100 



THIi: CIVIL ENGINEER AND ARCHITECTS JOURNAL. 



[April, 



of some of the stars at ditfercnt timps, the mean motion of the equi- 

 noctial ])(iints, or the precession of the i quinoxes, may be discovered. 

 M. Lalande, in his astrunoniy, lias computed the precession liy com- 

 paring ihe longitude of Spica Virginis, as assigned by Hipparchus, 

 with the longitude of thf same star computed in 175U. 



12S u.c. Longitude of Spica Virginis 5» 24° 0' 



1750 A.D. Ditto .. .. C 20 21 



Increase in 1878 years 0» 20= 21' 

 From this it appears that the annual mean precession is equal to 



2G° 21' 

 SO'- 5= . By a number of like comparisons, the same author 



fixed the secular precession — that is, the amount of accumulated pre- 

 cession for lOU years — to be 1° 23' ■M" ; the mean annual precession 

 corresponding to this is 50"* 34: and the sum of such annual preces- 

 sion amounts to f in 715 years. If we su[)pose the precession to be 



/360x(50x60xlO\ ^ ^ . , 



aO"' 1, then, in 2d,809 years! 1, the first point of 



Aries will have retrograded through an entire circle. The quantity 

 .'JU'" 1, which is the mean value ol the precession, is obtained from the 

 dirterences of the longitudes of a great many stars (three or four 

 hundred, for instance^, computed at different epochs. This mean 

 (juantitv may not agree with the mean qu.iiitily derived iruin the ob- 

 servations of a single star, however many, or accurately made, these 

 observations may be. It will be found the case with f'ollux, thesecond 

 star in the fidlciwing table. The dilVerences, however, between the 

 mean quantities of tile precession as they residt from three hundred 

 stars, or from a single one, is in all cases very small. 



Yet the difference which is found to exist, points out some pecu- 

 liarity in every star. For instance, Pollux cannot be like most of tlie 

 other stars, apparently entirely fixpd, but must have what is called, or 

 what we are obliged to call, from default of a knowledge of its cause, 

 a proper motion. However, the comparison of the longituiles of the 

 stars, computed to the epochs of 1756 and 1815, establishes, as we 

 have before observed, the important fact of the precession of the equi- 

 noxes. Because the mean longitude of a star is not altered solely by 

 the precession of the equinoxes, astronomers employ the term annual 

 variation, comprehending under it the effects both of precesaioit and 

 of annual proper motion. 



We shall now compare the latitudes of the stars mentioned iu the 

 above table, at tlie same epoclis. 



Latitudes. 



It appears from this table that the changes of the latitudes are very 

 small. III no case amounting to U-1" annually. " The astronomical 

 fact is," says Wood house, "a minute annual change of latitude, and 

 a considerable ehai.ge uf longitude. With regard to the former change, 

 we may conjecture that it aiisi s either partly from the precession of 

 the equinoxes, anil partly from otiier causes, or that it is altogether 

 independent cd llie precession." The succeeding tables show the 

 turiution buth in right ascension and declination of the stars whose 



latitudes and longitudes we have just compared. These variations 

 more clearly point out the general apparent change produced in the 

 heavens by Ik right motion of the tart/i's axis, than those of the lati- 

 tudes and longitudes, as the declin.itions and right ascensions of the 

 stars are reckoned in a manner similar to the latitudes and longitudes 

 of places on the earth. Yet they are not in complete accordance with 

 the right motion, as the latitudes of places have been supposed to re- 

 main fixed and trom the dissatisfactory theory of corrections, of 

 w liicli we shall speak hereafter. 



Right .ySscensions. 



Declinations. 



Stars. 



1843 



OC Arietis 

 Pollux 

 Spica Virg. 

 OC Aquilae 

 OC Pegasi 



N22 43 4-14 

 N28 23 58-81 

 S 10 20 23-55 

 N 8 27 26-37 

 N 14 21 41-97 



1817 



N 22 35 33-1 

 N28 27 31-7 

 S 10 12 6-5 

 N 8 23 37-6 

 N 14 13 26-0 



Difference 

 in 26 years. 



7 31-04 

 3 32-89 



8 17-05 

 3 48-77 

 8 15-97 



Mean annu- 

 al variation. 



+ 17- 34 

 - 8-188 

 + 19-117 

 + 8-779 

 + 19-075 



0/ the Collapsing of the Planes of the Equator and Ecliptic. 

 The angle contained between the plane of the equator and ecliptic 

 is what is denominated the obliquity of the ecliptic ; which is shown, 

 from repeated observations, to be variable. In this place it will be 

 sufficient to show the results of a long succession of such observations 

 by different astronomers, taken from the " Encyclopedia Metropoli- 

 tana" :— 



230 B.C., made the obliquity to be 23° .11' 20" 



A.D. 



"Oliver Byrne," 1843 „ „ 



The observations of Albatenius and Arazachel are here corrected for 

 refraction : those of Waltherus, De la Caille, De la Lande, computed. 

 The obliquity of Tycho is put down as correctly computed from his 

 observations ; also the obliquity as determined by Flamstead, is cor- 

 rected for the nutation of the earth's axis : these corrections L ilande 

 applied. It is manifest, from the above observations, that the obliqu- 

 ity of the ecliptic continually decreases; and the "irregularity which 

 here appears," says the writer, "in the diminution, we in.iy ascribe 

 to the inaccuracy of the ancient observations, as we know they are 

 subject to greater errors than the irregularity of this variation. If 

 we compare the first and last observations, they give a diminution of 

 70" in 100 years. If we compare the observations of Lalande with 

 that of Tycho, it gives 45". The same compared with Flamstead 

 gives 50". If wp compare that of Dr. Maskelyne with Dr. Bradley's 

 and Meyer's, it gives 50". The comparison of Dr. M askelyne's with 

 that of Lalande, which he took as the mean of several results, gives 



50", as determined from the most accurate observations. The ob- 

 servations of Pond, compared with those of Br.idley, give 44" for 

 the variation of the obliquity in 100 years, or 044" annually." 



