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 ASTRO N O M Y. 54^ tor to the fouth polar circle juft the reverfe. The fun not fet to any part of the north frigid zone, as Chap. VIII. The different Lengths of Days does by the candle’s ftiining on it, fo that the motion and Nights, and the Viciffttudet of Sea- Ihewn of the globe can carry no place of that zone into the fons, explained. The Explanation of the dark : And, at the fame time, the fouth frigid zone is involved in darknefs, and the turning of the globe brings Phenomena of Saturn’s Ring concluded. none of its places into the light. If the earth were to The following experiment will give a plain idea of the continue in the like part of its orbit, the fun would never diurnal and annual .motions of the earth, together with fet to the inhabitants of the north frigid zone, nor rife the different lengths of days and nights, and all the to thofe of the fouth. At the equator it would be albeautiful variety of feafons, depending on thofe mo- ways equal day and night; and as places are gradually tions. .more and more diftant from the equator, towards the Take about feven feet of ftrong wire, and bend it in- ardic circle, they would have longer days and fborter to a circular form, as abed, which being viewed oblique- nights ; whilft thofe on the fouth fide of the equator ly, appears elliptical, Plate XLI. fig. 3. Place a lighted would have their nights longer than their days. In this candle on a table, arid having fixed one end of a filk cafe there would be continual fummer on the north fide thread K, to the north pole of a fmall terreftrial globe H, of the equator, and continual, winter on the fouth fide about three inches diameter, caufe another perfon to of it. hold the wire circle, fo that it may be parallel to the But as the globe turns round its axis, move your hand table, and as high as the flame of the candle /, which flowly forward, fo as to carry the globe from H towards fliould be in or near the centre. Then, having twilled the E, and the boundary of light and darknefs will approach thread as towards the lef«r>diS.nd, that by untwiiling it towards the north pole, and recede towards the fouth may turn the globe round eaftward, or contrary to the the northern places will go through lefs and lefs way that the hands of a watch move; hang the globe pole; of the light, and the fouthern places through more and by the thread within this circle, almoft,contiguous to it; more of it; (hewing how the northern days decreafe in and as the thr£*d'untwills, the globe (which is enlight- length, and the fouthern days increafe, whilft the globe ened half round by the candle as the earth is by the fun) proceeds from H to E. When the globe is at E, it is at will turn round its axis, and the different places upon it a mean (late between the lowed and higheft part of its will be carried through the light and dark hemifpheres, orbit; the candle is diredlly over the equator, the bounand have the appearance of a regular fucceflion of days #dary of light and darknefs juft reaches to both the poles, and nights, as our earth has in reality by fuch a motion. and all places on the globe go equally through the light .As the'globe turns, move your hand (lowly, fo as to and dark hemifpheres. (hewing that the days and nights carry the globe round the candle according to the order are then equal at all places of the earth, the poles only of the letters abed, keeping its centre even with the excepted; fpr the fun is then fetting to the north pole, wire circle ; and you will perceive, that the candle be- and rifing to the fouth pole. ing Hill perpendicular to the equator, will enlighten the Continue moving the globe forward, arid as it goes thro’ globe from pole to pole in its whole motion round the the quarter rl, the north pole recedes dill farther into circle; and that every place on the globe goes equally the dark hemifphere, and the fouth pole advances more through the light and the dark, as it turns round by the into the light, as the globe comes nearer to 2B • And untwilling of the thread, and therefore has a perpetual when it comes there at F, the candle is direftly over the equinox. The globe, thus turning round, reprefents the tropic of Capricorn, the days are' at the (horteft, and earth turning round its axis ; and the motion of the nights at the longed, in the northern hemifphere, all the globe round the candle reprefents the earth’s annual mo- way from the equator to the arctic circle; and the retion round the fun, and Ihews, that if the earth’s orbit verfe in the fouthern hemifphere from the equator to the had no inclination to its axis, all the days and nights of antardlic circle; within which circles it is dark to the the year would be equally long, and there would be no north frigid zone, and light to the fouth. different feafons. But how, delire the perfon who holds Continue both motions, and as the globe moves the wire, to hold it obliquely in the pofition ABCD, through the quarter B, the north pole advances towards raifing the fide julf as much as he depreffes the fide the light, and the fouth pole recedes towards the dark ; rs. that the flame may be Hill in the plane of the circle ; the days lengthen in the northern hemifphere, and and twilling the thread as before, that the globe may ftiorten.in the fouthern ; and when the globe comes to turn round its axis, the fame way as you carry it round G, the candle will be again over the equator (as when thr candle,. that is, from- weft to eaft, let the globe the globe was at E) ana the days and nights will again be down into the lowermoft part of the wire circle at rs, equal as formerly; and the north pole will be juTFcoand if the circle be properly inclined, the candle will ming into the light, the fouth pole going out of it. fhine perpendicularly on the tropic of Cancer, and the Thus we fee the reafon why the days lengthen and frigid zone,Tying within the ardic or north polar circle, (horten from the equator to the polar circles every will be all in the light, as in the figure; and will keep in year;, why there is no day or night for fevera; turnings the light, let the globe turn round its axis ever fo often. of the earth, within the polar circles; why there is but From the equator to the north polar circle all the places one day and one night in the whole year at the poles ; have longer days and fhorter nights; but from the equa- and why the days and nights are equally long all the year.