away from its sun with inconceivable velocity30 during 1500 years, would, at the end of that vast period, seem to be no farther away than one-fourth of the distance separating the sun from its near companion. Look at the middle star of the Bear's tail on any clear night, and on its small satellite, remembering this fact, and the awful immensity of the star depths are strongly impressed upon the mind. But the observer must not fail to remember that the star really is many times more remote than we have here for a moment supposed, and that such a comet's range of travel would be proportionately reduced. Moreover, many among the stars are, doubtless, hundreds, even thousands, of times still farther away.
Fig. 5—Six-tailed Comet of 1744.
Let us turn lastly to the amazing comet of the year 1744, pictured, at the time, as shown in fig. 5 (though probably the drawing is greatly exaggerated). We find that though it had the longest period of any which has ever been assigned to a comet as the result of actual mathematical calculation, yet its range in space would scarcely suffice to change the position of the stars in such sort that the aspect of the familiar constellations31 would be materially altered. Euler, the eminent32 mathematician33, calculated for this comet a period of 122,683 years, which would correspond, I find, to a distance of recession equal to 2469 times the distance of the earth from the sun, or about eighty times the distance of Neptune. Yet this is but little more than twelve times the greatest distance of the comet of 1811. Probably the actual range of such an orbit from the middle star of the Bear's tail would be equal in appearance to the range described above on the supposition that the star is no farther from us than the nearest known star (Alpha Centauri). That is, such a comet, if it could be seen and watched during a period of about 122,000 years, would seem to recede34 from the star to a distance equal to about one-fourth the space separating it from its close companion, and then to return to the point of nearest approach to its ruling sun.
Such are the immensities of star-strewn space! The journey of a comet receding35 from the sun with inconceivable velocity during hundreds of thousands of years carries it but so small a distance from him compared with the distance of the nearest star as scarcely to change the appearance of the celestial landscape; and yet the distances separating the sun from the nearest of his fellow suns are but as hair-breadths to leagues when compared with the proportions of the scheme of suns to which he belongs. These distances, though so mighty that by comparison with them the inconceivable dimensions of our own earth sink into utter nothingness, do not bring us even to the threshold of the outermost court of that region of space to which the scrutiny36 of our telescopes extends. Yet the whole of that region is but an atom in the infinity37 of space.
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1
thereby
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| adv.因此，从而 | |
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mere
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| adj.纯粹的；仅仅，只不过 | |
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celestial
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| adj.天体的；天上的 | |
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parable
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| n.寓言，比喻 | |
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scooped
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| v.抢先报道( scoop的过去式和过去分词 )；（敏捷地）抱起；抢先获得；用铲[勺]等挖（洞等） | |
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mansion
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| n.大厦，大楼；宅第 | |
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drawn
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| v.拖，拉，拔出；adj.憔悴的，紧张的 | |
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scrap
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| n.碎片；废料；v.废弃，报废 | |
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fixed
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| adj.固定的，不变的，准备好的；(计算机)固定的 | |
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orb
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| n.太阳；星球；v.弄圆；成球形 | |
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insignificance
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| n.不重要；无价值；无意义 | |
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outermost
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| adj.最外面的，远离中心的 | |
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dwindle
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| v.逐渐变小（或减少） | |
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sentient
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| adj.有知觉的，知悉的；adv.有感觉能力地 | |
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mighty
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| adj.强有力的；巨大的 | |
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pointed
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| adj.尖的，直截了当的 | |
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lasting
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| adj.永久的，永恒的；vbl.持续，维持 | |
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domain
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| n.(活动等)领域，范围；领地，势力范围 | |
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astronomer
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| n.天文学家 | |
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fig
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| n.无花果(树) | |
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velocities
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| n.速度( velocity的名词复数 )；高速，快速 | |
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displacements
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| n.取代( displacement的名词复数 )；替代；移位；免职 | |
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remarkable
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| adj.显著的，异常的，非凡的，值得注意的 | |
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forth
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| adv.向前；向外，往外 | |
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Neptune
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| n.海王星 | |
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fully
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| adv.完全地，全部地，彻底地；充分地 | |
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predecessor
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| n.前辈，前任 | |
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interval
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| n.间隔，间距；幕间休息，中场休息 | |
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outwards
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| adj.外面的，公开的，向外的；adv.向外；n.外形 | |
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velocity
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| n.速度，速率 | |
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constellations
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| n.星座( constellation的名词复数 )；一群杰出人物；一系列（相关的想法、事物）；一群（相关的人） | |
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eminent
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| adj.显赫的，杰出的，有名的，优良的 | |
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mathematician
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| n.数学家 | |
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34
recede
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| vi.退(去)，渐渐远去；向后倾斜，缩进 | |
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receding
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| v.逐渐远离( recede的现在分词 )；向后倾斜；自原处后退或避开别人的注视；尤指问题 | |
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scrutiny
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| n.详细检查，仔细观察 | |
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infinity
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| n.无限，无穷，大量 | |
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