Page:Encyclopædia Britannica, first edition - Volume I, A-B.pdf/553

 ASTRO N O M 461 ecliptic : We now proceed to explain the other caufe of to both funs at the fame time, and therefore it is noon this difference, namely, the inequality of the fun’s ap- by them both at the.fame moment. parent motion, which is floweft in the fummer, when the But the increafing velocity of the real fun, now bethe fun is farthefl: from the earth, and fwiftelt in winter ing at the quickeft, carries him before the fictitious one ; when he is neareft to it. But the er. th’.s motion on its and therefore, the fame meridian will come to the fictiaxis is equable all the year round, and is performed tious fun fooner than to the real : For, whilft the fictifrom weft to eaft; which is the way that the fun appears tious fun moves from c to g, the real fun moves through a greater arc from C to G ; confequently the point K has to change his place in the ecliptic. If the fun’s motion were equable in the ecliptic, the its noon by the clock when it comes to I, but not its whole difference, between the equal time as fhewn by a noon by the fun till it come to /. And although the veloclock, and the unequal time as (hewn by the fun, would city of the real fun diminilhes all the way from C to A, arife from the obliquity of the ecliptic. But the fun’s and the fiCHtious fun by an equable motion is ftill coming motion fometimes exceeds a degree in 24 hours, though nearer to the real fun, yet they are not in conjunction till generally it is left: And when his motion is flowed, any the one comes to A and the other to a; and then it is particular njeridian will revolve fooner to him than when noon by them both at the fame moment. his motion is quickeft; for it will overtake him in left Thus it appears, that the folar noon is always later time when he advances a leftffpace than when he moves than noon by the clock, whilft the fun goes from F to A% fooner whilft he goes from A to C; and at thefe points through a larger. ^ Now, if there were two funs moving in the plane of the fun and clock being equal, it is noon by them both the ecliptic, fo as to go round it in a year ; the one defcri- at the fame moment. bing an equal arc every 24 hours, and the other defcri- The point A is called, the fun’s apogee, becaufe bing fometimes a left arc 24 hours, and at other times a when he is there he is at his greateft diftance from larger, gaining at one time of the year what it loft at the earth; the point C % perigee, becaufe when in it the oppofl.te; it is evident that either of thefe funs would he is at his leaft diftance from the earth ; and'a right come fooner or latter to the meridian than the other, line, as AEG, drawn through the earth’s centre, from as it happened to be behind or befor,e the other: and one of thefe points to the other, is called//;*- line of-the when they were both in conjunftion, they would come to apfides. the meridian at the fame moment. The diftance that the fun has gone in any time from As the real fun moves unequably^n the ecliptic, let us his apogee (not the diftariee he has to go to it, though fuppofe a fiAitious fun to move equably in a circle coinci- ever fo little) is called his mean anomaly, and is reckondent with the plane of the ecliptic. In Plate XLIII. fig. 1. ed in figns and degrees, allowing 30 degrees to a fign._ let ABCD be the ecliptic or orbit in which the real fun Thus, when the fun has gone, fuppofe 174 degrees from moves, and the doted circle abed the imaginary orbit of the his apogee at A, he is faid to be 5 figns 24 degrees from fi&itious fun ; each going round in a year according to the it, which is his mean anomaly: And when he is gone, order of letters, or from weft to eaft. Let HIKL be the fuppofe 35:5 degrees from his apogee, he is faid to be 11 earth turning round its axis the fame way'every 24 hours ; figns 25 degrees* from it, although he be but 5 degrees and fuppofe both funs to ftart frbm A and a, in a right ftiort of A in coming round to it again. line with the plane of the meridian EH, at the fame mo- From what was faid above, it appears, than when the1 ment; the real fun at ^ being then at his greateft di- fun’s anomaly is left than 6 figns, that is, when he is ftance from the earth, at which time his motion is any where between A and C, in the half ABC of his floweft ; and the fiAitiqus fun at a, whofe motion is al- orbit, the folar noon precedes the clock noon; but when ways equable, becaufe "his diftance from they earth is fup- his anomaly is more than 6 figns, that is, when he is apofed to be always the fame. In the time that the meridian ny where between C and A, in the half CD A of his revolves from H to H again, according to the order of orbit, the clock noon precedes.the folar. When his. anothe letters HIKL, the real fun has moved from A to F; maly is o figns o degrees, that is, when he is in his aand the iiAitious with a^quicker motion from a to f, pogee zt A or 6 figns o degrees, which is when he is . through a larger arc . Therefore, the meridian EH will in his perigee at C; he comes to the meridian at the^ morevolve fooner from H to h under the real fun at F, than ment that the fictitious fun dbes, and then it is noon by from H to k under the fictitious fun at f; and confequent- them both at the fame inftant. ly it will then be noon, by the fun-dial fooner than by The obliquity of the ecliptic to the equator, which is the firft mentioned caufe of the equation of time, would the clock. As the y;a! fun moves from A towards C, the fwi£t- make the fun and clocks agree on four days of the year ; nefs of his motion increafes all the way to C, where it is which are, when the fun enters Aries, Cancer, Libra, at the quickeft. But notwithftanding this, the fictitious an4 Capricorn: But the other caufe, now explained, fun gains fo much upon the real, foon after his departing wpuld- make the fun and clocks equal only twice a. year; from A, that the increafing velocity of the, real fun does that is, when the fun is in his apogee and perigee. Connot bring him up with the equally moving fictitious fun fequently, when thefe two points fall in the beginnings till -the former comes to C, and the latter to c, when each of Cancer and Capricorn, or of Aries and Libra, they has gone half round its refpeCtive orbit; and then being in concur in^naking the fun and clocks equal in thefe points. conjunction, the meridian EH relvolving to EK comes But the apogee at prefect is in the 9th degree of Cancer, 6A and Vol. I. No. 20.. 3