ABERRATION of the STARS, &c. 53 



As rid. or fine of 9c ° 



Is to 20 the eq\ial parts contained In rad. 



So is tlie line of the lat. 22° 28' 



To 7.6 tiie equal parts cint. in the femi-conjngate Log. 0.88326 



From the fame fcale of equal parts take 7.6, and from the 

 centre of the circle at E, apply this diftance each way on the 

 diameter AC : fuppofe thofe points to be at F, and G, then 

 will FG, be the conjugate diameter of the ellipfe BFDG ap- 

 parently difcribed by the ftar. The ellipfe mull be divided 

 funilar to the ecliptic into fi.^ns, &c. to fhew the Sun's place. 

 This divifion mull begin from the longitude of the flar, for 

 which the projeflion is made, which in the prefent cafe is i' 23'' 

 13' at the point F. — From the point A in the primitive circle 

 lay off 23° 13', (the excefs of the ftar's longitude above 1%) 

 towards B, to the point z : then from the point z, draw the 

 occult line si to the periphery of the ellipfe parallel to AC, 

 and the place of the firft fign will be had — next from the point 

 z, in the primitive lay off 30'' or one fign each way, and from 

 thofe points, as in the firfl: cafe, draw parallels to AC, meeting 

 the periphery of the ellipfe, and the pofitiorr of o", and 2' will 

 be had : In this manner the whole periphery of the ellipfe may 

 be graduated into figns, and degrees if the projeftion fliould be 

 fufEciently large. 



The next requifite is to draw the meridian of the ftar through 

 the centre of the projeftion. In order to do this, the angle 

 made by the interfeftion of the circle of the ftar's longitude, 

 with the circle of its right afcenfion, muft be determined ; 

 which in the prefent cafe is about 18" 11': this quantity mufi 

 be laid off in the primitive from A to M, toAvards B* : then 

 from M through the centre of the projeftion draw MEP cutting 

 the ellipfe in the point u, and it vv'ill be the meridian required. 



From a little confideration it will be eafy to conceive that the 

 effeft of aberration will always be found three figns behind the 

 Sun's place — hence the aberration anfwering to 2 of the Sun's 

 place, muft be eftimated at 11 and the occult line E 11, 



* It may be obferved for a general rule that when the right afcenfion of 

 the ft.ir is lefs than 3^^ and mnre than y^ the meridian muft he lai 1 off from 

 A towards B; when more than 3'^ and lefs than 9' from A towards D. 



H will 



