PRECESSION. 



This seeming change of place in the 

 stars was first observed by Hipparchus of 

 Rhodes, who 128 years before Christ, 

 found that the longitudes of the stars in 

 his time were greater than they had been 

 before observed by Tymochares, and 

 than they were in the sphere of Eudoxus, 

 who wrote 380 years before Christ. 

 Ptolemy also perceived the gradual 

 change in the longitudes of the stars ; 

 but he stated the quantity at too little, 

 making it but 1 in 100 years, which is 

 at the rate of only 36" per year. Y-hang, 

 a Chinese, in the year 721, stated the quan. 

 tily of this change at 1 in 83 years, which 

 is at the rate of 43" per year. Other 

 more modern astronomers have made this 

 precession still more, but with some 

 small differences from each other; and 

 it is now usually taken at 50"$ per year. 

 All these rates are deduced from a com- 

 parison of the longitude of certain stars, 

 as observed by more ancient astronomers, 

 with the later observations of the same 

 stars ; viz. by subtracting the former from 

 the latter, and dividing the remainder by 

 the number of years in the interval be- 

 tween the dates of the observations. 

 Thus, by a medium of a great number 

 of comparisons, the quantity of the an- 

 nual change has been fixed "at 50", ac- 

 cording to which rate it will require 

 25,791 years for the equinoxes to make 

 their revolutions westward quite around 

 the circle, and return to the same point 

 again. 



The phenomena of this retrograde mo- 

 tion of the equinoxes, or intersections of 

 the equinoctial with the ecliptic, and con- 

 sequently of the conical motion of the 

 earth's axis, by which the pole of the 

 equator describes a small circle in the 

 same period of time, may be understood 

 an illustrated as follows ;" Let NZSVL be 

 the earth. (See Plate Perspective, &c. 

 fig. 6.) SOX A its axis produced to the 

 starry heavens, and terminating in A, the 

 present north pole of the heavens, which 

 is vertical to N, the north pole of the 

 earth. Let EOQ be the equator, T25Z 

 the tropic of cancer, and VT^J the tro- 

 pic of Capricorn ; VOZ the ecliptic, and 

 BO its axis, both of which are imtnovea- 

 ble among the stars. But as the equi- 

 noctial points recede in the ecliptic, the 

 earth's axis SON is in motion upon the 

 earth's centre O, in such a manner as to 

 describe the double cone NOn and SOs, 

 round the axis of the ecliptic BO, in the 

 lime that the equinoctial points move 

 round the ecliptic, which is 25,791 years, 

 and in that length of time, the north pole 



of the earth's axis produced, describes 

 the circle ABCDA in the starry hea- 

 vens, round the pole of the ecliptic, 

 which keeps immoveable in the centre of 

 that circle. The earth's axis being now 

 23 28' inclined to the axis of the eclip- 

 tic, the circle ABGDA, described by the 

 north pole of the earth's axis produced 

 to A, is 46 56' in diameter, or double 

 the inclination of the earth's axis. In 

 consequence of this, the point A, which 

 is at present the north pole of the hea- 

 vens, and near to a star of the 2d mag- 

 nitude in the end of the Little Bear's tail, 

 must be deserted by the earth's axis ; 

 which, moving backwards one degree 

 every 711 years, will be directed towards 

 the star or point B in 6447| years hence ; 

 and in double of that time," or 12,895^ 

 years, it will be directed towards the stal- 

 er point C ; which will then be the nonh 

 pole of the heavens, although it is at 

 present 8$ degrees south of the zenith of 

 London L. The present position of the 

 equator EOQ will then be changed into 

 eOg, the tropic of cancer T25Z into 

 Vf23, and the tropic of Capricorn VTVJ 

 into/V^Z; as is evident by the figure- 

 And the sun, in the same part of the hea- 

 vens where he is now over the earthly 

 tropic of Capricorn, and makes the short- 

 est days and longest nights in the north- 

 ern hemisphere, will then be over the 

 earthly tropic of cancer, and make the 

 days longest and nights shortest. So 

 that it will require 12,895^ years yet 

 more, or from that time, to bring the 

 north pole N quite round, so as to be di- 

 rected towards that point of the heavens 

 which is vertical to it at present. And 

 then, and not till then, the same stars 

 which at present describe the equator, 

 tropics, and polar cricles, &c. by the 

 earth's diurnal motion, will describe them 

 over again. 



From this shifting of the equinoctial 

 points, and with them all the signs of the 

 ecliptic, it follows, that those stars, which 

 in the infancy of astronomy were in Aries, 

 are now found in Taurus ; those of Tau- 

 rus in Gemini, &c. Hence likewise it is, 

 that the stars which rose or set at any 

 particular season of the year, in the times 

 of Hesiod, Eudoxus, Virgil, Piiny, &c. 

 by no means answer at this time their de- 

 scriptions. 



As to the physical cause of the preces- 

 sion of the equinoxes, Sir Isaac Newton 

 demonstrates, that it arises from the 

 broad or fiat spheroidal figure of the 

 earth ; which itself arises from the earth's 

 rotation about its axis : for as more mat- 



