Imagini ale paginilor
PDF
ePub

found by Struve for the time of revolution and eccentricity, and the other elements of its orbit. Of the thirty-eight positions given from 1827 to 1874, only two cases occur in which the discordances amount to one tenth of a second of arc; and these causes, it is promised, will be, at least in part, explained away in a forthcoming memoir relating to the peculiar systematic errors that attach themselves to the observations made by Otto Struve in 1840–41. Of the remaining thirty-six discordances, eight slightly exceeded one twentieth of a second of arc. The remainder are less than that quantity. The probable error of a single observed distance or the result of a single night's work is 0.046. Observations of these stars made by other astronomers agree satisfactorily with the orbit determined by Otto Struve, although the average of the discordances is somewhat larger in their observations than in his own.-Notices of the Royal Astronomical Society, May, 1875, 372.

METHOD OF CONSTRUCTING CHARTS OF STARS. In constructing the new charts of the stars in the neighborhood of the ecliptic, the French astronomers, under the general direction of Le Verrier, have adopted some novel and excellent methods. The brothers Paul and Prosper Henry, in that portion of the work which they have performed, have made use of two equatorials, having apertures of about nine inches, and by a duplicate examination of each portion of the heavens have been able to discover many small planets and comets. The great equatorial of the observatory has been furnished with a micrometer of special construction, in which advantage is taken of the precision with which the telescope is made, by means of the regulator of Foucault, to follow the diurnal movements of the stars. This micrometer gives immediately the co-ordinates of any star comprised in the field of view of the telescope, with l'eference to a given standard point, and that in such shape that these figures may be entered directly upon the chart. This micrometer is also now being applied to the mapping of the individual stars in some of the clusters. The accuracy of the work done with this instrument is such that the star places given upon the charts are reliable within a second of time and one tenth of a minute of arc: a result somewhat surprising when we consider the extent of the work and the rapidity with which it is done. A portion of the zones will cross the Milky Way, and it will be attempted to give the position of every star visible in this region with the help of telescopes of ten inches' aperture.Bullet. Hebdomadaire, 1875, 335.

ON THE RECTILINEAR RELATIVE MOTION OF THE COMPONENTS

OF THE STAR 61 CYGNI. Mr. Wilson has examined the relative motion of the components of the double star 61 Cygni, with the intention of ascertaining how far recent measures confirm Struve's conclusions that this motion is rectilinear. If these stars were physically connected in a binary system, it would be highly improbable that their apparent motions as seen from the earth would be sensibly straight lines. And yet, during the past century, the observations, which have been numerous, show that their motions really are so. On the other hand, the fact that they both have very large proper motions, being respectively 517 and 509 seconds per century, and in the same directions, leads to the conclusion that in all probability there must be some connection between them. We have thus the remarkable phenomena of two stars close together, animated by an unusually great proper motion, yet whose physical connection is still in doubt. Mr. Wilson's studies upon this subject seem not to contribute any thing toward a solution of our present doubts. He is merely able to confirm the fact that all known observations may be sufficiently well explained by the assumption that the two stars are moving in straight lines.- Notices of the Royal Astronomical Society, April, 1875, 324.

THE TRIPLE STAR ZETA CANCRI.

The triple star Zeta Cancri has for many years formed an object of study on the part of Otto Struve, who has recently published an excellent memoir on the relative movements of its components. The first observations of this star were hy Tobias Mayer in 1756, who recognized it as double, and determined the relative position of the components. Similar observations were made in 1778 by Christian Mayer.

Sir William Herschel, in 1781, made the interesting discovery that the principal one of the stars observed by Mayer was itself double; which observation was again confirmed, in 1825, by Sir James South. The three stars are nearly equal in brightness, and are ranked as between the fifth and sixth magnitudes; but whether the masses of the three bodies are really nearly the same can only be determined by their own movements, as deduced from such observations as have been made by William and Otto Struve. The former astronomer observed them first in 1826, since which time they have been closely followed by himself and son. The remarkably accurate observations of Baron Dembowski at Gallarate, near Milan, in Italy, together with those made by Dawes in England, have been by Otto Struve combined with his own; and from the entire assemblage of all the appropriate observations he finds that the apparent orbit of the star B about the star A is completed in about 62.4 years, under the assumption that the apparent orbit is circular, which appears to be very nearly the case (the real orbit has an eccentricity of 0.35). The third star of the group, indicated by the letter C, is apparently about ten times as far from A as is the star B. Its angular movement relative to the former star is therefore correspondingly slow, it having described only 47° in ninety years, while the star B has entirely completed its revolution and described a portion of its second orbit. An interesting peculiarity of the motions of the star C consists, however, in this, that its movements are by no means regular. It is in fact subject to repeated alternations within periods of about ten years, within which time it moves sometimes forward rapidly, and at other times backward, and at other times it is stationary. Its apparent orbit around the star A is therefore essentially an epicycloid; but unbappily the present state of mathematical analysis does not enable us to say whether these irregularities in its movement are due to the perturbing attractions of the stars A and B, or whether we must assume that the star C describes an elliptical orbit about an invisible point central between it and a fourth invisible star, D, while the central point itself describes a much larger orbit about the stars A and B. Otto Struve states that if we refer the positions of the star C to a point half-way between the stars A and B, we can closely represerit all our observations by assuming C to move in a small ellipse, having a diameter of a third of a second, which latter is, at a distance of 5.5 seconds, carried uniformly about the point central between A and B.- Bullet. Hebdomadaire Assoc. Scientifique, 1875, 217.

