Gambar halaman
PDF
ePub
[ocr errors]

Comets.-The origin of comets, always a fruitful field for speculation, has received more than usual attention of late. Dr. Th. Bredichin has expressed the opinion that periodic comets owe their origin to the segmentation of ordinary parabolic comets, having been thrown off from the latter by an eruption, such as we have probably witnessed in the great comet of 1882 and in Biela's comet. A valuable paper on the capture theory of comets by M. Tisserand will be found in the "Bulletin Astronomique for June and July, 1889. In a recent paper by Dr. J. Holetschek it is claimed that the apparent systematic grouping of cometary perihelia in certain directions (270° and 90° of heliocentric longitude) has no connection with the general motion of the solar system, but is due to the position of the earth at the time that these discoveries are most readily made. Among the most important of recent contributions to cometary literature is Dr. H. Kreutz's monograph on the great September comet 1882 II, the comet that was seen in full daylight, and was followed by astronomers until it actually disappeared against the sun's disk. The formidable obstacles to an accurate determination of the orbit, presented by the disintegration of the nucleus into several points of condensation, seems to have been most skillfully surmounted by the computer. The final value for the period is 772-2 years. Dr. Kreutz has also finished a computation of the orbit of comet 1880 I, but its publication is delayed until the completion of Dr. Weiss's determination of that of 1843 I, as these three comets seem to belong to the same cometary system, distinguished for short perihelion distances; and we, no doubt, now have a fourth member of the same family in the headless comet 1887 I.

William R. Brooks, of Geneva, N. Y., reported the discovery, on the morning of Jan. 15, 1889, of a faint comet in the constellation Sagittarius. This comet, to which the designation Comet a, 1889, was given, in order to distinguish it as the first comet discovered in the year was diligently searched for by several observers, especially by Barnard and Swift, who examined the region carefully from Jan. 19 to Feb. 13, but without success. As the three observations necessary for determining the orbit were not secured, this comet is not catalogued among the comets of the year. A comet announced by Swift on July 15, is also omitted, as it proved to be identical with the comet discovered by Brooks on Aug. 7, 1888 (1888 III). The comets of 1889 (up to the end of October), arranged in the order in which they passed perihelion, are then as follows:

Comet 1889 I or Comet e 1888, designated as Comet V in the record of last year, was discovered by Barnard, at Lick Observatory, with a 4-inch comet seeker on Sept. 2, 1888, or the morning of Sept. 3, and was also independently discovered by Brooks at Geneva on the following morning. It was a round, nebulous mass, 1' in diameter, with a central condensation of between the eleventh and twelfth magnitude and no tail. At the end of November and beginning of December, it was visible to the naked eye and about as bright as a star of the sixth magnitude. Perihelion was passed on Jan. 31, 1889, and by the end of February it disappeared in the sun's rays, reappearing, however, about the middle of April, and re

maining visible for several months longer. Barnard remarked, on June 3 that there was an anomalous tail directly following the comet about, 1° in length and some 2' or 3' broad. There was no trace of a tail preceding or pointing away from the sun, the direction usually taken by these cometary appendages. The spectrum on Nov. 14, according to Dr. Ralph Copeland, instead of being composed of the usual feeble, separate bands, was continuous, brighter in the middle, and faded gradually at both ends; it resembled the spectrum of a close globular star-cluster or of a nongaseous nebula, rather than that of a self luminous gas. Faint patches of light were made out in the positions usually occupied by the second and third cometary bands. Similar observations were made later, and on Dec. 8 all three bands were distinctly visible, but on each occasion the continuous spectrum formed the ground on which the brighter spectrum was superposed. Dr. Copeland says, "It seems probable that the comet shines mainly by reflected light, . . . to which the action of the sun on the cometary material is slowly adding the usual bright bands."

Comet 1889 II. On the evening of March 31, E. E. Barnard discovered, with the 12-inch equatorial of Lick Observatory, a very small and extremely slender comet, the head being not over 10" in diameter and the tail about 15' in length; the nucleus was stellar and of about the thirteenth magnitude.

