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as respecting the real directions and velocities of the individual motions.
(858.) The velocity of the solar motion which results from M. Otto Struve's calculations is such as would carry it over an angular subtense of 0%.3392 if seen at right angles from the average distance of a star of the first magnitude. If we take, with M. Struve, senior, the parallax of such a star as probably equal to 0".209*, we shall at once be enabled to compare this annual motion with the radius of the earth's orbit, the result being 1.623 of such units. The sun then advances through space (relatively, at least, among the stars), carrying with it the whole planetary and cometary system with a velocity of 1.623 radii of the earth's orbit, or 154,185,000 miles per annum, or 422,000 miles (that is to say, nearly its own semi-diameter) per diem: in other words, with a velocity a very little greater than one-fourth of the earth's annual motion in its orbit.
(859.) Another generation of astronomers, perhaps many, must pass away before we are in a condition to decide from a more precise and extensive knowledge of the proper motions of the stars than we at present possess, how far the direction and velocity above assigned to the solar motion deviates from exactness, whether it continue uniform, and whether it show any sign of deflection from rectilinearity; so as to hold out a prospect of one day being enabled to trace out an arc of the solar orbit, and to indicate the direction in which the preponderant gravitation of the sidereal firmament is urging the central body of our system. An analogy for such deviation from uniformity would seem to present itself in the alleged existence of a similar deviation in the proper motions of Sirius and Procyon, both which stars are considered to have varied sensibly in this respect within the limits of authentic and dependable observation. Such, indeed, would appear to be the amount of evidence for this as a matter of fact as to have given rise to a speculation on the probable circulation of these stars round opaque (and therefore invisible) bodies at no great distances from them respectively, in
• Etudes d'Astronomie Stellaire, p. 107.
the manner of binary stars : [and it has been recently shown by M. Peters (Ast. Nachr. 748.) that, in the case of Sirius, such a circulation, performed in a period of 50·093 years in an ellipse whose excentricity is 0.7994, the perihelion passage taking place at the epoch A. D. 1791.431, would reconcile in a remarkable manner the observed anomalies, and reduce the residual motion to uniformity.]
(860.) The whole of the reasoning upon which the determination of the solar motion in space rests, is based upon the entire exclusion of any law either derived from observation or assumed in theory, affecting the amount and direction of the real motions both of the sun and stars. It supposes an absolute non-recognition, in those motions, of any general directive cause, such as, for example, a common circulation of all about a common center. Any such limitation introduced into the conditions of the problem of the solar motion would alter in toto both its nature and the form of its solution. Suppose for instance that, conformably to the speculations of several astronomers, the whole system of the Milky Way, including our sun, and the stars, our more immediate neighbours, which constitute our sidereal firmament, should have a general movement of rotation in the plane of the galactic circle (any other would be exceedingly improbable, indeed hardly reconcilable with dynamical principles), being held together in opposition to the centrifugal force thus generated by the mutual gravitation of its constituent stars. Except we at the same time admitted that the scale on which this movement proceeds is so enormous that all the stars whose proper motions we include in our calculations go together in a body, so far as that movement is concerned (as forming too small an integrant portion of the whole to differ sensibly in their relation to its central point); we stand precluded from drawing any conclusion whatever, not only respecting the absolute motion of the sun, but respecting even its relative movement among those stars, until we have established some law, or at all events framed some hypothesis having the provisional force of a law, connecting the whole, or a part of the motion of each individual with its situation in space.
i, situation, which he
(861.) Speculations of this kind have not been wanting in astronomy, and recently an attempt has been made by M. Mädler to assign the local center in space, round which the sun and stars revolve, which he places in the group of the Pleiades, a situation in itself improbable, lying as it does no less than 26° out of the plane of the galactic circle, out of which plane it is almost inconceivable that any general circulation can take place. In the present defective state of our knowledge respecting the proper motion of the smaller stars, especially in right ascension, (an element for the most part far less exactly ascertainable than the polar distance, or at least which has been hitherto far less accurately ascertained,) we cannot but regard all attempts of the kind as to a certain extent premature, though by no means to be discouraged as forerunners of something more decisive. The question, as a matter of fact, whether a rotation of the galaxy in its own plane exist or not might be at once resolved by the assiduous observation both in R. A. and polar distance of a considerable number of stars of the Milky Way, judiciously selected for the purpose, and including all magnitudes, down to the smallest distinctly identifiable, and capable of being observed with normal accuracy: and we would recommend the inquiry to the special attention of directors of permanent observatories, provided with adequate instrumental means, in both hemispheres. Thirty or forty years of observation perseveringly directed to the object in view, could not fail to settle the question.*
(862.) The solar motion through space, if real and not simply relative, must give rise to uranographical corrections analogous to parallax and aberration. The solar or systematic parallax is no other than that part of the proper motion of each star which is simply apparent, arising from the sun's motion, and until the distances of the stars be known, must remain inextricably mixed up with the other or rea portion. The systematic aberration, amounting at its maximum (for stars 90° from the solar apex) to about 5" displaces all the stars in great circles diverging from that apex through angles proportional to the sines of their respective distances from it. This displacement, however, is permanent, and therefore uncognizable by any phænomenon, so long as the solar motion remains invariable ; but should it, in the course of ages, alter its direction and velocity, both the direction and amount of the displacement in question would alter with it. The change, however, would become mixed up with other changes in the apparent proper motions of the stars, and it would seem hopeless to attempt disentangling them.
