is a list of the fixed stars, disposed in their several constellations; with the longitudes, latitudes, &c., of each; or according to their right ascensions, that is, the order of their passing over the meridian.
The first who undertook to reduce the fixed stars into a catalogue was Hipparchus Rhodius, about 120 years before Christ; in which he made use of the observations of Timocharis and Arityllus for about 180 years before him. Ptolemy retained Hipparchus's catalogue containing 1026 fixed stars; though he himself made abundance of observations, with a view to a new catalogue, A.D. 140. About the year of Christ 880, Albategni, a Syrian, brought down the same to his time. Anno 1437, Ulugh Beigh, king of Parthia and India, made a new catalogue of 1022 fixed stars, since translated out of Persian into Latin by Dr. Hyde. The third who made a catalogue from his own observations was Tycho Brahe, who determined the places of 777 stars for the year 1600, which Kepler from other observations of Tycho afterwards increased to the number of 1000 in the Rudolphine tables; adding those of Ptolemy omitted by Tycho, and of other authors; so that his catalogue amounts to above 1160. At the same time, William landgrave of Hesse, with his mathematicians Christopher Rothmannus and Julius Byrgius, determined the places of 400 fixed stars by his own observations, with their places rectified for the year 1593; which Havelius prefers to those of Tycho's. Ricciolus, in his Astronomia Reformata, determined the places of 101 stars for the year 1700, from his own observations; for the rest he followed Tycho's catalogue, altering it where he thought fit. Anno 1667, Dr. Halley, in the island of St. Helena, observed Catalogue observed 350 southern stars not visible in our horizon.
The same labour was repeated by F. Noel in 1710, who published a new catalogue of the same stars constructed for the year 1687.
Bayer, in his Uranometria, published a catalogue of 1160 stars, compiled chiefly from Ptolemy and Tycho, in which every star is marked with some letter of the Greek alphabet; the biggest star in any constellation being denoted by the first letter, the next by the second, &c., and if the number exceeds the Greek alphabet, the remaining stars are marked by letters of the Roman alphabet, which letters are preferred by Flamsteed, and by Senex on his globes. The celebrated Hevelius composed a catalogue of 1888 stars, 1553 of which were observed by himself; and their places were computed for the year 1660.
The last and greatest is the Britannic catalogue, compiled from the observations of the accurate Mr. Flamsteed; who for a long series of years devoted himself wholly thereto. As there was nothing wanting either in the observer or apparatus, we may look on this as a perfect work so far as it goes. It is to be regretted the impression had not passed through his own hands: that now extant was published by authority, but without the author's consent: it contains 2734 stars. There was another published in 1725, pursuant to his testament; containing no less than 3000 stars, with their places rectified for the year 1689: to which is added Mr Sharp's catalogue of the southern stars not visible in our hemisphere, adapted to the year 1726.
The first catalogue, we believe, that was printed in the new or second form, according to the order of the right ascensions, is that of De la Caille, given in his Ephemerides for the ten years between 1755 and 1765, and printed in 1755. It contains the right ascensions and declinations of 307 stars, adapted to the beginning of the year 1750. In 1757 De la Caille published his Astronomiae Fundamenta, containing a catalogue of the right ascensions and declinations of 398 stars, likewise adapted to the beginning of 1750. And in 1763, the year after his death, was published the Catalogus Astrale Stelliferum of the same author; containing a catalogue of the places of 1042 stars, all situated to the southward of the tropic of Capricorn, and observed by him while he was at the Cape of Good Hope in 1751 and 1752; their places being also adapted to the beginning of 1750. In the same year was published his Ephemerides for the ten years between 1765 and 1775; in the introduction to which are given the places of 515 zodiacal stars, all deduced from the observations of the same author; the places adapted to the beginning of the year 1765.
In the Nautical Almanac for 1773, is given a catalogue of 387 stars, in right ascension, declination, longitude and latitude, derived from the observations of the late celebrated Dr Bradley, and adjusted to the beginning of the year 1760. This small catalogue, and the results of about 1200 observations of the moon, are all that the public have yet seen of the multiplied labours of this most accurate and indefatigable observer, although he has now (1798) been dead upwards of 36 years.
