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enlargement of our base to the vast diameter of the earth's orbit, the next step in our survey (art. 227.) would be made at a great advantage; that our change of station, from side to side of it, would produce a perceptible and measurable amount of annual parallax in the stars, and that by its means we should come to a knowledge of their distance. But, after exhausting every refinement of observation, astronomers have been unable to come to any positive and coincident conclusion upon this head; and it seems, therefore, demonstrated, that the amount of such parallax, even for the nearest fixed star which has hitherto been examined with the requisite attention, reinains still mixed up with, and concealed among, the errors incidental to all astronomical determinations. Now, such is the nicety to which these have been carried, that did the quantity in ques. tion amount to a single second (i.e. did the radius of the earth's orbit subtend at the nearest fixed star that minute angle) it could not possibly have escaped detection and universal recognition.

(589.) Radius is to the sine of 1", in round numbers, as 200000 to 1. In this proportion, then, at least, must the distance of the fixed stars from the sun exceed that of the sun from the earth. The latter distance, as we have already seen, exceeds the earth's radius in the proportion of 24000 to 1; and, lastly, to descend to ordinary standards, the earth's radius is 4000 of our miles. The distance of the stars, then, cannot be so small as 4800000000 radii of the earth, or 19200000000000 miles! How much larger it may be, we know not.

(590.) In such numbers, the imagination is lost. The only mode we have of conceiving such intervals at all is by the time which it would require for light to traverse them. Now light, as we know,'travels at the rate of 192000 miles per second. It would, therefore, occupy 100000000 seconds, or upwards of three years, in such a journey, at the very lowest estimate. What, then, are we to allow for the distance of those

innumerable stars of the smaller magnitudes which the telescope discloses to us! If we admit the light of a star of each magnitude to be half that of the magnitude next above it, it will follow that a star of the first magnitude will require to be removed to 362 times its distance to appear no larger than one of the sixteenth. It follows, therefore, that among the countless multitude of such stars, visible in telescopes, there must be many whose light has taken at least a thousand years to reach us; and that when we observe their places, and note their changes, we are, in fact, reading only their history of a thousand years' date, thus wonderfully recorded. We cannot escape this conclusion, but by adopting as an alternative an intrinsic inferiority of light in all the smaller stars of the milky way. We shall be better able to estimate the probability of this alternative, when we have made acquaintance with other sidereal systems, whose existence the telescope discloses to us, and whose analogy will satisfy us that the view of the subject we have taken above is in perfect harmony with the general tenour of astronomical facts.

(591.) Quitting, however, the region of speculation, and confining ourselves within limits which we are sure are less than the truth, let us employ the negative knowledge we have obtained respecting the distances of the stars to form some conformable estimate of their real magnitudes. Of this, telescopes afford us no direct information. The discs which good telescopes show us of the stars are not real, but spurious— a mere optical illusion. * Their light, therefore, must be our only guide. Now Dr. Wollaston, by direct photometrical experiments, open, as it would seem, to no objectionst, has ascertained the light of Sirius, as received by us, to be to that of the sun as 1 to 20000000000. The sun, therefore, in order that it should appear to us no brighter than Sirius, would require to be removed to 141400 times its actual distance. We have seen, however, that the distance of Sirius cannot be so small as 200000 times

* See Cab. Cyc. Optics. + Phil. Trans. 1829, p. 24.

that of the sun. Hence it follows, that, upon the lowest possible computation, the light really thrown out by Sirius cannot be so little as double that emitted by the sun; or that Sirius must, in point of intrinsic splendour, be at least equal to two suns, and is in all probability vastly greater. *

(592.) Now, for what purpose are we to suppose such magnificent bodies scattered through the abyss of space ? Surely not to illuminate our nights, which an additional moon of the thousandth part of the size of our own would do much better, nor to sparkle as a pageant void of meaning and reality, and bewilder us among vain conjectures. Useful, it is true, they are to man as points of exact and permanent reference; but he must have studied astronomy to little purpose, who can suppose man to be the only object of his Creator's care, or who does not see in the vast and wonderful apparatus around us provision for other races of animated beings. The planets, as we have seen, derive their light from the sun; but that cannot be the case with the stars. These doubtless, then, are themselves suns, and may, perhaps, each in its sphere, be the presiding center round which other planets, or bodies of which we can form no conception from any analogy offered by our own system, may be circulating.

