ore, in would stance. not be follows really t emit

splenability,

hod of stars, f other

en if it till be

ll con

Let A and B (fig. 8) represent the earth at two opposite points in its orbit, and C and D two stars of different magnitudes. Then, if when the earth is at B, the two stars appear to us near each other, as at C and E, it was thought that when the earth arrived at A the two stars might appear farther apart, as at C and D; in other words, that the angles at which they would appear to us in the two cases would be different, the angle DAC being larger than the angle D B C, in which case the angle of parallax might be computed. But it does not appear that any difference in the angles referred to has yet been found, or that any definite conclusions respecting parallax have hitherto been deduced from this method, excepting the general position that the stars are at too great a distance to be subjected to our calculations, or that our angular instruments are still in too imperfect a state to detect so small an angle as that of the annual parallax.

While writing the above (December, 1838), I perceived an announcement in certain literary journals, that Professor Bessel, of Kongisberg, had addressed a letter to Sir John Herschel, which was immediately communicated to the Royal Astronomical Society, containing an account of the discovery of the annual parallax and the observations on which it was founded. In the introduction to this communication Professor Bessel says: "After so many unsuccessful attempts to determine the parallax of a fixed star, I thought it worth while to try what might be accomplished by means of the accuracy which my great Fraunhofer heliometer gives to the observations. I undertook to make this investigation upon the star 61 Cygni, which, by reason of its great proper motion, is perhaps the best of all, which affords the advantage of being a double star, and on that account may be observed with greater accuracy, and which is so near the pole that, with the exception of a small part of the year, it can always be observed at night at a sufficient distance from the horizon." The professor began his observations in September, 1834, but various circumstances prevented them from being regularly continued at that period. They were resumed in 1837 with certain hopes of success. He selected among the small stars which surround the double star 61 Cygni two stars between the ninth and tenth magnitudes, of which one (a) is nearly perpendicular to the line of direction of the double star, the other (b) nearly in this direction. He measured with the heliomo

F 2

ter the distances of these stars from the point which bisects the distance between the two stars of 61 Cygni, and generally repeated the observations sixteen times every night, and when the atmosphere was unusually steady he made more numerous repetitions. The places of both stars, referred to the middle point of the double star, he calculated, for the beginning of 1838, to be,

Distance. a 461".617

b 706".279

Angle of Position. 201° 29' 24"

109° 22' 10"

In these observations he concentrated his attention as far as he could on the distance of the small stars from the double star, as being the most important point to be ascertained. His communication contains tables of all his measures of distance, freed from the effects of refraction and aberration, and reduced to the beginning of 1838.

It would be uninteresting to the general reader to enter into all the details of observations, corrections, and calculations which Professor Bessel's communication contains, as they can only be understood by practical astronomers. I shall therefore only state his general conclusion, which seems to be legitimately deduced from his observations and reasonings, and may be considered at least as a very near approximation to the point, if not perfectly correct. The result then is, that the annual parallax of the star 61 Cygni is 0".3136; that is, somewhat less than one third of a second. It follows that the distance of this star from the sun is 657,700 times the mean distance of the earth from the sun; and as the distance of the sun from the earth is 95,000,000 of miles, this number multiplied by the former produces 62,481,500,000,000, or sixty-two BILLIONS, four hundred and eighty-one thousand five hundred MILLIONS of miles, which is the distance of the star 61 Cygni from the sun, and which, of course, nearly about the same distance from the earth; the earth being in one part of its course ninety-five millions of miles nearer the star than this distance, and in the opposite part of it ninetyfive millions of miles beyond it. This, I have no doubt, will be considered as one of the most interesting and splendid discoveries which have been made in astronomy for a century past. It lays a foundation for precise and definite conceptions of the distances of some of the starry orbs, of the am

plitude of the celestial regions, and of the magnitude and grandeur of those countless orbs which diversify the spaces of immensity. It likewise proves to a demonstration the annual motion of the earth round the sun, and all the principles and phenomena with which it is connected, as well as corroborates the general views of former astronomers respecting the immense distance of the fixed stars.

