| XXX | (446) | XXX |
^6 ASTRO N O M Y. earth, would be up to the inhabitants on the oppofite fide. and all thofe in the fouthern hemifphere-are inviFor, the inhabitants n, i, e, m, s, o, q, 1, (Plate XXXIX. fible. The like phenomena are feen by the obferver (s) fig. 6.) (land with their feet towards the earth’s centre 6’; on the fouth pole, with refped to the hemifphere ESg>_; and have the fame figure of Iky, N, /, E, M, S, 0, §>_, L and to him the oppofite hemifphere is always invifible. over their heads. Therefore the point S is as diredly Hence, under either pole, only half of the heavens is upward to the inhabitant (s) on the fouth pole, as N is to feen; for thofe parts which are once vifible never fet, the inhabitant n on the north pole ; fo is v? to the inha- and thofe which are once invifible never rife. But the bitant e, fuppofed to be on the north end of Peru; and ecliptic TCX, or orbit which the fun appears to defcribe ^to the oppofite inhabitant q on the middle of the ifland once a year by the earth’s annual motion, has the half of Sumatra. Each of thefe obfervers is furprifed that YC conftantly above the horizon EC§>_ of the north pole Ids oppofite or antipode can ftand with his head hanging n ; and the other half CX always below it. Therefore, downwards. But let either go to the other, and he will whilft the fun defcribes the northern half YC pf the etell him that he flood as upright and firm upon the place cliptic, he neither fets to the north pole, nor rifes to the where he was, as he now ftands where he is.' To all thefe fouth; and whilft he defcribes the fouthern half CX, he obfervers, the fun, moon, and ftars, feem to turn round neither fets to the fouth pole nor rifes to the north. the points N and S, as the poles of the fixed axis NCS; The fame things are true with refped to the moon; onbecaufe the earth does really turn round the mathematical ly with this difference, that as the fun defcribes the eline nCs as round an axis, of which n is the north pole, cliptic but once a year, he is for half that time vifible to and s the fouth pole. The inhabitant C/ (PlateXL. fig. i.) each pole in, its turn, and as long invifible; but as the affirms thathe is on the uppermofl fide of the earth, and moon goes round the ecliptic in 27 days 8 hours, fhe is wonders how another at L can ftand on the undermoft fide only yifible for 13 days 16 hours, and as long invifible with his head hanging downwards. But U, in the mean to each pole by turns. All the planets likewife rife and time, forgets that in twelve hours time he will be carried fet to the poles, becaufe their orbits are cut obliquely in half round with the earth’, and then be in the very fitu- halves by the horizon of the poles. When the fun (in ation that L now is, although as far from him as before. his apparent way from X) arrives at C, which is on the And yet, when U comes there, he will find no difference 20th of March, he is juft riling to an obferver n on the as to his manner of ftanding; only he will fee the oppo- north pole, and letting to another at s on the fouth pole. fite half of the heavens, and imagine the heavens to have From C he rifes higher and higher in every apparent diurnal revolution, till he comes to the higheft point of the gone half round the earth. Wheh we fee a globe hung up in a room, we cannot ecliptic y, on the 2 ift of June, and then he is at his help imagining it to have an upper and an under fide, greateft altitude, which is 234 degrees, or the arc and immediately form a like idea of the earth ; from Ey, equal to his greateft north, declination ; and from whence we conclude, that it is as impoffible for people to thence he feems to defcend gradually in every apparent ftand on the under fide of the earth, as for pebbles to lie circumvolution, till he fets at C on the 23d of Septemon the under fide of a common globe, which inftantly ber; and then he goes to exhibit the like appearances at fall down from it to the ground; and well they may, the fouth pole for the other half of the year. Hence, the becaufe the attraction of the earth, being greater than fun’s apparent motion round the earth is not in parallel the attradion of the globe, pulls them away. Juft fo circles, but in fpirals; fuch as might be reprefented by a would be the cafe with our earth, if it were placed near thread wound round a globe from tropic to tropic; the a globe much bigger than itfelf, fuch as Jupiter; for fpirals being at fame diftance from one another about the then it would really have an upper and an under fide, equator, and gradually nearer to each other as they apwith refped to that large globe; which, by its attrac- proach toward the tropics. tion, would pull away every thing from the fide of the If the obferver be any where on the terreftrial equaearth next to it; and only thofe on the top of the oppo- tor eCq, as fuppofe at e, he is in the plane of the celefite or upper fide could remain upon it. But there is ftial equator; or under the equinoxial EC^_; and the .no larger globe, near enough our earth to overcome its axis of the earth nfis is coincident with the plane of his central attradion ; and therefore it has no fuch thing as horizon, extended out to N and S, the north and fouth an upper and an under fide; for all bodies, on or near poles of the heavens. As the earth turns round th“ line its furface, even to the moon, gravitate-^towards its NCS, the whole heavens MOLl feem to turn round the centre. fame line, but the contrary way. It is plain that this The earth’s bulk is but a . point, as that atC, com- obferver has the celeftial poles conftantly in his horizon; pared to the heavens; and therefore every inhabitant and that his horizon cuts the diurnal paths of all the ceupon it, let him be where he will, as at n, e, m, s> &c. leftial bodies perpendicularly and in .halves. Therefore fees half of the heavens. The inhabitant: n, on the the fun. planets and ftars, rife every day, and afeend pernorth pole of the earth, conftantly fees the hemifphere pendicularly above the horizon for fix hours ; and, pafEiViT; and having the north pole N of the heavens- fing over the meridian, defcend in the Tame manner for juft over his head, his horizon coincides with the celeftial the fix following hours; then fet in the horizon, and equator ECQj. Therefore,.. all the ftars in the northern; continue twelve hours-below it. Confequently at the hemifphere EN^J, between the equator and north pole,, equator the days.and nights are equally long throughout appear to turn round the line JVC, moving parallel to the year. When the obferver is in the fituation e, he tbsJiorizon.,. The e quatoreal ftars keep m the horizon, fees the hemilphere SEN) but in twelve .hours after,.