Comet 1889 III. Mr. Barnard discovered, with the 6-inch equatorial of Lick Observatory, another faint comet, at about 2 o'clock on the morning of June 24, in the constellation Andromeda. According to elements computed by W. W. Campbell, it had passed perihelion on June 20, and was receding from the earth as well as from the sun when discovered. The last observation seems to have been obtained at Lick Observatory on Aug. 6.

Comet 1889 IV. A tolerably bright comet was discovered in Centaurus by Davidson at the Melbourne Observatory on July 21. On the 25th it was easily visible to the naked eye, and in a small telescope showed a bushy tail. A photograph of this comet was secured on July 30 at Lick Observatory by Mr. Barnard. The time of exposure was one hour and thirty minutes, and the negative showed a wide, fan-shaped tail, with borders concave to its axis and pretty bright for 20' from the head and traceable to 53'. Prof. Holden finds that the brightest part of the tail was of the brightness of the brighest part of the solar corona on Jan. 1, 1889, and 000000 that of the full moon.

Comet 1889 V. William R. Brooks, while sweeping the southwestern heavens with a 10-inch equatorial on the morning of July 6, discovered a suspicious-looking nebulous object, the cometary character of which he was able to confirm by observations on the following morning. The position at the time of discovery was right ascension 23h 45m, declination -9° 10. It was then faint, with a short, wide tail. The comet attracted no special attention on the part of astronomers till Aug. 1, when Barnard discovered that it had apparently given birth to two small nebulous bodies. The next morning showed that both objects, which he designated as B and C, were moving with the parent comet through space. Mr. Barnard says: "On Aug. 3 they were

examined with the 36-inch equatorial, which showed the whole group very beautifully. Each of the companions had a very small nucleus and condensation in a very small head and a short, faint tail, presenting a perfect miniature of the larger one, which was pretty bright and well developed, with small nucleus and slightly fanshaped tail long. There was then absolutely no nebulous connection with the larger, nor has there been at any time since, either in the 12-inch or in the 36-inch telescopes. Nothing whatever has been seen here of the nebulous envelope spoken of by the Vienna observers as apparently inclosing the whole group (A. N., 2,914). I have from the first carefully looked for a nebulous connection. Under unfavorable circumstances the tails of B and C might be imagined to be a connecting nebulosity, but the tail of B falls short of A, and that of C does not nearly reach B. Each comet is in appearance absolutely independent of the other. The tails of all three have lain in the line of the nucleus of A, and therefore have not sensibly deviated from the position-angle 241°.

"On Aug. 4 two other companions were detected with the great telescope, one of which was measured, the other being too elusive to set the wires on. I have numbered these four companions B, C, D, E, in the order of increasing right ascension, A being the larger comet, D and E being the two last discovered. D has been seen several times since the moon withdrew, but has always been too faint to observe; it has not sensibly changed its position. E has only been seen once; its position-angle referred to C would be the same as that of D, and its distance twice as great. Four or five other nebulous bodies observed near the comet Aug. 2 have not since been seen, and were probably nebulous.

"The results of the observations of the two brighter companions are extremely interesting, Measures of B have been made on eighteen, and of C on seventeen nights. These two have almost exactly the same position-angles, which have been sensibly constant; their distances from the main body have, however, been increasing. At the last observations, B seems to be stationary, the distance from A remaining constant, while C continues to recede.'

According to elements published by Dr. Knopf, the main comet passed its perihelion on Sept. 27 at a distance from the sun of 1.96 in terms of the earth's mean distance, and its period of revolution is 7.286 years.

Such resolution into several points of extreme condensation was well exemplified in the case of the great comet of 1882, but there the separate condensations were contiguous, and all were enveloped in one common nebulosity. But in the present case the two components were far apart, and each was surrounded by its own nebulosity, a phenomenon of which we also have a precedent in Biela's comet. This comet, which was discovered in 1772, continued single till November, 1845, and then, by the end of December, separated into two distinct nebulosities. In 1852 the two companions were seen, but the distance had increased eightfold, and at the next return they had apparently been dissipated in a shower of meteors; diligent search has failed to reveal the least trace of the comet since.

[ocr errors]

Star-Catalogues.-Dr. Auwers published in 1880 a provisional list of 303 reference stars for the southern zones of the Astronomische Gesellschaft between 2° and 23°, and although the material accumulated since that time is not sufficient to give the most accurate places, he has prepared a catalogue of positions which will probably require but slight correction. The places have been reduced to 1885 by carefully determined proper motions.

