| XXX | (447) | XXX |
447 ASTRO N O M ?. and come nearer one another, until he comes to he is carried half round the earth’s axis to q , and then meters, the pole; and then they meet and coincide in the equithe hemifphere S^JV becomes vifible to him ; and SEN difappears. Thus we find, that to an obferver at either noctial. On different fides df the equator, to obfervers of the poles, one half of the Iky is always vifible, and at equal diftances from it, the circle of perpetual apparithe other half never feen ; but to an obferver on the e- tion to one is the circle of perpetual occultation to the other. quator, the whole Iky is feen every 24 hours. The figure here referred to, reprefents a celefKal Becaufe the ftars never vary their diftances from the globe of glafs, hating a terreftial globe within it ; after equinoctial, fo as to be fenfible in an age, the lengths of the manner of the glafs-fphere invented by Dr Long, their diurnal and noCturnal arcs are always the fame to the fame places on the earth. But as the earth goes Lowndes’s profeflbr of aftronomy in Cambridge. If a globe be held fidewife to the eye, at feme diftance, round the fun every year in the ecliptic, one half of and fo that neither of its poles can be feen, the equator which is orrthe north fide of the equinoctial, and the ECQ_, and all circles parallel to it, as DL, yzx, abX, other half on its fouth fide, the fun appears to change MO, &c. will appear to be ftraight lines, as proje&ed his place every day, fo as to go once round the circle in this figure; which is requifite to be mentioned here, TCX every year. Therefore whilft the fun appears to becaufe we fhall have occafion to call them circles in the advance northward, from having deferibed the paralleL abA touching the ecliptic in X, the days continually following articles of this chapter. Let us now fuppofe that the obferver has gone from lengthen and the nights Ihorten, until he comes to y and the equator e towards the north pole n, and that he Hops deferibes the parallel ^zx, when the days are at the longat i, from which place he then fees the hemifphere eft and the nights at the Ih'orteft; for then, as the MEINL; his horizon MCL having fhifted as many fun goes no farther northward, the greateft portion that degrees from the celeftial poles N and S, as he has tra- is poifible of the diurnal arc yz is above the horizon of velled from under the equinoftial E. And as the heavens the inhabitant i, and the fmalleft portion zx below it. feem conftantly to turn round the line NCS as an axis, As the fun declines fouthward from y, he deferibes all thofe ftars which are not fo many degrees from the fmaller diurnal and greater noCturna! arcs,, or portions of north pole N as the obferver is from the equino&ial, circles every day; which caufeth the days to Ihorten and namely, the ftars north of the dotted parallel DL, never nights to lengthen, until he arrives again at the parallel fet below the horizon ; and thofe which are fouth of the abX; which having only the fmall part ab above the hodotted parallel MO never rife above it. Hence the for- rizon MCL, and the great part bX below it, the days mer of thefe two parallel circles is called the circle of are at the ftiorteft and the nights at the longeft; becaufe perpetual apparition, and the latter the circle of perpe- the fun recedes no'farther fouth, but returns northward tual eccultation but all the ftars between thefe two cir- as before. It is eafy to fee that the fun muft be in the cles rife and fet every day. Let us imagine many circles equinoctial twice every year, and then the days to be drawn between thefe two, and parallel to them; and nights are equally long; that is, 12 hours each thofe which are on the north fide of the equinoctial will Thefe hints ferve at prefent to give an idea of fome of4 be unequally cut by the horizon MCL, having larger the appearances refulting from the motions of the earth ; . portions above the horizon than below it; and the more which will be more particularly deferibed in the tenth , fo, as they are nearer to the circle of perpetual appari- • chapter. tion; but the reverfe happens to thofe on the fouth fide To an-obferrer at either pole, the horizon and equi-of the equiqoCtial, whilft the equinoctial is divided in notftial are coincident; and the fun and ftars feem to two equal parts by the horizon. Hence, by the appa- move parallel to the horizon; therefore, fuch an obferrent turning of the heavens, the northern ftars deferibe ver is faid to have a parallel pofition of the fphere. To greater arcs or portions of circles above the horizon than an obferver any w'here between either pole and equator, the below it; and the.greater, as they are farther from the parallels deferibed by the fun and ftars are cut obliquely equinoctial towards the circle of perpetual apparition; by the horizon, and therefore he is faid to,have an obwhilft the contrary happens to all ftars fouth of the equL lique pofition of the fphere. To an obferver any where noCtial; but thofe. upon it deferibe equal arcs both above on the equator, the parallels of motion, deferibed by the and bqjow the horizon, and therefore they are juft as fun and ftars, are cut perpendicularly, or at right angles, long above as below it. by the horizon ; and therefore he is faid to have a right An obferver on the equator has no circle of perpetual pofition of the fphere. And thefe three are all the dif- apparition or occultation, becaufe all the ftars, together, ferent ways that the fphere can .be pofited to all people with the fun and moon, rife and fet to him every day. on the earth.. But, as a bare view of the figure is fufficient to ftrew tbat^hefe two circles DL and MO are juft as far from the poles N and S as the obferver at i (or one oppofite Chap. IV. The Phenomena of the Heavens to him at o) is from the equator EC§f, it is plain, that at feen from different farts of the Solar if an obferver begins to travel from the equator towards Syjlem,. either pole, his circle of perpetuil apparition rifes from that pole as from a point, and his circle of perpetual oc-r cultation from the other. As the obferver advances to- So vaftly great is the diftance of the ftarry heavens* ward the nearer pole, thefe two circles enlarge their dia- that if viewed from any part of the folar fyftem, or even many -
