Page:Encyclopædia Britannica, first edition - Volume I, A-B.pdf/554

 5 62 ASTRO N O M Y. and the perigee in the 9th degree of Capricorn, and there- 6 hours: So that the civil year is almoft a mean bethe fydereal and tropical. fore the fun and clocks cannot be equal about the begin- twixt ning of thefe figns, nor at any time of the year, except As the fun deferibes the whole ecliptic, or 360 dewhen the fwiftnefs or flownefs of equation refulting from grees, in a tropical year, he moves 59 minutes 8 feOne caufe juft balances the flownefs or fwiftnefs arilmg conds of a degree -.very day at a mean rate ; and confequently 50 feconds of a degree in 20 minutes 174 fefrom the other. conds of time : Therefore, he will arrive at the fame equinox or folftice when he is 50 feconds of a degree Chap. XII. Of the PreceJJion of the fliort of the fame ftar or fixed point in the heavens from which he fet out in the year before. So that, with reEquinoxes. fpedt to they fixed ftars, the fun and equinoftial points back (as it were) 30 degrees in 2160 years; which It is a known fa<ft, that there is a greater quantity fall will make the ftars appear to have gone 30 degrees of matter accumulated -all round the equatoreal parts of forward, with refpeft to the figns of the ecliptic in that the earth than any where elfe. the fame figns always keep in the fame points The fun and moon, by attracting this redundancy of timethe: For ecliptic," without regard to the conftellations. matter, bring the equator fooner under them in every of To this by a figure, (Plate XLIII. fig. 1.) let return towards it, than if there was no fuch accumula- the funexplain be in conjunction with a fixed ftar at 5, fuppofe tion. Therefore, if the fun fets out, as from any ftar, in the 30th of ^ on the 21ft of May 1756. Then, or other fixed point in the heavens, the moment when making 2160degree through theecliptic VIVX, at the he is departing from the equinoctial or from either tropic, end of fo manyrevolutions years, he will be found again at he will come to the fame equinox or tropic again 20 S: But at the fydereal end of fo many Julian years, he will be min. 174 fee. of time, or 50 feconds of a degree, be- found at M, ihort of S; and at the end of fo many fore he completes his courfe, fo as to arrive at the tropical years, he will found Ihort of M in the 30th fame fixed ftar or point from whence he fet out. For, degrees of Taurus at T,be which has receded back from tire equinoctial points recede 50 feconds of a degree $ to T in that time, by the preceflion •weftward every year, contrary to the fun’s annual pro- points V5 Aries and dX Libra. The arcofSTthewillequinoctial be equal greflive motion. of the preceflion of the equinox in 2160 When the fun arrives at the fame equinoctial or folfti- to the amount at the rate of jo feconds of a degree, or 20 miti'al point, he finilhes what we call the tropical year ; years, 174 feconds of time, annually : This, in fo many which, by obfervation, is found to contain 365 days nutes 30 days 104 hours ; which is the difference 5 hours 48 minutes 57 feconds : And, when he ar- years, makes 2160 fydereal and tropical years : And the arc rives at the fame fixed ftar again, as feen from the earth, between MT will be equal to the fpace moved through by the he completes the fyderealyear, which contains 365 days fun in 2160 times 11 minutes 3 feconds, or 16 days 6 hours 9 minutes 144 feconds. The fydereal year is 13 hours 48 minutes, which is the difference between therefore 20 minutes 174 feconds longer than the folar 2x60 Julian and tropical years. or tropical year; and 9 minutes 144 feconds longer th.an the Julian or civil year, which we-ftate 'at 365 days A