A collection of all available meridian observations of stars that will be within 1° of the north pole in 1900 has been prepared by Miss Anna Winlock, and published as No. 9, of vol. xviii, of the "Harvard Observatory Annals." Prof. Safford has published a catalogue of right ascensions of 261 stars, mostly within 10° of the pole, observed with the 44-inch Repsold meridian circle of Field Memorial Observatory at Williamstown from 1882 to 1887.

The third volume of Auwers's new reduction of Bradley, which has been five years in going through the press, was finally published in 1888. This volume contains, in addition to the catalogue proper, tables giving the quantities in the reduction to the apparent place that depend upon the star's position, and a comparison of Bradley's positions, reduced to 1865, with Berlin and Greenwich observations of about the same date. The catalogue contains 3,268 stars.

Star Charts.-A series of charts embracing all the stars visible to the naked eye-that is, down to about the six-and-a-half magnitudehas been published by Mr. Cottam, and has been highly complimented. There are thirtysix sheets, the scale being one third of an inch to one degree of a great circle. Another most useful set of star maps is Klein's new star atlas, which has appeared in both English and German editions. In the latter there are eighteen maps, containing about the same number of stars as Mr. Cottam's, and giving also all the nebulæ and clusters visible in telescopes of moderate power-a great help to comet hunters.

Stellar Spectra.-The researches that constitute the Henry Draper Memorial, at Harvard Observatory, have consisted for the past three years in the photographic study of stellar spectra; and, while this will continue to be the principal subject for investigation, Mrs. Draper has decided to extend the field of work undertaken so as to include the study of other physical properties of the stars by photography. The first research is now rapidly approaching completion. The catalogue of spectra of bright stars as far south as -25° declination is practically ready for the printer; the photographs for the catalogue of the spectra of faint stars are nearly finished, and the detailed study of the spectra of the brighter stars by means of the 11-inch refractor, with one, two, or four large prisms over its object-glass, will probably be completed within the year. The 8-inch Bache telescope, with which the observations for the first catalogue referred to have been made, remained in California after the New Year's eclipse until Feb. 2, 1889, and was then sent to Peru to continue work upon the spectra of bright stars from -25° to the south pole; the resulting photographs will be sent to Cambridge for reduction. The research on the spectra of faint stars will also be continued to

the south pole, and it is expected that this work in the southern hemisphere will be completed in two years. The Bache telescope will be replaced at Cambridge by an instrument of similar construction provided by Mrs. Draper.

Photography has been applied very successfully, by Dr. Vogel, of Potsdam, to the determination of the velocities of stars in the direction of the line of sight from the observer to the star under observation. The displacement of the lines of the spectrum due to this motion of the star to or from the observer, or of the observer to or from the star, is extremely minute, and the accordance of the measurements made by Dr. Vogel is quite remarkable. His mean results for the motion referred to the sun-that is, after the observed motion has been corrected for the known motion of the earth in its orbit-are as follow:

[ocr errors]

Capella receding from the sun 16 miles a second. Aldebaran receding from the sun 30 miles a second. Polaris approaching the sun 16 miles a second. Algol approaching the sun 7 miles a second. Procyon approaching the sun 7 miles a second. Stellar Parallax.-Prof. Pritchard, of Oxford University Observatory, has continued his investigations of stellar parallax by means of photography, his aim being to examine all stars of the second magnitude suitably situated for observation at Oxford, in the hope of contributing somewhat to our knowledge of what Herschel called the "construction of the heavens." With reference to the differences between the results obtained by different observers, Prof. Pritchard says: "Guided by the suggestions of recent experience, I now think that such differences of parallax' might very reasonably have been anticipated, and may probably be accepted as matters of fact, without in any degree impugning the accuracy of the observations. For in process of this work on parallax, and also from the general history of similar inquiries, it has been made abundantly evident that no necessary connection exists between the brightness of a star and its position in space or distance from the sun. Nevertheless, it is this very difference of brightness mainly which guides us in the selection of comparison stars. The 'parallax' is, in fact, and is becoming more and more generally recognized to be, a differential quantity, fainter stars being in very many instances much nearer to us than others possessing incomparably greater brightness. In passing, I may here instance a Lyræ as compared with 61 Cygni: 8 Centauri as compared with e Indi. In fact, the position in space of the faint comparison stars in relation to that of the star whose parallax is sought is, if not a matter of accident, at all events wholly unknown until the observations and computations are complete."