In 1775 was published a thin volume, entitled, Opera Medita, containing several papers of the late Tobias Mayer, and among them a catalogue of the right ascensions and declinations of 998 stars, which may be occulted by the moon and planets; the places being adapted to the beginning of the year 1756.
At the end of the first volume of "Astronomical Observations made at the Royal Observatory at Greenwich," published in 1776, Dr Maskelyne, the present astronomer royal, has given a catalogue of the places of 34 principal stars, in right ascension and north polar distance, adapted to the beginning of the year 1770.
These being the result of several years repeated observations, made with the utmost care, and the best instruments, it may be presumed are exceedingly accurate.
In 1782, M. Bode of Berlin published a very extensive catalogue of 1058 of the fixed stars, collected from the observations of Flamsteed, Bradley, Hevelius, Mayer, De la Caille, Messier, Monnier, D'Arquier, and other astronomers; all adapted to the beginning of the year 1780; and accompanied with a celestial atlas or set of maps of the constellations, engraved in a most delicate and beautiful manner.
To these may be added Dr Herschel's catalogue of double stars, printed in the Phil. Trans. for 1782 and 1783; Messier's nebulae and clusters of stars, published in the Connaissance des Temps for 1784; and Herschel's catalogue of the same kind, given in the Phil. Trans. for 1786.
In 1789 Mr Francis Wollaston published "A Specimen of a General Astronomical Catalogue, in Zones of North-polar Distance, and adapted to January 1, 1790." These stars are collected from all the catalogues before-mentioned, from that of Hevelius downwards. This work contains five distinct catalogues; viz.: Dr Maskelyne's new catalogue of 36 principal stars; a general catalogue of all the stars, in zones of north polar distance; an index to the general catalogue; a catalogue of all the stars in the order in which they pass the meridian; and a catalogue of zodiacal stars, in longitude and latitude.
Finally, in 1792, Dr Zach published at Gotha, Tabula Motuum Solis; to which is annexed a new catalogue of the principal fixed stars, from his own observations made in the years 1787, 1788, 1789, 1790. This catalogue contains the right ascensions and declinations of 381 principal stars, adapted to the beginning of the year 1800. Hutton's Math. Dict.
Besides these two methods of forming catalogues of the stars, Dr Herschel has proposed a new one, in which the comparative brightness of the stars is accurately expressed. It is long since astronomers were first led to arrange the stars in classes of different magnitudes by their various degrees of brilliancy or lustre. Brightness and size have at all times been considered as synonymous terms; so that the brightest stars have been referred to the class comprehending those of the first magnitude; and as the subsequent orders of stars have been supposed to decrease in lustre, their magnitude has been determined in the same decreasing progression: but the want of some fixed and satisfactory standard of lustre has been the source of considerable confusion and uncertainty in settling the relative magnitudes of the stars. A star marked 1. 2m. is supposed to be between the first and second magnitude; but 2. 1m. intimates, that the star is nearly of the second magnitude, Catalogue magnitude, and that it partakes somewhat of the lustre of the Stars, of a star of the first order. Such subdivisions may be of some use in ascertaining stars of the first, second, and third classes; but the expressions 5m, 6m, 7m, 8m, must be very vague and indefinite. Dr Herchel observes that he has found them so in fact; and he therefore considers this method of pointing out the different lustre of stars as a reference to an imaginary standard. If any dependence could be placed on this method of magnitudes, "it would follow, that no less than 11 stars in the constellation of the Lion, namely β, σ, π, ξ, A, b, c, d, 54, 48, 72, had all undergone a change in their lustre since Flamsteed's time: For if the idea of magnitudes had been a clear one, our author, who marked β 1.2m. and γ 2m. ought to be understood to mean that β is larger than γ; but we now find that actually γ is larger than β. Every one of the eleven stars (says Dr Herchel) which I have pointed out may be reduced to the same contradiction."