Professor Bessel concludes his communication in these words: "As the annual proper motion of a Cygni amounts to 5".123 of a great circle, the relative motion of this star and the sun must be considerably more than sixteen semidiameters of the earth's orbit [that is, one thousand five hundred and twenty millions of miles], and the star must have a constant aberration of more than 52". When we shall have succeeded in determining the elements of the motion of both the stars forming the double star, round their common centre of gravity, we shall be able to determine the sum of their masses. I have attentively considered the preceding observations of their relative positions, but I consider them as yet very inadequate to afford the elements of the orbit. I consider them as sufficient only to show that the annual angular motion is somewhere about two thirds of a degree, and that the distance at the beginning of this century had a minimum of about 15". We are enabled hence to conclude that the time of a revolution is more than 540 years, and that the semi-major axis of the orbit is seen under an angle of more than 15". If, however, we proceed from these numbers, which are merely limits, we find the sum of the masses of both stars less than half the sun's mass. But this point, which is deserving of attention, cannot be established till the observations shall be sufficient to determine the elements accurately. When long-continued observations of the places which the double star occupies among the small stars which surround it shall have led to the knowledge of its centre of gravity, we shall be enabled to determine the two masses separately; but we cannot anticipate the time of these farther researches. I have here troubled you with many particulars; but I trust it is not necessary to offer any excuse for this, since a correct opinion as to whether the investigation of the parallax of 61 Cygni has already led to an approximate result, or must still be carried farther before this can be affirmed of them, can only be formed from a knowledge of these particulars. Had I merely com

municated to you the result, I could not have expected that you would attribute to it that certainty which, according to my own judgment, it possessed."

The distance inferred from the parallax ascertained by Bessel is more than three times greater than what was formerly considered the least distance of any of the fixed stars. In order to acquire some rude conceptions of this distance, it may not be inexpedient to illustrate it by the times which certain moving bodies would require to move along such a space. Light is the swiftest moving body with which we are acquainted; it flies from the sun to the earth, a distance of ninety-five million of miles, in about eight minutes, or at the rate of 192,000 miles every moment of time; yet light, incomprehensively swift as its motion is, would require 10 years and 114 days to fly across this mighty interval; so that if the star 61 Cygni were supposed to be only just now launched into existence, it would be more than ten years before its light could reach the distant globe on which we dwell, so as to appear like a small star twinkling in our sky. Suppose a cannon ball to move 500 miles every hour without intermission, it would require fourteen millions, two hundred and fifty-five thousand, four hundred and eighteen years before it could move across the same interval. But to come to motions with which we are more familiar: suppose a steam carriage to set out from the earth with a velocity of twenty miles an hour, or 480 miles a day; at this rate of motion, continued without intermission, it would require 356,385,466, or three hundred and fifty-six millions, three hundred and eighty-five thousand, four hundred and sixty-six years before it could pass from our globe to the star alluded to above; a number of years sixty-one thousand times greater than the whole period which has elapsed since the Mosaic creation.

Such distances are amazing, and almost terrifying to the human imagination. The mind is bewildered, confounded, and almost overwhelmed, when attempting to form a conception of such portions of immensity, and feels its own littleness, the limited nature of its powers, and its utter incapacity for grasping the amplitudes of creation; but although it were possible for us to wing our flight to such a distant orb as that to which we have referred, we should still find ourselves standing on the extreme verge of the starry firmament, where ten thousands of other orbs, a thousand times more distant, would

meet our view. We have reason to believe that a space nearly equal to that which we are now considering intervenes between most of the stars which diversify our nocturnal sky. The stars appear of different magnitudes; but we have the strongest reason to conclude that in the majority of instances this is owing, not to the difference of their real magnitudes, but to the different distances at which they are placed from our globe. If, then, the distance of a star of the first or second magnitude, or those which are nearest us, be so immensely great, what must be the distance of stars of the sixteenth or twentieth magnitude, which can be distinguished only by the most powerful telescopes? Some of these must be several thousands of times more distant than the star 61 Cygni, whose distance now appears to be determined. And what shall we think of the distance of those which lie beyond the reach of the most powerful telescopes that have yet been constructed, stretching beyond the utmost limits of mortal vision, within the unexplored regions of immensity? Here even the most vigorous imagination drops its wing, and feels itself utterly unable to penetrate this mysterious and boundless unknown.

The vastness of the spaces and the greatness of the distances to which we have adverted ought not, however, to prevent any one from acquiescing in the statements we have now made; for space is boundless, absolutely infinite. A seraph might wing his flight with the swiftness of light for millions of years through the regions of immensity, and never arrive at a boundary where it might be said, "Hitherto mayest thou approach, but no farther;" and we have reason to believe, from what we already know of the Creator and his works, that during the whole course of such an excursion, new objects and new scenes of glory and magnificence would be continually rising to his view. To suppose otherwise would be to set boundaries to space, and to prescribe limits to the infinite perfections of the Divinity. That incomprehensible Being who formed the universe fills immensity with his presence; his power and wisdom, and all his other perfections, are infinite; and therefore we should expect that the plans on which he has constructed the systems of the universe should be like himself, vast, boundless, and inconceivable by mortals. Were we to find the plans of the universe circumscribed like those which were represented by the ancient astronomers, who