Page:Encyclopædia Britannica, first edition - Volume I, A-B.pdf/543

 ASTRO year round at the equator, which; is always equally cut by the circle bounding'light and darknefs. The inclination of ;in axis or orbit is merely relative, becanfe we compare it with fome' other axis or orbit which we confider as not inclined, at all. T. hus, mur horizon being level to us whatever place of the earth we are upon, we confider it as having no inclination ; and yet, if we travel 90 degrees from that place, we fhall then have an horizon perpendicular to the former, but •it will llill be level to us. And if this book be held lb that the circle 4B€D‘be parallel to the horizon, both the circle abed, and the thread or axis K, will be inclined to it. But if the book or plate be held fo that the thread be perpendicular to the horizon, then the orbit 4BCD will be inclined to the thread, and the orbit abed perpendicular to it,-and parallel to the horizon. We generally-confider the earth’s annual ’orbit as having no inclination, and the orbits of - all the other planets as inclined to it. Let us now take a view of the earth in its annual courfe round the fun, Confidering its orbit as Jraving no inclination, and its axis as inclining 23-I- degrees from a line perpendicular to the plane of its orbit, and keeping the fame oblique diredlion in all parts of its annual courfe; or, as commonly termed, keeping always parallel to itfelf. In Plate XLI. fig. 4. let abccfefgh be the. earth in eight different parts of its orbit, equidillant from one another. Ns its axis, N the north pole, s the fouth pole, and S the fun nearly in the centre of the earth’s orbit. As the earth goes round the fun according to the order of the letters abed, &c. its axis Ns keeps the fame obliquity, and is Hill parallel to the line MNs. When the earth is at a, its north pole inclines toward the fun S, and-brings all the northern places more into the light than at any other time of the year. But when the earth is at e in the oppofite time of the year, the north pole declines from the fun, which occafions the northern places to be more in the dark than in the light; and the reverfe at the fouthern places, as is evident by the figure. When the earth is either at c or g, its axis inclines not either to or from the fun, but lies lidewife to him, and then the poles are in the boundary of light and darknefs; and the fun, being dire&ly over the equator, makes equal day and night at all places When the earth is at b, it is half way between the fum-4 mer folftice and harveft equinox; when it is at d, it is half way from the harveft equinox to the winter folftice; at f, half way from the winter folftice to the fpring er quinox; and at half way from the fpring equinox to. the fummer folftice. From this oblique view of the earth’s orbit, let us fuppofe ourfelves to be raifed far above it, and. placed juft over its centre 5; looking down, upon it from its north pole; and as the earth’s orbit differs but very little from a circle, we lhalLhave its figure in fuch .a view reprefented by the circle 4BCDEFGH (Plate XL1L fig.. 1.). Let us fuppofe this circle to be divided into-12 equal parts, called Jigns, having their names affixed to them ; and each fign into 30 equal parts, called degrees, xmmbered 10, 20, 30, as in the outermoft circle of. the fi-

N O M ¥. 455 gure,: which reprefents the great ecliptic in the heavens. The earth is ftiewn in eight different pofitions in thiscircle, and in each pofition JE is the equator, T the tiopic of Cancer, the dotted circle the parallel of London, U the anftic or north polar circle, and P the north pole,. where all the meridians or hour-circles meet. As the earth goes round the fun, the north pole keeps conftantly towards one part of the heavens, as it keeps in the figure towards the right-hand fide of the plate. When the earth is at the beginning of Libra, namely, on the 20th of March, in this figure (as at g in Plate XLI. fig. 4.) the fun S as feen from the earth appears at the beginning of Aries in the oppofite part of the heavens, the north pole is juft coming into the light, and the fun is vertical to the equator; which, together with the tropic of Cancer, parallel of London, and arffic circle, are all equally cut by the circle bounding light and darknefs, coinciding with the fix o’clock hour-circle, and therefore the days and nights are equally long at all places ; for every part of the meridian JE TLa. comes into the light at fix in the morning, and revolving with the earth according to the order of the hour-letters, goes into the dark at fix in the evening. There are 24 meridians orhour-cirdes drawn on the earth in this figure, to ftiew the time of fun-rifing and fetting at different feafons of the year. As the earth moves in the ecliptic according to the order of the letters ABCD, See. through the figns Libr’a, Scorpio, and Sagittarius, the north pole comes more and more into the light; the days increafe as the nights decreafe in length, at all places north of the equator JE; which is plain by viewing the earth at b on the jth of May, when it is in the xyth degree of Scorpio, and the fun as feen from the earth appears in the 15th degree of Taurus ; for then the tropic of Cancer 7" is in the light from a little after five in the morning till almoft feven in the evening; the parallel of London from half an hour paft four till half an hour paft feven; the polar circle 1/ from three till nine; and a large track round the north pole P has-day. all the 24 hours, for many rotations of the. earth on its axis-. When the earth comes to c at the beginning of Capricorn; and the fun as feen from the earth appears at the beginning of Cancer on the 21ft of June, as in this figure, it is in the pofition a in Plate XLI. fig. 4.; and its north pole inclines towards the fun, fo as to bring all tbe-north frigid zone into the light, and the northern parallels of latitude more into the light than the dark from-the equator to the polar circle, and the more fo as they are farther from the equator. The tropic of Cancer is m the light from five in the morning till feven at night; the parallel of London from a quarter before four till a quarter after eight; and the polar circle juft touches the dark, fo that the fun has only the lower half of his di(k hid from the inhabitants on that circle for a few minutes about midnight, fuppofing no-inequalities in the horizon, and no refractions. A bare view of the figure is enough’to (hew, that as the earth advances-from Capricorn towards Aries, and the fun appears to move from Cancer towards Libra, the north pole recedes towards the dark, which caufes the days to decreafe, and the nights to increafe in length, "’till