Prof. Pritchard's final results for stellar parallax, as published in the third volume of the "Oxford Observations," are as follows:

[blocks in formation]
[blocks in formation]

From which Dr. Oudemans concludes that "stars with proper motions greater than '05" have probably an annual parallax of 10" to 50".

Variable Stars.-Several important papers by S. C. Chandler have been published in the Astronomical Journal." In one of these, Mr. Chandler describes an ingenious method of estimating star colors, which he has used with good effect. It consists in estimating the relative change of brightness effected in two stars by the interposition, first of a blue and then of a red shade-glass. If a red and a white star appear of the same brightness when viewed directly, the red star will seem the fainter when the blue glass is interposed, but the brighter with the red glass. These differences of brightness can be precisely estimated by Argelander's method, and they thus afford definite measures of the differences in color of the two stars on an arbitrary scale depending upon the glasses employed. The effect of brightness upon the scale estimates seem to be imperceptible, at least between the second and ninth magnitudes. An important result of Mr. Chandler's investigations is the intimate connection shown between the length of period and the depth of color of the star; the very short period variables are nearly white; those of longer period somewhat redder, the tint growing deeper the longer the period. The possessors of large refractors are strongly urged to devote a portion of their time to the observation of the minima of variables

that become too faint for ordinary telescopes, our knowledge of such variables being extremely deficient. Argelander's method of observation is recommended.

Mr. Chandler has collected the observations of

U Ophiuchi (of all variables, the one with shortest period and most rapid fluctuations of light), and he finds a curious but well-marked retardation in the increase of brilliancy some half-hour or so after minimum is passed. A similar irregularity has been noticed in the light-curve of S Caneri and occasionally in that of Algol. Further observations of the Algol variable Y Cygni, the period of which has hitherto been uncertain, have fixed this element at 1d 11h 56m 48; the period of R Canis Majoris, an interesting variable detected by Mr. E. F. Sawyer, is undoubtedly very close to 14 3h 15m 56s.

An "Index to Observations of Variable Stars" forms No. 8 of vol. xviii of the "Annals of the Harvard Observatory." A large number of unpublished observations are referred to, particularly three extensive series of observations by Argelander, Heis, and Schmidt.

Extension of the Law of Gravitation to Stellar Systems.-Prof. Hall, in a discussion of this question in the " Astronomical Journal,” after a review of the various speculations upon the subject, says: "The weakness of the proof that the Newtonian law governs the motions of double stars arises from two sources. In the first place, the errors of observation have a large ratio to the quantities measured. This condition makes it difficult to compute the orbits with much accuracy, or we may satisfy the observations with very different elements. . . The insufficiency in the data can only be removed by further observation. Since there is no theoretical difficulty in the way, the continuation of the observations of double stars and the improvement of methods of observation will, in time, give the means for the accurate determination of their apparent orbits. The theoretical difficulty in proving the law of Newton for double stars can not be overcome, but we can increase the probability of the existence of this law by determining more orbits and those that are very differently situated. If the law proves satisfactory in all cases, we shall have a probability of its universality increasing with the progress of astronomy." But, although this probability may be very great, it can not constitute a proof offering the character of experimental certainty which clothes the law of Newton itself in our planetary system. A serious difficulty is encountered in the enormous velocities with which quite a number of stars appear to be moving through space, "runaway Groombridge-1830, μ Cassiopeia, 8 Hydri, a Boötis, and others. Some of these velocities are comparable to that of a comet in close proximity to the sun, but in most cases there is no visible object near the one in motion to which we can ascribe an attractive force, acting according to the Newtonian law, which would produce the velocity observed, unless we assume

enormous masses.

[ocr errors]

An interesting article on this subject is contributed by M. Tisserand to the "Bulletin Astronomique" for January, 1887.

