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LXXV. On the nelv Tables of Aberration, Nutation and 

 Precession. 



To the Editors of the Philosophical Magazine and Journal. 



TN reply to the request of your correspondent T. M. ex- 

 -*■ pressed in your last Number, I shall trouble you with the 

 I'ollovving explanation of M. Sciiumacher's tables. 



The term " January " refers to the noon of December 3 1 st : 

 and the intervals are carried on, for every ten days, to Decem- 

 ber 36; which latter date is meant to designate the 5th of 

 January in the ensuing year. The date of January has been 

 long adopted by astronomers; and is inserted in Dr. Maske- 

 lyne's tables published many years ago : so that it is by no 

 means a new term. I acknowledge, however, that it may 

 sometimes lead to confusion. 



With respect to the formation of the tables, they are con- 

 structed on the principles detailed by M. Bessel in his Astro- 

 nomic Fundamenta, page 67 : and are so contrived that they 

 show the corrections applicable to the star, at the juo>ncnt of 

 observation ; which is made the argument, wherewitli to enter 

 the table. I have not time at present to enter into a full ex- 

 })lanation of those principles, or of their mode of application : 

 but, the following brief outline may perhaps suffice. At some 

 future time I may probably resume the subject, more at lengtJi^ 



The year is supposed to commence at tlie moment when tlie 

 mean* longitude of the sun is 280°; and to consist of 366|^ 

 sidereal days. Now, a given star will, at some tiixus in the 

 year, culminate tijaice in the period of a day, according to the 

 civil motle of reckoning : and an error of one day, according 

 to such computation, would arise in the argument if no cor- 

 rection were applied to counteract it. The correction, which 

 M. Bessel has introduced for this purpose, is denoted by tlie 

 letter i. 



Again ; since the mean longitude of the sun is never exactly 

 the same at the commencement of each successive year; and 

 since, in fiu;t, the year seldom or never begins when the mean 

 longitude of the sun is exactly 280°, a correction for the Argu- 

 ment, is also applied to counteract the effect of this assum,p- 

 tion. This united correction for the Argument, is denoted by 

 the letter /- : and a small table is given, showing its value for 

 certain years. Against the leaji year (J820) are inserted two 

 numbers, differing from each other by unity: the first is to be 



* To explain the mode, by which M. Bessel passes from the mean to the 

 true iongitiulc of the sun, would extend this letter to an inconvenient 

 lenjlh. 



taken 