The author has pointed out the insufficiency of the insufficiency of this method, and of the uncertain conclusions that are deduced from it, in determining the comparative brightness of stars found not only in Mr Flamsteed's catalogue, but also in the catalogues of other astronomers. It is sufficiently apparent that the present method of expressing the brightness of the stars is very defective. Dr Herchel therefore proposes a different mode, that is more precise and satisfactory.
"I place each star (he says), instead of giving its magnitude, into a short series, constructed upon the order of brightness of the nearest proper stars. For instance, to express the lustre of D, I say CDE. By this short notation, instead of referring the star D to an imaginary uncertain standard, I refer it to a precise and determined existing one. C is a star that has a greater lustre than D, and E is another of less brightness than D. Both C and E are neighbouring stars, chosen in such a manner that I may see them at the same time with D, and therefore may be able to compare them properly. The lustre of C is in the same manner ascertained by BCD; that of B by ABC; and also the brightness of E by DEF; and that of F by EFG.
"That this is the most natural, as well as the most effectual way to express the brightness of a star, and by that means to detect any change that may happen in its lustre, will appear, when we consider what is requisite to ascertain such a change. We can certainly not wish for a more decisive evidence, than to be assured, by actual inspection, that a certain star is now no longer more or less bright than such other stars to which it has been formerly compared; provided we are at the same time assured that those other stars remain still in their former unaltered lustre. But if the star D will no longer stand in its former order CDE, it must have undergone a change; and if that order is now to be expressed by CED, the star has lost some part of its lustre; if, on the contrary, it ought now to be denoted by DCE, its brightness must have had some addition. Then, if we should doubt the stability of C and E, we have recourse to the orders BCD and DEF, which express their lustre; or even to ABC and EFG, which continue the series both ways. Now having before us the series BCDEF, or if necessary even the more extended one ABCDEFG, it will be impossible to mistake a change of brightness in D, when every member of the series is found in its proper order except D."
In the author's journal or catalogue, in which the order of the lustre of the stars is fixed, each star bears its own proper name or number, e.g., "the brightness of the star δ Leonis may be expressed by δ & Leonis, or better by 94—68—17 Leonis; these being the numbers which the three above stars bear in the British catalogue of fixed stars."
This method of arrangement occurred to Dr Herchel so early as the year 1782; but he was diverted from the regular pursuit of it by a variety of other astronomical engagements. After many trials, he proposed, in the Transactions of the Royal Society of London for 1796, the plan which appeared to him the most eligible. It is as follows:—Instead of denoting particular stars by letters, he makes use of numbers; and in his choice of the stars which are to express the lustre of any particular one, he directs his first view to perfect equality. When two stars seem to be similar both in brightness and magnitude, he puts down their numbers together, separated merely by a point, as 30.24 Leonis; but if two stars, which at first seemed alike in their lustre, appeared on a longer inspection to be different, and the preference should be always decidedly in favour of the same star, he separates these stars by a comma, thus, 41.94 Leonis. This order must not be varied; nor can three such stars, as 20, 40, 39 Librae, admit of a different arrangement. If the state of the heavens should be such as to require a different order in these numbers, we may certainly infer that a change has taken place in the lustre of one or more of them. When two stars differ very little in brightness, but so that the preference of the one to the other is indisputable, the numbers that express them are separated by a short line, as—17—70 Leonis, or 68—17—70 Leonis. When two stars differ so much in brightness, that one or two other stars might be interposed between them, and still leave sufficient room for distinction, they are distinguished by a line and comma, thus,—, or by two lines, as 32—41 Leonis. A greater difference than this is denoted by a broken line, thus——29 Bootis. On the whole, the author observes the marks and distinctions which he has adopted cannot possibly be mistaken; "a point denoting equality of lustre; a comma indicating the least perceptible difference; a short line to mark a decided but small superiority; a line and comma, or double line, to express a considerable and striking excess of brightness; and a broken line to mark any other superiority which is to be looked upon as of no use in estimations that are intended for the purpose of directing changes."
