Dreyer — New Determination of Constant of Precession . 617 



LXXXIV. — A New Deieemination of the Constant of Peecession. 

 Ey J. L. E. Deetee, Ph.D. 



[Eead, June 26, 1882.] 



Among tte various constants of Astronomy one of the most important 

 is the Constant of Precession. It is remarkable how few determina- 

 tions of the exact quantity of this constant have appeared during this 

 century, considering the great number of investigations called forth by 

 the progress of Astronomy. JS'ot counting theoretical researches on 

 the subject, such as those of Laplace, Leverrier, Lehmann, and Stock- 

 well, which possess great interest, but cannot, as regards accuracy, 

 compare with those deduced from observations, we have in reality 

 only three recent determinations of the Constant of Precession, by 

 Bessel, Struve, and Nyren. My reasons for undertaking a new deter- 

 mination will be best described by giving a rapid sketch of the re- 

 searches of these three astronomers. 



Bessel founded his investigation^ on about 2400 stars which occurred 

 both in Piazzi's Catalogue and in the catalogue which he had himself 

 deduced from Bradley's observations. As the epochs of the two 

 catalogues are 1755 and 1800, he thus found for 1777-5 the value of 

 the two constants commonly designated as m and n, with which the 

 variation of the right ascension (a) and declination (S) of a star is com- 

 puted by the well-known formulae 



da i. o • 



■^ = m + n tan 6 sm a, 



(to 



dh 



-vr = w cos a, 



at 



and from m and n the constant of the "General Precession." The 

 result thus found was some years after modified by applying certain 

 corrections to Bradley's and Piazzi's Eight Ascensions, which were 

 made necessary by the better value for the Constant of Nutation which 

 Lindenau in the mean time had produced, as also by the two Funda- 

 mental Catalogues for 1815 and 1825, deduced from the Kdnigsberg 

 observations. Bessel's modified result for the year 1800 is'^ 



50" -2235. 



Otto Struve used for his determination^ a very small number of 



1 Fundamenta Astronomiae, p. 285, et seq. 



• Astronomische Nachrichten, vol. iv. (1826). 



3 Memoires de I'Academie de St. Petersbourg, 1842. 



