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M. Arago, in his autobiography, gives movably in the heavens, it might have an amusing, but perhaps an exaggerated, been easy, or, at least, it might have sketch of his own share in these labours. been deemed easy, to compare his He tells us that he commenced by pacing distance with the size of the earth. to and fro, for the space of six months, But the sun wanders among the stars on the narrow platform of a rock which and rolls round the earth, and thus overlooks the Mediterranean, to watch seems to defy the efforts of the measurer. for the signal-light from the island of It was the good fortune of James Iviza. From this airy spot he was Gregory to point out a method by which transferred to the closer atmosphere of his distance may be determined, spite the castle of Belver, wounded, and a of his unsteadiness. The orbits of the prisoner. Here he had the satisfaction two planets, Mercury and Venus, lie of reading in the Spanish papers a de- between the sun and the earth, so that tailed account of his own execution. those planets occasionally cross the face Judging that the announcement was of the sun-Mercury frequently, Venus but the prelude to the event, he looked more rarely. It occurred to Gregory about for the means of escape. From that observers at different parts of the the window of his prison he finds he earth's surface would witness a transit can leap into the sea, and he resolves across different parts of the sun-one on doing so ; conceiving, as he says, seeing it cross the centre, another observ" that it is as well to be drowned as to ing it graze the edge. And, as the time it be hanged.” But he is not drowned. took in crossing might be readily ascerHe reaches a ship, and is conveyed to tained in either case, the places at which the coast of Africa, where he finds the it crossed would be thereby determined. Moors almost as uncivilized as the And thus, knowing the positions of the Spaniards. So he is not sorry when he two places of observation, and the coris allowed to return to his work. Once responding positions of the projection more in Spain, he is not long in dis- of the planet on the sun's disk, the covering that brigandage is one of the determination of the distance of the institutions of the country. His tem- sun would, by a little help from theory, porary station, on the top of a mountain be reduced to a mere matter of triangles. near Culléra, is visited, one stormy Perhaps Gregory hardly appreciated the night, by the chief bandit of the district. full value of the suggestion he was The astronomer makes him his friend, making. At any rate, nothing followed and the work proceeds merrily under the publication of his hint for a great his protection.
number of years. At length, about the Enough. We have measured the earth, beginning of the last century, it assumed, but we are a great way from the stars in the mind of Halley, the definite and still. Our yard measure has brought practicable form which renders it now us thousands of miles on our journey; the corner-stone of astronomy. Halley but the stars are millions of millions of perceived that the planet Venus was miles away, and how are we to get at greatly to be preferred to Mercury for them? We shall see. Remember, then, the determination of the sun's distance that, when we had a base line of a few from the earth. His lucid statements miles, we could determine the distance and earnest exhortations aroused the of an object seen from either end, by whole astronomical world, and a transit means of angles alone. In the same of Venus was anxiously awaited. Halley way, we get at the distance of the sun, himself, indeed, when he directed ator of a planet, by the longer base-line tention to the importance of the method, of the earth itself. We get at it roughly, had no hope of living to see it tested. it must be confessed. Copernicus, He stood like Moses on the top of Tycho, even Kepler himself, had no idea Pisgah, and looked on the Promised that the sun is so far from us as he Land; but to cross the Jordan was not really is. Had the sun been fixed im- his earthly lot. He had been laid with
his fathers many a year before the oc- for the second transit. But, alas ! alas! currence of the transit from which he after eight years of weary waiting, a had prepared men to expect so much. little cloud effectually hid the phenomeAt length, in 1761, the looked-for time non from his sight, and Le Gentil had arrived. Now transits, which are of to return to France empty as he left it. very rare occurrence, when they do Poor Le Gentil! for him there is no happen, occur in pairs, at an interval of cross of honour in life, no national only eight years. Thus, when, after monument at death. He is like the anxious waiting, astronomers beheld the poor subaltern who leads the forlorn transit of 1761, they knew that in eight hope, and perishes in an unsuccessful years they should witness another. It attack. Let us drop a tear to his was probably this circumstance of a memory and that of