The difficulties that attend this arrangement are not disguised; but the importance and utility of it more than compensate for the labour which it must necessarily require. By a method of this kind, many discoveries of changeable and periodical stars might probably have been made, which have escaped the most diligent and accurate observers. We might then, as the author suggests, be enabled to resolve a problem in which we are all immediately concerned.
"Who, for instance, would not wish to know what degree of permanency we ought to ascribe to the lustre of our sun? Not only the stability of our climates, but..." Catalogue the very existence of the whole animal and vegetable creation itself, is involved in the question. Where can we hope to receive information upon this subject but from astronomical observations? If it be allowed to admit the similarity of stars with our own as a point established, how necessary will it be to take notice of the fate of our neighbouring suns, in order to guess at that of our own? That star, which among the multitude we have dignified by the name of sun, to-morrow may slowly begin to undergo a gradual decay of brightness, like α Leonis, α Ceti, α Draconis, α Ursae majoris, and many other diminishing stars that will be mentioned in my catalogues. It may suddenly increase, like the wonderful star in the back of Cassiopeia's chair, and the no less remarkable one in the foot of Serpentarius; or gradually come on, like β Geminorum, β Ceti, γ Sagittarii, and many other increasing stars, for which I also refer to my catalogues; and, lastly, it may turn into a periodical one of 25 days duration, as Algol is one of three days, δ Cephei of five, β Lyrae of six, α Antiohi of seven days, and as many others as are of various periods.
Having thus explained the general principle on which this catalogue is formed, as we find it in the author's first memoir on the subject, we must refer the reader to the doctor's own account for its particular arrangement, observing only that the catalogue subjoined comprehends nine constellations, which are arranged in alphabetical order, with the comparative brightness of the stars accurately stated. In a subsequent paper, published in the same volume, he has completely verified the utility of his method by experience, and shown that there is no permanent change of lustre in the stars. In the notes to his first catalogue he mentioned α Herculis as a periodical star. By a series of observations on this star, compared with α Ophiuchi, which was most conveniently situated for his purpose, he has been able not only to confirm this opinion, but to ascertain its period. His observations are arranged in a table, by means of which he determines that this star had gone through four successive changes in an interval of 241 days; and therefore the duration of its period must be about 60 days and a quarter. This fact concurs with other circumstances in evincing the rotatory motion of the stars on their axes. "Dark spots, or large portions of the surface less luminous than the rest, turned alternately in certain directions, either towards or from us, will account for the phenomena of periodical changes in the lustre of the stars, so satisfactorily, that we certainly need not look out for any other cause." If it be alleged that the periods in the change of lustre of some stars, such as Algol, β Lyrae, δ Cephei, and α Antiohi, are short, being only 3, 5, 6, and 7 days respectively; while those of α Ceti, and of the changeable star in Hydra, and that in the neck of the Swan, are long, amounting to 331, 394, and 497 days; and that we cannot ascribe phenomena so different in their duration to the same cause—it may be answered to this objection, that the force of it is founded on our limited acquaintance with the state of the heavens. To the 7 stars, the periodical changes of which were before known, we may now add α Herculis, which performs a revolution of its changes in 60 days.
"The step from the rotation of α Herculis to that of α Ceti is far less considerable than that from the period of Algol to the rotation of α Herculis; and thus a link in the chain is now supplied, which removes the objection that arose from the vacancy." The rotation of the fifth satellite of Saturn is proved by the change observable in its light; and "this variation of light, owing to the alternate exposition of a more or less bright hemisphere of this periodical satellite, plainly indicates, that the similar phenomenon of a changeable star arises from the various lustre of the different parts of its surface, successively turned to us by its rotatory motion."
Besides, we perceive a greater similarity between the sun and the stars, by means of the spots that must be admitted to exist on their surfaces, as well as on that of the sun.
Dr Herschel farther observes, that the stars, besides a rotatory motion on their axes, may have other movements; "such as nutations or changes in the inclination of their axes; which, added to bodies much flattened by quick rotatory motions, or surrounded by rings like Saturn, will easily account for many new phenomena that then offer themselves to our extended views."