Admitting this, the sun, the centres of planetary systems. multitudes of fixed stars, that may be discovered with the most inferior telescopes, shew us an extent of the universe, that our imagination can scarcely comprehend; but what is even this, compared to the extent that the discoveries and conjectures of Herschel point out? 167. We cease to have distinct ideas, when we enumerate by ordinary measures the distances of the fixed stars, and we require the aid of other circumstances to enable us to comprehend them. Thus, we compute that the nearest of the fixed stars is so far distant, that light will take above a year in coming from the star to the earth; that the light of many telescopic stars may have been many hundred years in reaching us; and still farther, that, according to Dr. Herschel, the light of some of the nebulæ, just perceptible in his forty-feet telescope, has been above a million of years on its passage. 168. We know, from the eclipses of Jupiter's satellites, that the velocity of light is so great, that it takes only about eight minutes in travelling from the sun to the earth; while the earth itself, moving with its velocity of nineteen miles in a second, would take nearly two months to pass over the same space. We also know, as will be explained farther on, that the light of the fixed stars moves with the same velocity as the reflected light of the sun. Hence, as we are certain that the distance of the nearest of the fixed stars exceeds 80,000 times the distance of the sun from the earth, the distance of the nearest star is such, that light must be above 400 days in passing from it to the earth. 169. The limit of the distance of the nearest fixed star may be considered as well ascertained; but any thing advanced with respect to the distances of the others, must be in a manner conjectural. The brighter fixed stars have been supposed to be nearer to us than the rest. Besides their superior lustre leading to this conclusion, many of them were discovered to have small motions called proper motions, that only could be explained by supposing them to arise from a real motion in the stars themselves, or in the sun and solar system, or from a motion compounded of both these circumstances. Now whichsoever of these suppositions was adopted, it was reasonable to suppose, that the cause of the smaller stars not appearing to be affected, could only arise from the greater distance of those stars. However it is now ascertained that some of the smaller stars appear to have proper motions, much greater than those of the brightest stars. Hence conclusions deduced from the proper motions of the bright stars, respecting the relative distances of those stars must tend to weaken conclusions that might be deduced from their brightness and apparent magnitudes. There is a double star of the sixth magnitude, the 61st star of the constellation of the Swan, which consists of two stars, within a few seconds of each other. Both of these stars are moving nearly at the same rate, at the rate of about 6" in a year. It is likely they are also moving about their common centre of gravity. At present they preserve nearly the same distance from each other. This proper motion is far greater than has been observed in any of the brighter stars, or indeed in any star. It might be supposed, on this account, that these stars (61 Cygni) are nearer to us than the brighter stars. To ascertain this point, I have made observations of the zenith distances, at the opposite seasons, to endeavour to discover any sensible parallax in these stars. But there appears to be no sensible parallax. Mr. Bessel has compared these and some of the neighbouring stars by observations on the right ascensions, and found no sensible parallax. Still the arguments formerly adduced, for the brighter fixed stars being nearer to us, are considerably weakened by the great proper motions observed in some of the smaller stars. The star 40 Eridani has a proper motion of about four seconds in a year. The annual proper motion of Arcturus is about two seconds. In many of the stars there is no proper motion perceptible. Besides the proper motions, it has been remarked by Dr. Herschel, that in several instances, the line joining two stars very near together, changes its position. This is in some cases explained by a proper motion in the brighter star; in other cases it seems to indicate the revolution of one star round another. The double star Castor is a striking instance during the last fifty years, the line joining the two stars, which are about five seconds asunder, has had a motion of rotation at the rate of about a degree in a year, while the interval between the stars has remained nearly the same. Of the three circumstances which explain the apparent motion of a star, that which supposes it to arise from a combination of the motion of the solar system and of the star is most probable. The sun and nearest fixed stars are probably all in motion round a centre, the centre of gravity, perhaps of a nebula, or cluster of stars, of which the sun is one, and the milky way a part, as Dr. Herschel supposes, while this nebula revolves with other nebulæ about a common centre. 170. The direction of the motion of our system cannot with certainty be ascertained, because, from the whole motion we observe in a fixed star, we have nothing to help us in assigning that which belongs to the sun. Dr. Herschel has particularly considered this subject (Phil. Trans. 1805 and 1806), and has concluded that our sun is moving towards a point in the constellation Hercules, the declination of which is 40°, and right ascension 246°. His arguments are very ingenious, but there is necessarily so much hypothetical in them, that the mind cannot feel much confidence in his conclusion. That our system is in motion, there can be no doubt; the difficulty is to ascertain the precise direction and velocity: and from the circumstances of the case, there seems to be little probability that the knowledge will ever be here attained to by man. Dr. Herschel conjectures that the distances of the fixed stars are nearly inversely as their apparent magnitudes. From thence, and a train of ingenious reasoning, relative to the faintest nebula discoverable by his 40 feet telescope, he has concluded that the distances of these nebulæ are so great, that light issuing from them must have been two millions of years in reaching the earth. But the recent discoveries relative to the proper motions of the smaller fixed stars must, as has been said, in some measure weaken the conclusions formerly adopted respecting the relative distances of the fixed stars. The discoveries of Dr, Herschel have also made us acquainted with many nebulæ, which are not resolvable into stars, but apparently formed of luminous matter, gradually condensing, by the principle of universal attraction, into masses, as if about to form the suns of future systems. Distant ages only can appreciate these conjectures. I CHAPTER X. OBSERVATIONS FOR ASCERTAINING THE DECLINATION-DISTANCE OF THE POLE FROM ZENITH-OBLIQUITY OF ECLIPTIC-RIGHT 171. PREVIOUSLY to a more minute statement of the motions of the celestial bodies, it will be necessary to give some account of the nature of the principal observations, by which these motions are ascertained, and of the instruments by which the observations are made. The most important observations, and which admit of the greatest accuracy, are those for the declination and right ascension. Having obtained the declination and right ascension, or the position with respect to the celestial equator, we can by spherical trigonometry obtain the longitude and latitude, or the position with respect to the ecliptic. The latitude and longitude of any of the bodies of the solar system, as they would be observed from the centre of the earth, are called their geocentric latitude and longitude. The tables give the distance of the body from the sun, and its place, as seen from the sun, or its heliocentric longitude and latitude, from whence we can compute its geocentric latitude and longitude, and compare them with those observed. 172. The declination of an object is best found by observing its distance, when on the meridian, from the zenith or from the horizon. Either of these distances being found, if we previously |