Green ere we prosecond transit to fall back upon that claim that the stronghold has fallen! rendered the observations of 1761 so The solar system is now measured. little worth. That date being past, and The distance of the sun is now ascertained the occasion lost, the succeeding transit with positive certainty. Seven different of 1769 was all that the world had to base-lines, a host of independent obserrely on for another century. Had this vations, all concur in giving the distance opportunity been again lost, what a dif- of the sun from the earth (in round ferent position would our astronomy and numbers) as ninety-five millions of miles. our navigation have been in from that It is a grand era in astronomy. What which they now occupy! Happily, all would Copernicus, what would Tycho Europe was astir. Men were sent out have said? They, worthy men, great north and south, east and west, to make astronomers as they were, never dreamt the whole length and breadth of the that the sun is a tenth part as far away. globe available base - lines. England Even Halley, when he proposed this fitted out an expedition to the South most successful problem, laboured under Seas, and placed it under the command the delusion that he was some thirty of Captain Cook. Who has not read millions of miles nearer the sun than he Cook's first voyage ? Most of us have actually was. devoured it, every part but the account Well, we have extended our yardof the observation of the transit, the measure to a pretty good length now. real object of the expedition. Possibly As the earth goes round the sun every it would have been otherwise had the year in an orbit nearly circular, the astronomer Green returned to tell his position we shall occupy six months own tale. But it was not so to be. hence will be just a hundred and ninety His body was consigned to the deep millions of miles from where we now are. during the homeward voyage. But his And we can observe a star from both ends observation was made under favourable of this line, just as we observed a steeple circumstances, and is invaluable. In previously from the two ends of a field. this respect, Green was happier than Our measuring tape for the stars is a some of his fellow - labourers. The hundred and ninety millions of miles. Abbé Chappe erected his observatory Yet, great as this distance is, so inconin California, and died ere his work was ceivably far away are the stars, that all well complete. M. Le Gentil had been the refinements of modern science were sent out to Pondicherry to observe the unable, half a century ago, to deduce previous transit of 1761; but the winds anything about them but this negative and the waves detained him on ship- conclusion—that the nearest of them is board until after the event had taken at least a hundred thousand times as far place. But Le Gentil was a man of from us as spring is from autumn, or spirit, not easily discouraged. Ac- summer from winter-a hundred thoucordingly, he resolved to lessen the sand times a hundred and ninety milchance of a second disappointment, by lions of miles; no star nearer than
tances as these—the mind is unable to grasp them. Dobrizhoffer, the Jesuit missionary, tells us that the Abipones of Paraguay, ainongst whom he laboured, have no better mode of expressing numbers. above a score or so, than by taking up a handful of sand or grass and exhibiting it. They had to pass through a deal of schooling to learn to count up to a thousand. The Professor at Angers, wishing to exhibit to his class the relative magnitudes of the sun and the earth, poured sixteen pecks of wheat on his lecture table. “This,” said he,“ represents the sun, and one of the grains represents the earth.” If we try a similar method, we shall not succeed so well. Let us, however, try. You have some faint idea of three thousand miles, from having painfully measured it on the Atlantic, it may be The thirtieth of an inch, on the other hand, you can estimate well enough. It is the dot you place over the letter i, as you write. Well, suppose this dot to represent the distance between Liverpool and New York; then will the actual distance-three thousand miles—represent the interval, nearer than which there is no fixed star. Three thousand miles of dots, when each separate dot stands for three thousand miles ! Or you may help your mind, or cheat yourself into the belief that you do so, by some such process as the following. Light travels with such a velocity, that it would fly round the earth, at the equator, eight times in a second. Yet there is no star so near us, but that its light occupies more than three years on its journey to the earth. The whole starry firmament, seemingly so bright, may, for ought we know, have been quenched in everlasting darkness, three years ago. Were such a catastrophe conceivable, the lamps of heaven would go out, one by one, to mortal eyes, year after year, and century after century, until, some two thousand years hence, the faint light of stars of the sixth and seventh magnitude would alone hold on its journey.