Double and Multiple Stars.-The star 7 Ophiuchi of the second or third magnitude has been divided into two nearly equal components by Burnham with the 36-inch Lick refractor, and he is of the opinion that it will undoubtedly prove to be a binary of short period; the distance is about 0.35′′ He has also found companions for Aldebaran and y Cassiopeia, and a very minute companion for Cygni distant about 3.6′′

In measuring the double stare Hydra at Pulkowa, in 1860, and again in 1864, Otto Struve suspected an elongation in the principal star. In 1877-'78, Burnham carefully examined the pair, but always found the larger star round. In April, 1888. J. V. Schiaparelli, with the 18-inch refractor at Milan, found it plainly elongated, and Burnham has been able to separate and measure this faint companion with the 36-inch Lick glass, using a magnifying power of 3,300. It seems very probable, therefore, that this will prove to be a physical system, and perhaps one in rapid motion.

Herr H. Seeliger has published a paper on the peculiar stellar system Cancri, in which he finds that more recent observations and further study confirm his former result and the still earlier

conclusions of Otto Struve. Cancri consists of a star A, 50 magnitude, a close visible companion B, 57 magnitude, and a more distant companion C, 55 magnitude. Treating A, B, and C as three bodies, Herr Seeliger by a strict analysis of their motions, shows that to make theory conform to observation, C must revolve about a point S, which can be no other than the center of gravity of C and a dark companion D while their common center of gravity s2 revolves about the center of gravity of the two stars A and B.

The Great Nebula in Orion.-In order to test the efficacy of photography in the discovery of new nebula, Prof. Pickering has compared the number of nebulæ shown in a series of photographs of the regions about the great nebulæ in Orion, with the number in the same region given by Dreyer's catalogue. Fourteen of the objects photographed are contained in Dreyer's catalogue; four in the catalogue are not photographed; twelve that are photographed are not in the catalogue. Prof. Pickering concludes that in carrying out the same proportion we might expect to discover four or five thousand such objects by photographing the whole sky; but, he adds, "there is one consideration that may seriously modify this conclusion. The successive improvements in photography have continually increased the limits of the nebula in Orion. These plates show that it not only includes the sword-handle c,, and 0, but a long nebulosity extends south from, others surround this star, while others, both north and south, indicate that perhaps the next increase in sensitiveness of our plates will join them all in a vast nebula many degrees in length."

In the March number of the "Monthly Notices," Isaac Roberts gives a brief paper on what he terms photographic analyses of the great nebula of Orion, and the nebule M 42, M 43, and h 1180 in Orion. This was done by exposing negatives between five seconds and three hours, twentyfive minutes, and studying the gradations of the nebulosity obtained in order to compare the relative actinic power of the light in different parts of the nebula." An exposure of five secconds showed the four stars of the trapezium; one of thirty seconds increased the diameter and density of these stars, and a third exposure of one minute intensified the same effect, and showed the beginning of nebulosity around the star 0. Photographs obtained under longer exposures, upon comparison with the drawings of Lord Rosse and Bond, seem to show that changes of the relative position of certain stars have taken place since 1866; while still longer exposures have brought out evidence that the three objects, M 42, M 43, and h 1180, all belong to one gigantic nebula.

When the 36-inch refractor of Lick Observatory was mounted, one of the first objects examined by Mr. Alvan G. Clark was the trapezium of Orion; and he detected at once an exceedingly faint point of light within the trapezium which Mr. Burnham has seen and measured frequently since; it is probably fainter than the sixteenth magnitude. In October, 1888, Barnard saw that the star was really double, forming the severest possible test for the defining and illuminating power of the great telescope. Barnard

has detected a second star within the trapezium, and also another of about the same magnitude as the Clark star just preceding the trapezium. Dr. William Huggins has photographed new lines within the spectrum of the nebula, and he has also noticed that at least three groups of lines in the spectra of two of the stars of the trapezium extend into the adjoining nebular matter, and so show that these stars are not merely optically, but truly and physically connected with the nebula. These observations seem to point to the conclusion that the nebula is now in a state of gas, though we have no knowledge of the anterior conditions which have brought it into this condition.