* An examination of the proper motions of the stars of the B. Assoc. Catal, in the portion of the Milky Way nearest either pole (where the motion should be almost wholly in R A) indicates no distinct symptom of such a rotation. If the question be taken up fundamentally, it will involve a redetermination from the recorded proper motions, both of the precession of the equinoxes and the change of obliquity of the ecliptic.
(863.) A singular, and at first sight paradoxical effect of the progressive movement of light, combined with the proper motion of the stars, is, that it alters the apparent periodic time in which the individuals of a binary star circulate about each other.* To make this apparent, suppose them to circulate round each other in a plane perpendicular to the visual ray in a period of 10,000 days. Then if both the sun and the center of gravity of the binary system remained fixed in space, the relative apparent situation of the stars would be exactly restored to its former state after the lapse of this interval, and if the angle of position were 0° at first, after 10,000 days it would again be so. But now suppose that the center of gravity of the star were in the act of receding in a direct line from the sun with a velocity of one-tenth part of the radius of the earth's orbit per diem. Then at the expiration of 10,000 days it would be more remote from us by 1000 such radii, a space which light would require 57 days to traverse. Although really, therefore, the stars would have arrived at the position 0° at the exact expiration of 10,000 days, it would require 57 days more for the notice of that fact to reach our system. In other words, the period would appear to us to be 10,057 days, since we could only conclude the period to be completed when to us as observers the original angle of position was again restored. A contrary motion would produce a contrary effect.
• Astronomische Nachrichten, No. 520.
OF CLUSTERS OF STARS AND NEBULÆ.
OF CLUSTERING GROUPS OF STARS. — GLOBULAR CLUSTERS. — THEIR
STABILITY DYNAMICALLY POSSIBLE. - LIST OF THE MOST REMARKABLE. — CLASSIFICATION OF NEBULÆ AND CLUSTERS. THEIR DISTRIBUTION OVER THE HEAVENS. — IRREGULAR CLUSTERS. — RESOLVABILITY OF NEBULÆ. — THEORY OF THE FORMATION OF CLUSTERS BY NEBULOUS SUBSIDENCE. ~ OF ELLIPTIC NEBULÆ. —THAT OF ANDROMEDA. — ANNULAR AND PLANETARY NEBULÆ. -DOUBLE NEBULÆ. — NEBULOUS STARS. —CONNEXION OF NEBULÆ WITH DOUBLE STARS. — INSULATED NEBULÆ OF FORMS NOT WHOLLY IRREGULAR. — OF AMORPHOUS NEBULÆ. — THEIR LAW OF DISTRIBUTION MARKS THEN AS OUTLIERS OF THE GALAXY. — NEBULE, AND NEBULOUS GROUP OF ORION — OF ARGO - OF SAGITTARIUS — OF CYGNUS. — THE MAGELLANIC CLOUDS. — SINGULAR NEBULA IN THE GREATER OF THEM. THE ZODIACAL LIGHT. - SHOOTING STARS.
(864.) WHEN we cast our eyes over the concave of the heavens in a clear night, we do not fail to observe that here and there are groups of stars which seem to be compressed together in a more condensed manner than in the neighbouring parts, forming bright patches and clusters, which attract attention, as if they were there brought together by some general cause other than casual distribution. There is a group, called the Pleiades, in which six or seven stars may be noticed, if the eye be directed full upon it; and many more if the eye be turned carelessly aside, while the attention is kept directed* upon the group. Telescopes show fifty or sixty
It is a very remarkable fact, that the center of the visual area is far less sensible to feeble impressions of light, than the exterior portions of the retina. Few persons are aware of the extent to which this comparative insensibility extends, previous to trial. To estimate it, let the reader look alternately full at a star of the fifth magnitude, and beside it; or choose two, equally bright, and about 30 or 4o apart, and look full at one of them, the probability is, he will see only the other. The fact accounts for the multitude of stars with which we are impressed by a general view of the heavens; their paucity when we come to count them.