(593.) Analogies, however, more than conjectural, are not wanting to indicate a correspondence between the dynamical laws which prevail in the remote regions of the stars and those which govern the motions of our own system. Wherever we can trace the law of periodicity — the regular recurrence of the same phænomena in the same times — we are strongly impressed with the idea of rotatory or orbitual motion. Among the stars are several which, though no way distinguishable from others by any apparent change of place, nor by any difference of appearance in telescopes, yet un

* Dr. Wollaston, assuming, as we think he is perfectly justified in doing, a much lower limit of possible parallax in Sirius than we have adopted in the text, has concluded the intrinsic light of Sirius to be nearly that of fourteen suns.

dergo a regular periodical increase and diminution of lustre, involving, in one or two cases, a complete extinction and revival. These are called periodical stars. One of the most remarkable is the star Omicron, in the constellation Cetus, first noticed by Fabricius in 1596. It appears about twelve times in eleven years, — or, more exactly, in a period of 334 days; remains at its greatest brightness about a fortnight, being then, on some occasions, equal to a large star of the second magnitude; decreases during about three months, till it becomes completely invisible, in which state it remains during about five months, when it again becomes visible, and continues increasing during the remaining three months of its period. Such is the general course of its phases. It does not always, however, return to the same degree of brightness, nor increase and diminish by the same gradations. Hevelius, indeed, relates (Lalande, art. 794.) that during the four years between October, 1672, and December, 1676, it did not appear at all.

(594.) Another very remarkable periodical star is that called Algol, or B Persei. It is usually visible as a star of the second magnitude, and such it continues for the space of 24 14h, when it suddenly begins to diminish in splendour, and in about 3 hours is reduced to the fourth magnitude. It then begins again to increase, and in 3 hours more is restored to its usual brightness, going through all its changes in 24 204 48m, or thereabouts. This remarkable law of variation certainly appears strongly to suggest the revolution round it of some opaque body, which, when interposed between us and Algol, cuts off a large portion of its light; and this is accordingly the view taken of the matter by Goodricke, to whom we owe the discovery of this remarkable fact *, in the year 1782; since which time the same phænomena have continued to be observed, though with much less diligence, than their high interest would appear to merit. Taken any how, it is an indication of a high degree of activity, in regions where, but for such evidences, we might conclude all lifeless. Our own sun requires nine times this period to perform a revolution on its own axis. On the other hand, the periodic time of an opaque revolving body, sufficiently large, which should produce a similar temporary obscuration of the sun, seen from a fixed star, would be less than fourteen hours.

* The same discovery appears to have been made nearly about the same time by Palitzch, a farmer of Prolitz, near Dresden, - a peasant by station, an astronomer by nature, - who, from his familiar acquaintance with the aspect of the heavens, had been led to notice among so many thousand stars this one as distinguished from the rest by its variation, and had ascertained its period. The same Palitzch was also the first to rediscover the predicted comet of Halley in 1759, which he saw

(595.) The following list exhibits specimens of periodical stars of every variety of period, so far as they can be considered to be at present ascertained :

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The variations of these stars, however, appear to be affected, perhaps in duration of period, but certainly in extent of change, by physical causes at present unknown.

nearly a month before any of the astronomers, who, armed with their telescopes, were anxiously watching its return. These anecdotes carry us back to the era of the Chaldean shepherds.

* These letters B. Fl. and M. refer to the Catalogues of Bode, Flamsteed, and Mayer.

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