[ocr errors]

The Great Nebula in Andromeda. - The "Observatory for February contains a reproduction of a remarkable photograph of the great nebula in Andromeda, taken by Mr. Roberts with a 20-inch silver-on-glass reflector. A regular, connected and highly suggestive form is given to the nebula, which has not been brought out by any drawings hitherto made.

New Astronomical Societies.-The friendly relations which were established between the professional astronomers at Mount Hamilton and amateur photographers and astronomers of the Pacific coast on the occasion of the total solar eclipse on Jan. 1, 1889, resulted in the formation of the Astronomical Society of the Pacific, of which Prof. Holden was elected president, and Messrs. Schaeberle and Burckhalter, secretaries. The new society is designed to be popular in the best sense of the term, and any person who takes a genuine interest in astronomy is invited to join its membership, whether he has made special studies in this direction or not. The "Publications" are printed in octavo form at irregular intervals. They contain papers read before the society, either in full or in abstract, the minutes of the meetings, and also notes from the Lick Observatory, which are brief and popular accounts of the current work of that establishment, prepared by members of the observatory staff. A fund has been established known as the "Donahoe Fund for the maintenance of the Comet Medal of the Astronomical

Square miles.

[ocr errors]

lar lectures are given on scientific subjects, and a very well-written popular journal Himmel und Erde," is published, especial attention being paid to astronomy. Another new society, the Société Astronomique de France," founded Jan. 28, 1887, by M. Flammarion, has met with well-deserved success. Reports are published in the "Observatory" and in "L'Astronomie," and the proceedings appear in an annual bulletin.

Astronomical Prizes.—The Lalande prize of the Paris Academy has been awarded to M. Bossert for his useful work as vice-director of the computing division of the Paris Observatory; the Valz prize, to Prof. E. C. Pickering for his work in stellar photometry; and the Janssen prize, to Dr. Huggins. The Rumford medal of the Royal Society was awarded, Nov. 30, 1888, to Tacchini, for important and longcontinued investigations which have largely advanced our knowledge of the physics of the sun. New Treatises on Astronomy.-The first volume of a treatise on celestial mechanics by M. Tisserand, of Paris, has appeared. It contains the general theory of perturbations, and is to be followed by another volume on the figures and rotation of celestial bodies and a third on the lunar theory, theory of Jupiter's satellites, Hansen's method for the calculation of perturbations and other methods of recent date. Another work which will be found in many respects useful as a text-book is "Die mathematischen Theorien der Planeten-Berwegungen," by Dr. Dziobek. A fourth edition of Chambers's "Astronomy" is being published in three volumes; the first volume only has been issued, and Proctor's" Old and New Astronomy" is to be finished under the editorship of A. C. Ranyard.

AUSTRALASIA, a division of the globe embracing the continent of Australia and the colonial possessions of England in the Pacific Ocean. The area and population of the British Australasian colonies, with their public debts and their budgets for the financial year 1887, which ended on June 30 in Victoria, Queensland, and South Australia, on March 31 in New Zealand and Fiji, and on Dec. 31 in the other colonies, are given in the following table:

New South Wales

Victoria

Queensland

South Australia

Western Australia

Tasmania.

New Zealand Fiji...

COLONIES.

810.700

87.884

[blocks in formation]
[blocks in formation]

3,368,883

25,820,850

[blocks in formation]

Society of the Pacific," the principal conditions of the gift, a medal of bronze, being the discovery of a new comet or the first precise determination of position of a periodic comet at any one of its expected returns. The discoverer is to make his discovery known in the usual way and also to communicate it immediately to the director of Lick Observatory. No application for the bestowal of the medal is required.

A new society for encouraging the study of nature, the "Gesellschaft Urania," has been established at Berlin under the presidency of Prof. Förster, of the Berlin Observatory, Popu

255,389

Inclusive of British New Guinea and various small islands in the Pacific, the total area of the Australasian colonies and dependencies of Great Britain is about 3,135,000 square miles, and the population exceeds 3,800,000.

Federation. The idea of imperial federation, which is popular in England, finds little or no acceptance in Australia, because the advantages would be almost entirely on the side of the mother-country, while the sacrifices would be borne by the colonies. Customs furnish nearly one third of the colonial revenues, and are the main dependence for the permanent expenses of

« SebelumnyaLanjutkan »