Page:Encyclopædia Britannica, first edition - Volume I, A-B.pdf/526

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ASTRO Period in eleven of our years 314 days and 12 hours. He is about 1000 times as big as the earth, for his diameter is 8i,ooo-mileS; which is more than ten times the diameter of the earth. Jupiter turns round his axis in 9 hours 56 minutes; fo that his year contains 10 thoufand 470 days; and the diurnal velocity of his equatoreal parts is greater than the 'fwiftnefswith which he moves in his annual orbit; a lingular circumftance, as far as we know By this prodigious quick rotation, his equatoreal inhabitants are carried 25 thoufand 920 miles every hour, (which is 920 miles an hour more than an inhabitant of our earth’s equator moves in twenty-four hours), befides the 25 thoufand above mentioned, which is common to all parts of his furface, by his annual motion. Jupiter is furrounded by faint fubMnces, called belts, in which fo many changes appear, that they are generally thought to be clouds ; for forhe of them have been iirlt interrupted and broken, and then have vanifhed entirely. They have fometimes been obferved of different breadths, 'and afterwards have all become nearly of the fame breadth. Large fpots have been feen in thefe belts'; and when a belt vanifhes, the contiguous fpots diftppear with it. The broken ends of fome belts have been generally obferved to revolve in the fame time with the fpots; only thofe nearer the equator in fomewhat lefs time than thofe near the poles, perhaps on account of the fun’s greater heat near the equator, which is parallel to the belts and courfe of the fpots. Several large fpots, which, appeaf round at one time, grow oblong by degrees, and then divide into two or three round fpots. The periodical time of the fpots near the equator is 9 hours 50 minutes, but of thofe near the poles 9 sours 56 minutes. The axis of Jupiter is fo nearly perpendicular to his orbit, that he has no fenfible change of feafons; which is a great advantage, and wifely ordered by the Author of nature. For if the axis of this planet were inclined any confiderable number of degrees, juft fo many degrees round each pole would in their turn be almoft fix of our years together in darknefs. And as each degree of a great circle on Jupiter contains 706 of our miles at a mean rate, it is eafy to judge what vaft tradls of land would be rendered uninhabitable by any confiderable inclination of his axis. The fun appears but ^ part fo big to Jupiter as to us ; and his light and heat are in the fame fmall proportion, but compenfated by the quick returns thereof, and by four moons (fome larger and fome lefs than our earth) which revolve about him; fo that there is fcarce any part of this huge-planet but what is, 'during the whole night, enlightened by one or more of thefe moons, except his poles, whence only the fartheft moons can be feen, and where their light is not wanted, becaufe the fun conftantly circulates in or near the horizon, and is very probably kept in view of both poles by the refraction of Jupiter’s atmofphere, which, if it be like ours, has certainly refractive ppwer enough for that purpofe. 'I he orbits of thefe moons are reprefented in the fcheme’of the folar fyftem by four finall- circles marked1, 2, 3, 4, on, Jupitet’s orbit ) but they are drawn fifty

N O M Y. times too large in proportion to it. The firft moon, or that neareft to Jupiter, goes round him in 1 day 18 hours and 36 minutes of our time; and is 229 thoufand miles diftant from his centre; the fecond performs its revolution in three days 13 hours and 15: minutes, at 364 thoufand miles diftance; the third in feven days three hours and 59 minutes, at the diftance of 580 thoufand miles; and the fourth, or outermoft, in 16 days 18 hours and 30 minutes, at the diftance of one million of miles from Ins centre. The periods of thefe moons are fo incommenfurate to one another, that if ever they were all in a right line between Jupiter and the fun, it will require more than 3,000,000,000,000 years from that'time to bring them all into the fame right line again, as any one will find who reduces all their periods into feconds, then multiplies them into one another, and divides the produT by 432 ; which is the higheft number that will divide the produft of all their periodical times, namely, 42,085,303,376,931,994,955,904 feconds, without a remainder. The angles under which the orbits of Jupiter’s moons are feen from the earth, at its mean diftance from Jupiter, are as follow: The firft, 3' 55": the fecond, 6' 14"; the third, 9' $8"; and the fourth, iy' ^o". And their diftances from Jupiter, meafured by his femidiameters, are thus : The firft, 5-J; the fecond, 9; the third, 14IJ; and the fourth, 25|-f- This planet, feen from its neareft moon, appears 1000 times as large as our moon does to us; waxing and waning in ail her monthly ftiapes every 424- hours. Jupiter’s three heareft moons fall into his fhadow, and are eclipfed in every revolution; but the orbit of the fourth moon is fo much inclined, that ft paiTeth by its oppofition to Jupiter, without falling into his lhadow, two years in every fix. By thefe eclipfes, aftronomers have not only difcovered that the fun’s light takes itp eight minutes of time in coming to us, but they have alfo determined the longitudes of places on this eartn with greater certainty and facility than by any other method yet known. The difference between the eqnatoreal and polar diameters of Jupiter is 6230 miles; for his equatoreal diameter is to his polar, as 13 to 12. So that his poles are 3115 miles hearer his centre than his equator is. Jupiter’s orbit is 1 degree 20 minutes inclined to the ecliptic. His north node is in the 7th degree of Cancer, and his fouth- node in the 7th degree of Capricorn. Sat or if, the remoteft of all the planets, is about 78a millions -of miles from the fun; and, travelling at the rate of 18 thoufand miles every hour, as in the circle marked fj, performs its annual circuit in 29 years 167 days and 5 hours of our time; which makes only one year to that planet. Its diameter is 67,600 miles; and therefore it is near 600 times as big as the earth. This planet is furrounded by a thin broad ring, as an artificial globe is by a horizon,, fig. 5. The ring appears double when feen through a good telefcope, and is reprefented by the figure in fuch an oblique view as it is generally feen. It is inclined 30 degrees to the ecliptic, and is about 21 thbufand miles in breadth; which is equal to its diftance from Saturn on all fides. There is reafon