[ocr errors]

ON THE CHEMISTRY OF THE SOLAR SYSTEM. Observation and theory have led Lockyer to the conclusion that the various elements constituting the sun are arranged in layers according to the atomic weight of their vapors. Thus outside of all is hydrogen, with an atomic weight of one. Then follow, in regular order, magnesium, calcium, sodium, chromium, manganese, iron, nickel, etc. At • the centre the nobler and rarer metals must be found, constituting the substance of the sun. In this same order should the nebulous mass have been arranged from which the solar system was developed, according to La Place's hypothesis, consequently the exterior planets of the solar system should be principally formed by the condensation of the metalloids, and the inferior planets be composed of the metallic elements; thus Lockyer explains the feeble specific gravity of the former, and the greater mass of the latter planets. The composition of the atmospheres of the planets, which give only a few rays of absorption in the spectrum, seem to confirm this view of Lockyer. Even the composition of the outer shell of the carth would seem to accord therewith, since it is formed in the following proportion : Oxygen, 500; silicium, 250; the other metalloids, 227; and of other simple bodies, 23 parts out of a thousand. If, on the other hand, we add the liquid portion of water, it will be found that hydrogen enters in a still larger proportion, and, with oxygen, acquires a predominance; so that it may be said our earth is composed principally of oxygen and hydrogen, with a small percentage of metals and metalloids.- La Nature, III., 206.

A FAMOUS SOLAR ECLIPSE.

The total solar eclipse of the 3d of June, 1239, was a memorable event in Central Europe, and has formed the subject of an interesting memoir by Celoria, one of the astronomers at the Observatory of Milan. This gentleman has collected together all the accessible notes with reference to observations made at that time upon the eclipse, from

which it appears that the line of central totality passed through Northern Italy and Southern Spain. Our author has sought, by a very careful discussion of the observations at command, to determine the precise northern limit of visibility of the total eclipse; and, by comparing the observations with the tables of Hansen, to deduce some positive addition to our knowledge of the secular changes in the orbit of the earth and moon. He expresses his results by an equation which shows that we need but one more similar eclipse in order to arrive by means of a second such equation at more correct elements of the lunar orbit.Publication of the Royal Observatory, Milun, No. 10, 1875.

STUDIES UPON THE DIAMETER OF THE SUN. As the extensive work of Father Rosa upon the solar diameter is likely to provoke much further investigation of this subject, notwithstanding all that has been done by Auwers, Wagner, Newcomb, and many others, we quote the following conclusions to which he lias been led, as published in the posthumous work recently edited by Father Secchi. First, the body of the sun must be considered as composed of two masses nearly independent of each other, viz., of a solid nucleus enveloped by a gascous matter. The expression “solid nucleus” can even be applied to the central portion of a gaseous mass whose condensation is such that it is necessary that it should be nearly independent of its envelope. Second, the deformations of the photosphere are not due directly to the force of gravity. Third, the continuous force, that which especially deforms the photosphere, is connected with that which produces actual secular movements of the centre of gravity of the sun as demonstrated by Le Verrier. Fourth, the vertical diameter of the sun experiences an annual variation or a semi-annual period, such that it is greater when the sun is north of the equator. Fifth, the mechanical theory of the sun's motions demands that its centre of gravity should describe, in its apparent movement, a great circle of the celestial sphere. The centre of figure, according to Airy, describes a parallel circle, lying northward therefrom. Sixth, it results evidently from the two preceding sentences that the plane of the ecliptic is not parallel to that which cuts the photosphere into two symmet

« ÎnapoiContinuați »