All that was known about the dis
ago, was this negative fact. No star nearer than the parallactic unit, as it is called, of twenty millions of millions of miles! Whether any were so near, or anything approaching the distance, nobody could say. At length the question of distance was resolved. And here occurs one of those singular duplications—twins in the births of thought
—with which the history of science abounds. The first determination of the distance of a star from the earth was worked out simultaneously by two men, under circumstances which precluded the possibility of mutual assistance; and the results were presented to the world within a few days of each other. The memoir of Bessel, which announced a sensible parallax for 61 Cygni, appeared on the 13th of December, 1838. That of Professor Henderson, in which the parallax of a Centauri was established, was read to the Astronomical Society on the 6th of January, 1839, and had of course been in the hands of the Society some days previously. There was no desire on the part of either astronomer to contest the claims of the other. Many years subsequently it was my good fortune to unite with Professor Henderson in entertaining his illustrious friend, Bessel ; and it was a gratifying sight to witness the warmth of affection with which these two good men welcomed each other as fellow-workers in the same field. They have both gone to their rest-Henderson too early for science ; Bessel at an advanced age, and full of honours.
The stars which Henderson and Bessel selected were in one respect very unlike. That of Henderson is a bright star in the southern hemisphere; that of Bessel is a faint inconspicuous star in the northern. But the stars have one thing in common--both have large proper motions. They are not fixed stars, in the strict sense of the word ; they move on by a few seconds annually. And this circumstance of a proper motion was an argument in the minds of the astronomers, that those stars are in close proximity to our was the ground on which they were selected. Professor Henderson commenced his calculations with a different object, and only diverted them into the channel of distance when he ascertained the amount of proper motion which the star has. His observations were not undertaken with a view to this question; they were ordinary meridian observa tions. And it is not to be wondered at that astronomers were very cautious in admitting results so obtained, when it is considered that observations of this kind are beset with such numerous sources of error, in refraction, aberration, and the like. The method adopted by Bessel, on the other hand, obviates those sources of error. It has some analogy to the method of obtaining the distance of the run by means of a transit of Venus, inasmuch as the observations are not those of the absolute position of one body, but of the relative positions of two.
The basis on which the operations are conducted is this : Certain stars are so nearly in the same direction in the heavens as not to be easily separated. Some of these are in reality doubletwin stars revolving about each other at any rate, physically connected. Others have no such connexion; and it is argued that, in certain cases, the smaller of the two is likely to be at an enormous distance behind the other. When such is actually the case, there will be a change of the relative positions of the two as viewed from different parts of the earth's orbit, and the amount of that change will depend on the proximity of the nearer star to our system, in precisely the same way as a tree will shift its place more or less rapidly, with respect to a distant hill, as the spectator is carried along in his journey. It is on stars so circumstanced that observations with the view of detecting a parallax were in stituted by Bessel. No absolute measures of position of either star are required; simply the relative distances and directions of the one with respect to the other. Thus all sources of error due to refraction, aberration, and many other causes, which equally effect both stars,
The conclusion may be stated in a single sentence. The star selected by Henderson is only a little beyond the parallactic unit (twenty millions of millions of miles); that selected by Bessel is about three times as far away. Other stars have been reached, but these two are the nearest known. With a trembling and uncertain hand, astronomers have stretched out their line to one or two stars ten times as far away as the farthest of these. But the great host of heaven lie incalculably farther back. Shall we ever reach them ? Judging from present appearances, we are compelled to answer in the negative. The stars, as we gaze into the sky, seem to defy us. For what do we see there? Close around us we see bright lamps pretty