Page:Encyclopædia Britannica, first edition - Volume I, A-B.pdf/563

 469 ASTRO N O M Y. eaft. But let the globe be turned half round < its) axis’ ground; and there the moon rifes about 48 minutes till rs Capricorn comes to the meridian and Y Aries later every day or night than on the former. At confiderable didances from the equator, where the weather rifes in the eaft, and then the ecliptic will have the low and feafons are more uncertain, the autumnal full moons elevation NL above the horizon, making only an angle of 26! degrees with it; which is 47 degrees lefs rife very foon after fun-fet for feveral evenings together. NUL than the former angle, equal to the diftance between the At the polar circles, where the mild feafon is of very tropics. Ihort duration, the autumnal full moon rifes at fun-fet from the firft to the third quarter. And at the poles, In northern latitudes, the fmalleft angle made by the where the fun is for half a year abfent, the winter full ecliptic and horizon is when Aries rifes, at which time Libra fets ; the greateft when Libra rifes, at which time moons fliine conftantly without fetting from the firft to Aries fets. From the riling of Aries to the rifing of the third quarter. It is foon faid that all thefe phenomena are owing to Libra, (which,is ttvelve fydereal hours), the angle inthe different angles made by the horizon and different creafes; and from the rifing of Libra to the rifing of parts of the moon’s orbit; and that the moon can be full Aries, it decreafes in the fame proportion. By this arbut once or twice in a year in thofe parts of her orbit ticle and the preceding, it appears that the ecliptic rifes which rife with the leaft angles. But to explain this fafteft about Aries, and floweft about Libra. fubjedt intelligibly, we muft dwell much longer upon it. On the parallel of London, as much of the ecliptic The plane of the equinoctial is perpendicular to the ' rifes about Pifces and Aries in two heturs as the moon earth’s axis : and therefore, as the earth turns round goes through in fix days; and therefore whilft the moon its axis, all parts of the equinodtial make equal angles is irt thefe figns, fhe differs but two hours in rifing for with the horizon both at riling and fetting ; fo that equal fix days together ; that is, about 20 minutes later every portions of it always rife or fet in equal times. Confe- day or night than on the preceding, at a mean rate. quently, if the moon’s motion were equable, and in the But in 14 days afterwards, the moon comes to Virgo and equinodlial, at the rate of 12 degrees from the fun every Libra, which are the oppofite figns to Pifces and Aries ; day, as it is in her orbit, fire would rife and fet 48 mi- and then fhe differs almoft four times as much in rifing ; nutes later every day than on the preceding : for 12 de- namely, one hour and about fifteen minutes later every grees of the equinoctial rife or fet in 48 minutes of time, day or night than the former, whilft fhe is in thefe figns. in all latitudes. But the moon’s motion is fb nearly in the ecliptic, All thefe things will be made plain by putting fmall that we may confider her at prefent as moving in it. ' patches on the ecliptic of a globe, as far from one anoNow the different parts of the ecliptic, on account of ther as the moon moves from any point of the celeftial its obliquity to the earth’s axis, make very different ecliptic in 24 hours, which at a mean rate is 13!-deangles with the horizon as they rife or fet. Thofe parts grees ; and then in turning the globe round, obferve the or figns which rife with the fmalleft angles fet .with the rifing and fetting of the patches in the horizon, as the greateft, and vice verfa. In equal times, whenever this index points out the different times in the hour-circle. angle is leaft, a greater portion of the ecliptic rifes than A few of thefe patches are reprefented by dots at o 1 when the angle is larger; as may be feen by elevating the 2 3, 6v. on the ecliptic, which has the pofitiop LUI pole of a globe to any confiderable latitude, and then when Aries rifes in the eaft; and by the dots 0123, &c. turning it round its axis in the horizon. Confequently, when Libra rifes in the eaft; at which time the ecliptic when the moon is in thofe figns which rjfe or fet with the has the pofition EUrs making an angle of 62 degrees fmalleft angles, fhe rifes or fets with the leaft difference with the horizon in the latter cafe, and an angle of no of time ; and with the greateft difference in thofe figns more than 15 degrees with it in the former; fuppofing which rife or fet with the greateft angles. the globe redfified to the latitude of London. But, becaufe all who readuhis treatife may not be pro- Having redtified the globe, turn it until the patch at vided with globes, though in this cafe it is requifite to o, about the beginning of X Pifces in the half LUI of know how to ufe them, we fhall fubftitute the figure of the ecliptic, comes to the eaftern .fide of the horizon ; a globe; (Plate XLIV. fig. 2.) in which FUP is the axis, and then keeping the ball fteady, fet the hour-index to 25TR the tropic of Cancer, LTrs the tropic of Ca- XII, becaufe that hour may perhaps be more eafily repricorn, 2LEUrs the ecliptic touching both the tro- membered than any other. Then turn the globe round pics, which are 47 degrees from each other, and AB the weftward, and in that time, fuppofe the patch o to have horizon. The equator, being in the middle between the moved thence to 1, 13^- degrees, whilft the earth turns tropics, is cut by the ecliptic in two oppofite points, once round its axis, and you will fee that 1 rifes only which are the beginnings of T Aries and ^ Libra. K about 20 minutes later than o did on the day before. is the hour-circle with its index, F the north pole of Turn the globe round again, and in that time fuppofe the the globe elevated to a confiderable latitude, fuppofe 40 fame patch to have moved from 1 to 2 ; and it will rife degrees above the horizon, and P the fouth pole de- only 20 minutes later by the hour-index than it did at 1 preffed as much below it. Becaufe of the oblique pofi- on the day or turn before. At the end of the next turn, tion of the fphere in this latitude, the ecliptic has the fuppofe the patch to have gone from 2 to 3 at U, and it high elevation N^L above the horizon, making the will rife 20 minutes later than it did at 2. And foon angle NUS£> of 73-!. degrees with it when 05 Cancer for fix turns, in which time there will fcarce be two is on the meridian, at which time Libra rifes in the hours difference : nor would there have been fo much if Vol. I. No., 20. 3 6C the