38 Corrections in Right Ascension of [Jan. 



The province of the practical astronomer is to determine the 

 apparent place of all sidereal bodies which come within the 

 reach of his instruments, and to observe such phenomena as 

 from time to time present themselves ; in the present instance, 

 we shall confine ourselves to the former. It is scarcely neces- 

 sary to mention, that by the place of anybody in the sidereal 

 heavens is understood its right ascension and north polar dist- 

 ance ; each is determined generally at the moment in which the 

 object passes the meridian of the observer, by the aid of instru- 

 ments fixed in its plane ; the transit instrument (with its 

 appendage, the clock) giving the former, while the quadrant or 

 circle indicate the latter. 



But by the successive labours of Bradley, Maskelyne, and 

 Pond, the places of 48 stars have been determined with extreme 

 accuracy, these we consider as Zero points when we would 

 assign to any celestial body, its right ascension or north polar 

 distance. Accordingly the business of the practical astronomer 

 among us, as far as right ascensions are concerned, is to secure 

 the meridian passage of each of these stars, or as many of them 

 as possible, and also of as many other stars, planets, or comets, 

 as opportunity will allow ; he then finds the error of his clock 

 by each Zero star, at the time of observation, thence deduces its 

 mean error at a corresponding time ; he next determines the 

 clock's daily rate by comparisons with previous observations of 

 the same stars ; and hence obtains a mean rate. With these 

 materials he is now prepared, by the aid of a little calculation, to 

 apply the clock's error to each observed transit, and is thus fur- 

 nished with the observed right ascension of each sidereal object 

 at the time of its passing the meridian of his observatory. 



Of all these calculations, however, that whereby he arrives at 

 the error of his clock is by far the most troublesome ; for before 

 he can find its error by a single star, he must apply corrections 

 to the star's mean right ascension, brought up to the 1st of Jan. 

 of the current year; and these he must seek by reference to the 

 17th and 18th tables of Dr. Maskelyne's ; the first of which 

 gives him the sum of the corrections for aberration, precession, 

 and solar inequality of precession. Nine times out often, how- 

 ever, he has to find the equation by proportional calculation, 

 and when gotten, it is sometimes positive, sometimes negative. 

 Having proceeded thus far, he refers to the Nautical Almanac 

 for the place of the moon's node, and consults Table 18, which 

 affords him, rarely without calculation, the correction for lunar 

 nutation; again sometimes a positive, sometimes a negative quan- 

 tity : he now applies one correction to the other, and procures a 

 result which, added to or subtracted from the star s mean right 

 ascension, affords him the star's apparent right ascension at the 

 time required; and which compared with the observed transit, 

 presents him with the clock's error, by that particular star. 

 Thus has he to hunt out corrections for every one of the 36 stars 

 before lie can convert its observation to any useful purpose; and 



