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ASTRO rill the earth comes to the beginning of Aries, and then they are equal as before; for the boundary of light and darknefs cut the equator and all its parallels equally or in halves. The north pole then goes into the dark, and continues therein until the earth goes half way round its orbit, or from the 23d of September till the 20th of March. In the middle between thefe times, viz. on the 2 2d of December, the north pole is as far as it can be in the dark, which is 234 degrees, equal to the inclination of the earth’s axis from a perpendicular to its orbit; and then the northern parallels are as much in the dark as they were in the light on the 21ft of June; the winter nights being as long as the fummer days, and the winter days as (hort as the fummer nights. It is needlefs to enlarge farther on this fubjedt, as we lhall have occafion to mention the feafons again in defcribing the orrery. Only this mull be noted, that all that has been faid of the northern hemifphere, the contrary mult be underltood of the fouthern; for on different fides of the equator the feafons are contrary, becaufe when the northern hemifphere inclines towards the fun, the fouthern declines from him. As Saturn goes round the fun, his obliquely polited ring, like our earth’s axis, keeps parallel.to itfelf, and is therefore turned edgewife to the fun twice in a Saturnian year, which is almolt as long as 30 of our years. But the ring, though confiderably broad, is too thin to be feen by us when it is turned round edgewife to the fun, at which time it is alfo edgewife to the earth, and therefore it difappears once in every fifteen years to us. As the fun fhines half a year together on the north pole of our earth, then difappears to it, and Ihines as long on the fouth pole; fo, during one half of Saturn’s year, the fun Ihines on the north fide of his ring, then difappears to it, and Ihines as long on its fouth fide. When the earth’s axis inclines neither to nor from the fun, but lidewife to him, he inftantly ceafes to Urine on one pole, and begins to enlighten the other; and when Saturn’s ring inclines neither to nor from the fun, but fidewife to him, he ceafes tolhine-on the one fide of it, and begins to Ihine upon the other. The earth’s orbit being elliptical, and the fun conllantly keeping in its lower focus, which is 1,377,000 miles from the middle point of the longer axis, the earth comes twice fo much, or 2,754,000 miles nearer the fun at one time of the year than at another; for the fun appearing under a larger angle in our winter than fummer, proves that the earth is nearer the fun in winter. But here this natural queftion will arife, Why have we not the hotteft weather when the earth is neareft the fun ? In anfwer, it mult be obferved, that the excentricity of the earth’s orbit, or 1 million 377 miles, bears no greater proportion to the earth’s mean dillance from the fun than 17 does to 1000; and therefore this fmall difference of dillance cannot occafion any great difference of heat or cold. But the principal caufe of this difference is, that in winter the funs rays fall fo obliquely upon ns, that any given number of them is fpread over a much greater portion of the earth’s furface where we live, and 'therefore each point mull then have fewer rays than in i ummer. Moreover, there conies a greater degree of cold
N O M Y. in the long winter nights than there can return of heat in fo Ihort days; and on both thefe accounts the cold mull increafe. But in fummer, the rays fall more perpendicularly upon us, and therefore come with greater force, and in greater numbers on the fame place; and by their long continuance, a much greater degree of heat is imparted by day than can fly off by night. Chap. IX. The Method of finding the Lon~ gitude by the Eclipfes of Jupiter's Satellites: The amazing Velocity of Light dmonjlrated by thefe Eclipfes. Geographers arbitrarily chufe to call the meridian of fome remarkable place the firft meridian. There they begin their reckoning ; and juft fo many degrees and minutes as any other place is to the eaftward or weftward of that meridian, fo much eaft or weft longitude they fay it has. A degree is the 360th part of a circle, be it great or fmali; and a minute the 60th part of a degree. The Englifh geographers reckon the longitude from the meridian of the Royal Obfervatory at Greenwich, and the French from the meridian of Paris. If we imagine 12 great circles, (Plate XLII. fig. 1.) ■one of which is the meridian of any given place, to interfeci each other in the two poles of the earth, and to cut the equator JE at every 15th degree, they will be divided by the poles into 24 femicircles which divide the equator into 24 equal parts ; and as the earth turns on its axis, the planes of thefe femicircles come fucceflivdy after one another every hour to the fun. As in an hour of time tj*re is a revolution of 15 degrees of the equator, in a minute of time there will be a revolution of 15 minutes of the equator, and in a fecond of time a revolution of x j feconds. Becaufe the fun enlightens only one half of the earth at once, as it turns round its axis, he rifes to fome places at the fame moments of abfolute time that he fets to others; and when it is mid-day to fome places, it is midnight to others. The XII on the middle of the earth’s enlightened fide, next the fun, ftands for mid-day; and the oppofite XII on the middle of the dark fide, for midnight. If we fuppofe this circle of hours to be fixed in the plane of the equino&ial, and the earth to turn round within it, any particular meridian will come to the different hours fo as to fhew the true time of the day or night at all places on that meridian. Therefore, To every place 15 degrees eaftward from any given meridian, it is noon an hour fooner than on that meridian, becaufe their meridian comes to the fun hour fooner; and to all places 15 degrees weftward, it is noon an hour later, becaufe their meridian comes an hour later to the fun, and fo on; every 15 degrees of motion caufing an hour’s difference in time. Therefore, they who have noon an hour later than we, have their meridian, that is, their longitude, 15 degrees weftward from us; and they who have qoon an hour fooner than we, have their meridian 15 degrees eaftward from ours ; and fo for every hour’s difference of time 15 degrees difference
