Page:Encyclopædia Britannica, first edition - Volume I, A-B.pdf/528

 444 ASTRO M O M Y. than in that of our earth : And we find that all our heat 7 times larger to Mercury than to us, 90 times larger to depends not on the rays of the fun; for if it did, we us than to Saturn, and 630 times as large to Mercury as fliould always have the fame months equally hot or cold to Saturn. at their annual returns. But it is far otherwife, for Fe- In fig. 5. we have a view of the bulks of the planets bruary is fometimes warmer than May; which mull; be In proportion to each other, and to a fuppofed globe of owing to vapours and exhalations from the earth. diameter for the fun. The earth is 27 times Every perfon who looks upon, and compares the fyf- astwobigfeet as Mercury, very little bigger than Venus, five times terns of moons together, which belong to Jupiter and Sa- as big as Mars ; but Jupiter is 1049 times as big as the turn, mull: be amazed at the vaft magnitude of thefe two earth ; Saturn 586 times as big, exclufive of his ring ; planets, and the noble attendance they have in refpeft of and the fun is 877 thoufand 650 times as big as the our little earth; and can never bring bimfelf to think, earth. If the planets in this figure were fet at their that an infinitely wife Creator Ihould difpofe of all his due diftances from a fun of two feet diameter, according animals and vegetables here, leaving the other planets, to their proportional bulks, as in our fyftem, Mercury bare and deftitute of rational creatures. To fuppofe that would be 28 yards from the fun’s centre ; Venus y t he had any view to our benefit, in creating thefe moons, yards 1 foot; the earth 70 yards 2 feet; Mars 107 and giving them their motions round Jupiter and Saturn ; yards 2 feet; Jupiter 370 yards 2 feet; and Saturn 760 to imagine that he intended thefe vaft bodies for any ad- yards two feet; the comet of the year 1680, at its vantage to us, when he well knew that they could never greateft diftance, 10 thoufand 760 yards. In this probe feen but by a few aflronomers peeping through tele- portion, the moon's diftance from the centre of the earth fcopes ; and that he gave to the planets regular returns of would be only 74 inches. days and nights, and different feafons to all where they To affift the imagination in forming an idea of the vafl: would be convenient; but of no manner of fervice to us, diftances of the fun, planets, and ftars, let us fuppofe, except only what immediately regards our own planet the that a body projedled from the fun fliould continue to earth ; to imagine that he did all this on our account, would fly with the fwiftnefs of a cannon-ball, i. e. 480 miles be charging him impioufly with having done much in vain; every hour; this body would reach the orbit of Mercuand as abfurd, as to imagine that he has created a lit- ry, in 7 years 221 days ; of Venus, in 14 years 8 days; tle fun and a planetary fyftem within the fhell of our of the earth, in 19 years 91 days; of Mars, in 29 years earth, and intended them-for our ufe. Thefe confide- 8y days ; of Jupiter, in 100 years 280 days; of Sarations amount to little lefs than a pofitive proof, that turn, in 184 years 240 days ; to the comet of 1680, at all the planets are inhabited : For if they are not, why its greateft diftance f/om the fun, in 2660 years; and all this care in furnifhing them with fo many moons, to to the neareft fixed ftars, in about 7 million 600 thoufupply thofe with light which are at the greater diftances fand years. from the fun ? Do we not fee, that the farther a planet As the earth i« not the centre of the orbits in which is from the fun, the greater apparatus it has for that pur- the planets move, they come nearer to it and go farther pofe ? fave only Mars, which being but a fmall planet, from it, and at different times; on which account they may have moons too fmall to be feen by us. We know appear bigger and .lefs by tutns. Hence, the apparent that the earth goes round the fun, and turns round its magnitudes of the planets are not always a certain rule own axis, to produce the viciflitudes „ of fummer and to know them by. winter by the former, and of day and night by the latter Under fig. 3. are the names and chara&ers of the motion, for the benefit of its inhabitants. May we not twelves figns of the zodiac, which the reader fliould be then fairly conclude, by parity of reafoh, that the end perfeftly well acquainted with, fo as to know the chaand defign of all the other planets is the fame ? and is radters without feeing the names. Every fign contains not this agreeable to the beautiful harmony which exifts 30 degrees, as in the circle bounding the folar fyftem; throughout the univerfe ? to which the chara&ers of the figns are fet in their proIn fig. 2. we have a view of the proportional breadth per places. of the fun’s face or difle, as feen from the different The Comets are folid opaque bodies, with long planets. The fun is reprefented, N° i, as feen from tranfparent trains or tails, iffuing from that fide which is Mercury; N° 2, as feen from Venus; N° 3, as feen turned away from the fun. They niove about the fun in from the earth ; N° 4, as feen from Mars; N° j, as very excentric ellipfes ; and are of a much greater denfeen from Jupiter; and N° 6, as feen from Saturn. fity than the earth; for fome of them are heated in eveLet the circle B, (fig. 3.) be the fun as feen from any pla- ry period to fuch a degree, as would vitrify or diffipate ner, at a given diftance ; to another planet, at double that any fubftance known to uj. Sir Ifaac Newton computed diftance, the fun will appear juft of half that breadth, the heat of the comet which appeared in the year 1680, as V, which contains only one fourth part of the area, when neareft the fun, to be 2000 times hotter than redor furface of B. For all circles, as well as fquare hot iron ; and that, being thus heated, it muft retain its furfaces, are to one another as the fquares of their dia- heat until it conies round again, although its period meters. Thus, (fig. 4.) the fquare A is juft half as broad as fliould be more than twenty thoufand years ; and it is the fquare B ; and yet it is plain to fight, that B contains computed to be only 575. four times as much furface as A. Hence, by comparing. Part of the paths of three comets are delineated in the diameters of the above circles (fig. 2.) together, it the fcheme of the folar fyftem, and the years marked will be found, that, in round numbers, the fun appears in which they made their appearance. It is believed that