equally distributed over the vault of heaven. They twinkle and dance before us, as though conscious of the close proximity of our gaze. But let us look again. Clasping the whole vault of heaven, we see a belt of faint light, some twelve degrees in breadth. This is the milky way, the galactic circle. To the ancients, it was part of the milk which washed the purple stains from the lily ; to the moderns, it is the universe itself-the stupendous whole, of which the brighter stars are but the portions which lie nearest to this little spot of earth. You may understand this if you bear in mind that the spherical appearance of the heavens is a necessary consequence of vast and unknown distance. There is no reality in this appearance. The arrangement of the stars is somewhat like an extended sheet of cardboard, of small thickness. Or, rather, you should imagine a vast plain planted with orange trees, all loaded with yellow fruit These oranges in countless myriads are the stars. We are situated near the centre of this grove. Our sun is a small orange; the earth and the planets are tiny buds grouped around it. The neighbouring branches are thinly supplied with fruit, and few fruit-stalks bear more than a single orange. But the grove is of boundless extent. Look
myriads of golden balls, extending away What imagination shall wing its flight right and left, until individual oranges to those still more shadowy groups are no longer distinguishable, except by which constitute the unresolved nebulæ ? the glow of light which they send to The yard - measure is too puny; the the eye. This glow is the milky way. hand of man is too feeble. An angel's Looking upwards, or downwards, from hand must grasp the rod that shall the milky way, there is no such pro- mete out the length and breadth of fusion of scattering. Much bright fruit this golden grove. Man has gone up does, indeed, cluster on the upper and through the immensity of space and lower branches ; and an unpractised eye strained his line till it will bear no is deceived into the belief that the more. Other generations may mount number is infinite. But the eye of an higher, but only to find the vast circles astronomer, armed with proper instru- ever widening beyond. The position ments, finds it far otherwise. He can which we have reached is a lofty one; count the stars; he can gauge the but, lofty as it is, future ages shall use heavens; and the conclusion to which it as their point of departure. It is an he will arrive is, that the number ennobling thought to console us amid which the eye takes in diminishes our many failures. Man rises by the gradually from the galactic circle up- aid of that Divine faculty which pertains wards or downwards. And this dimi to him alone of all created beings—the nution is not only regular, but is very faculty of accumulating stores of knowgreat indeed. From such considerations ledge, of working in succession, of as these, conjecture has ripened into acting on intelligence transmitted from conviction, that the solar system is a age to age. The great English philosopart of the milky way; that the scat- pher, Bacon, describes man as the “intered bright stars are those parts of the terpreter of nature.” But this is not same which lie in our immediate neigh- his highest, not his characteristic desigbourhood; and that the whole group nation ; for, are not the beasts, are not forms a vast, extended, rolling prairie of the birds, are not the very insects stars. The milky way is, therefore, to interpreters of nature? It is as the human apprehension, nothing less than interpreter of man, the interpreter of the universe itself. True, there may be man's records, that man stands disother galactic systems, other prairies, tinguished. Herein reason transcends other orange groves, as far separated instinct, that its gifts are transmissive from ours as the prairies of America are and cumulative. Mind does not stand from the groves of Europe. Some of the supported by the mind which exists remarkable nebulæ seem to hint at the around it, not simply, not mainly. possibility of the thing. On such a There is a higher and a broader support. subject it is premature to speculate. The minds of the great of bygone ages Now, it is only those oranges that live and work in the breasts of their cluster round us, those which grow on successors. The old Greeks, I suppose, the same branch with our sun, that we knew this, and embodied it in the fable have succeeded in stretching out our of Athene, the goddess of knowledge, hand to. What arithmetic shall suffice who sprang into existence not as a to count the distance of those which lie naked, helpless child, but as a grown-up on the remoter trees of our grove, the being, clad in complete armour, from the faintest groups of the milky way? head of Zeus.