NUTATION OF THE EARTH'S AXIS, &c. 6^ 



tcration of the equiuoftial points amounts i".r5 or the -ji^ of a fe- 

 cond in time. The obliquity of the ecliptic is greater by i'', 

 when the Sun is in the equinoxes, than in the folftices. The 

 right afcenfion of tlie ftars will be infenfibly aftefted, unlefs the 

 declinations fliould be very great : the declination of 88'"' 6' will 

 produce but i" in time, and 8i" 15' but ^ of a fecond. From 

 the theory the apparent didance of every ilar from the pole of 

 the equator will be fubjeft to a variation of i" twice a year, and 

 there being but three months between the greateft inclination, 

 and reclination, it will fenfibly afleft the obfervations made with 

 a good 8 feet zenith feftor. — For a further explanation, and 

 in aid to the calculations, take from any fcale of equal parts 5, 

 with that diflance for a radius defcribe a circle, which divide 

 into 13 equal parts for iigns, (fee Fig. 5. Plate I.) From what 

 has been already obferved it follows that, when the Sun is at o, 

 the pole will be at 3, when the Sun is at 3, the pole will be 

 at 9, and when the Sun is at 6, the pole will be again at o. For 

 an example : Suppofe it fliould be required to find the effeft of the 

 femi-annual equation in declination for fi Medufre, anfwering 

 to 3' of the Sun's place — lay off i' 13° 43', the right afcenfion 

 of ^ Medufic from 0% to M ; from M, through the centre C, 

 draw the meridian MD ; at right angles to which, draw the 

 diameter EF. Then from the theory, whilft the Sun is ad- 

 vancing 3% the pole will advance 6', and therefore be at 9" : and 

 the difl:ance 9 ;«, in the direftion of the meridian, will be the 

 quantity required, and when applied to the fcale, will be .34. 

 — this quantity may be readily calculated, being to the fine of 

 arc 9 F zz 43^" 43' as .5 is to radius, therefore 

 To .5 - - Log. 1.69897 



Add 43° 43' - Log. S. ^3^54 



9-5385^ 



As radius cannot be dedufted, the log. mufl be exprefled 

 "1.538^1 := .2,4-:= gm: in this mannner the calculations may 

 be made for any other points in the circle, and the quantities 

 ■will be additive to the declination of a northern ftar ; when the 

 pole is on the fame fide of a diameter at right angles to the 

 meridian with the point M, of the fl:ar's right afcenfion ; but 

 negative for a fouthern ftar ; — the contrary is to be obftTved 

 when tlic pole is on the other fide of the diameter. 



The 



