In the subsequent columns, under H. for Hevelius, are found — 19" and +18", which, when applied to the mean longitude and latitude, as in the preceding exam ple, give the longitude and latitude of Capella for the 1st January, 1820, according to Hevelius. By applying the numbers under Fl. we shall have the longitude ac cording to Flamstead ; and so on with the rest, the num bers to be applied being always seconds. When there are no numbers after the star, it then appears that the star is found only in one catalogue. The places of the star may be found for any other time before or after 1820, by the Table of the precession of the equinoxes at the end of the catalogue. Thus the longitude of Ca pella, according to Hevelius, for the 1st of June, 1830, -will be When the fixed stars are examined through a good telescope, they do not seem to have any sensible disc ; and their size rather diminishes by increasing the mag nifying power of the instrument. This circumstance alone would have been a striking proof of the immea surable distance of these celestial bodies, had we not been in possession of evidence still more convincing. If the earth's diameter had subtended any sensible an gle at the nearest fixed star, astronomers would have been able to determine the distance by the observed change in its place when viewed from the two extremi ties of the earth's diameter. This base, however, being found to be too small, they have attempted to discover a change in the position of the stars when viewed from the earth in two opposite points of its orbit, or, in other words, to find the angle subtended by the diameter of the earth's orbit at the fted stars, which is called their t.nnual Parallax. If a star is viewed, when the earth is at B, Plate XXXVII. Fig. 8. we should expect that it would appear in a different part of the heavens than when it is viewed from the earth at D: But notwith standing all the attempts of astronomers to discover this change of position or annual parallax, with the most accurate instruments, they have hitherto been unable to detect it. The observations which have been made for this purpose were so extremely exact, that the parallax of the stars, if it does exist, must be less than I", so that if we arc unable to determine the distance of these celestial bodies, we can at least fix the limits beyond which it must lie. If we suppose, then, that the paral lax of the stars is 1", and that the mean distance of the earth from the sun, or the scmidiameter of the earth's orbit, is 95,000,000 miles, we shall have a triangle whose base is 95,000,000, and the angle at its vertex I", to find its side, or the distance of the nearest star, which will be 20,159,665,000,000 miles, or 20 billions of miles, a distance so immensely great, that light cannot pass through it in less than three years. It is very probable, that the parallax of the stars is much less than 1", and that the nearest of these bodies is placed at a much greater distance from us than 20 billions of miles. Some of them, perhaps, are so remote, that since they were created the first beam of light which they emitted has not yet arrived within the limits of our system; while others which have disappeared, or have been destroyed for many ages, will continue to shine in the heavens till the last ray which they emitted has reached our earth.
For an account of Dr Herschel's theory of the annual parallax of the fixed stars, and his method of observing it, see Phil. Trans. 1782, p. 82. See also Roberts, Phil. Trans. 1697, vol. xviii. p. 18 ; and Flamstead, Phil. Trans. 1701, vol. xxii. p. 815. Cassini, Alan. Acad. Par. 1717, p. 256. Halley, Phil. Tree. 1720, p. 637. Brad ley, Phil. Trans. 1728, p. 637. Clairaut, /1frni. ..4eaa. Par. 1739, p. 358. Maskelync, Phil. Trans. 1760, p. 889. Michel, Phil. Trans. 1767, p. 234; 1784, p. 35. Her schel, Phil. Trans. 1795, p. 46. See also ABERRATION, and STAR.
By comparing the places of some of the fixed stars, as determined from ancient and modern observations, Dr Halley discovered that they had a motion of their own, which could not arise from parallax, precession, or aberration. This remarkable circumstance was af terwards noticed by Cassini and Le Monnier, and was completely confirmed by Tobias Mayer, who compared the places of 80 stars, as determined by Roemer, with his own observations, and found that the greater part of them had a proper motion. He suggested that the
change of place might arise from a progressive motion of the sun towards one quarter of the heavens; but as the result of his observations did not accord with this theory, he remarks that many centuries must elapse be fore the true cause of this motion can be explained.
The probability of a progressive motion of the sun was suggested upon theoretical principles by the late Dr Wilson of Glasgow ; and La Lande deduced a similar opinion from the rotatory motion of the sun, by suppo sing, that the same mechanical force which gave it a motion round its axis would also displace its centre, and give it a motion of translation in absolute space.
If the sun has a motion in absolute space, directed towards any quarter of the heavens, it is obvious that the stars in that quarter must appear to recede front each other, while those in the opposite region seem gra dually approaching, in the same manner as when walk ing through a forest, the trees to which we advance arc constantly widening, while the distance of those which we leave behind is gradually contracting. The proper motion of the stars, therefore, in those opposite regions, as ascertained by a comparison of ancient with modern observations, ought to correspond with this hypothesis.
The following Table contains the proper motions of some of the principal fixed stars, as determined by the accurate observations of Dr Maskelyne.
By comparing the proper motions given in the pre ceding Table, Dr Herschel has found, that the greater part of them are nearly in the direction which would result from a motion of the sun towards the constella tion Hercules, or rather to a part of the heavens whose right ascension is 250° 52' 30", and whose north polar distance is 40° 22'. Klugel found the right ascension of this point to be 260°, and Prevot makes it 230°, with 65° of north polar distance. Herschel supposes that the motion of the sun and the system is not slower than that of the earth in its orbit, and that it is performed round some distant centre. The attractive force capable of producing such an effect, he does not suppose to be lodged in one large body, but in the centre of gravity of a cluster of stars, or in the common centre of gravity of several adjacent clusters. La Lande, however, is of opinion, that there is a kind of equilibrium among all the systems of the universe, and that they have a pe riodical circulation round their common centre of gra vity. See Mayer, Opera Inedita, vol. i. p. 77. Lambert, Systeme du Monde, p. 87, 152. La Lande, Mem. 4cad. Par. 1776. Prevot, them. Acad. Berl. 1781, p. 145. Ephem. de Berl. 1786, 1789. Herschel, Phil. Trans. 1783, p. 247 ; Id. 1805, p. 23 ; Id. 1806.
On New and Variable Stars.
While the stars exhibit these apparent variations, they are affected with others of a different kind, which seem to arise from some great physical changes that are going on in these bodies. Several new stars have appeared for a time, and then vanished ; some that are given in the ancient catalogues can no longer be found ; while others are constantly and distinctly visible, which have not been described by the ancients. Some stars, like p in the Whale, have gradually increased in brilliancy ; others, like in the Great Bear, have been constantly diminishing in brightness ; and a great number sustain a periodical variation in their brilliancy. The new star which was seen by Tycho in 1572, in the constellation Cassiopeia, suffered very remarkable changes. On a sudden it became so brilliant, that it surpassed in bright ness even Venus and Mercury, and was visible on the meridian in the day time. Its light then began to di minish, till it disappeared sixteen months after it had been first seen. The new star which appeared in 1603, in the constellation Serpentarius, exhibited similar phe nomena, and disappeared after having been visible for some months.