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ASTRONOMY.
the fame nature with our earth, and deſtined for the like purpoſes. For they are ſolid opaque globes, capable of ſupporting animals and vegetable. Some of them are larger, ſome leſs, and ſome much about the ſize of our earth. They all circulate round the ſun, as the earth does, in a ſhorter or longer time, according to their reſpective diſtances from him; and have, where it would not be inconvenient, regular returns of ſummer and winter, ſpring and autumn. They have warmer and colder climates, as the various productions of our earth require: And, in ſuch as afford a poſſibility of diſcovering it, we obſerve a regular motion round their axes like that of our earth, cauſing an alternate return of day and night; which is neceſſary for labour, reſt, and vegetation, and that all parts of their ſurfaces may be expoſed to the rays of the ſun.
Such of the planets as are fartheſt from the ſun, and therefore enjoy leaſt of his light, have that deficiency made up by ſeveral moons, which conſlantly accompany and revolve about them, as our moon revolves about the earth. The remoteſt planet has, over and above, a broad ring encompaſſing it; which like a lucid zone in the heavens reflects the ſun’s light very copiouſly on that planet; ſo that if the remoter planets have the ſun’s light fainter by day than we, they have an addition made to it morning and evening by one or more of their moons, and a greater quantity of light in the night-time.
On the ſurface of the moon, becauſe it is nearer us than any other of the celeſtial bodies are, we diſcover a nearer reſemblance of our earth. For, by the aſſiſtance of teleſcopes, we obſerve the moon to be full of high mountains, large valleys, and deep cavities. Theſe ſimilarities leave us no room to doubt, but that all the planets and moons in the ſyſtem are deſigned as commodious habitations for creatures endued with capacities of knowing and adoring their beneficent Creator.
Since the fixed ſtars are prodigious ſpheres of fire like our ſun, and at inconceivable diſtances from one another as well as from us, it is reaſonable to conclude they are made for the ſame purpoſes that the ſun is ; each to beſtow light, heat, and vegetation, on a certain number of inhabited planets, kept by gravitation within the ſphere of its activity.
Chap. II. Of the Solar System.
The planets and comets which move round the ſun as their centre, conſtitute the Solar Syſtem. Thoſe planets which are near the ſun not only finiſh their circuits ſooner, but like wiſe move faſter in their reſpective orbits, than thoſe which are more remote from him. Their motions are all performed from weſt to eaſt, in orbits nearly circular. Their names, difiances, bulks, and periodical revolutions, are as follow.
The Sun, an immenſe globe of fire, is placed near the common centre, or rather in the lower focus, of the orbits of all the planets and comets; and turns round his axis in 25 days 6 hours, as is evident by the motion of ſpots ſeen on his ſurface. His diameter is computed to be 763,000 miles; and, by the various attractions of the circumvolving planets, he is agitated by a ſmall motion round the centre of gravity of the ſyſtem. All the planets, as ſeen from him, move the ſame way, and according to the order of ſigns in the graduated circle 'V5 V U -2c> Plate XL. fig. 2. which repreſents the great ecliptic the heavens: But, as ſeen from any one planet, the reſt appear ſometimes to go backward, ſometimes forward, and ſometimes to ſtand ſtill; not in circles nor ellipſes, but in looped curves which never return into themſelves. The comets come from all parts of the heavens, and move in all forts of directions.
The axis of a planet is a line conceived to be drawn through its centre, about which it revolves as on a real axis. The extremities of this line, terminating in oppoſite points of the planet’s ſurface, are called its poles. That which points towards the northern part of the heavens, is called the north pole; and the other, pointing towards the ſouthern part, is called the ſouth pole. A bowl whirled from one’s hand into the open air turns round ſuch a line within itſelf, whilſt it moves forward; and ſuch are the lines we mean, when we ſpeak of the axes of the heavenly bodies.
Let us ſuppoſe the earth’s orbit to be a thin, even, ſolid plane; cutting the ſun through the centre, and extended out as far as the ſtarry heavens, where it will mark the great circle called the ecliptic. This circle we ſuppoſe to be divided into 12 equal parts, called ſigns; each ſign into 30 equal parts, called degrees; each degree into 60 equal parts, called minutes; and every minute into 60 equal parts, called ſeconds: So that a ſecond is the 60th part of a minute; a minute the 60th part of a degree; and a degree the 360th part of a circle, or 30th part of a ſign. The planes of the orbits of all the other planets likewiſe cut the ſun in halves; but, extended to the heavens, form circles different from one another, and from the ecliptic; one half of each being on the north ſide, and the other on the ſouth ſide of it. Conſequently the orbit of each planet croſſes the ecliptic in two oppoſite points, which are called the planet’s nodes. Theſe nodes are all in different parts of the ecliptic; and therefore, if the planetary tracks remained viſible in the heavens, they would in ſome meaſure reſemble the different ruts of waggon-wheels croſſing one another in different parts, but never going far aſunder. That node, or interſection of the orbit of any planet with the earth’s, orbit, from which the planet aſcends northward above the ecliptic, is called the aſcending node of the planet; and the other, which is directly oppoſite thereto, is called its deſending node. Saturn’s aſcending node is in 21 deg. 13 min. of Cancer 05-, Jupiter's in 7 deg. 29 min. of the ſame ſign, Mars’s in 17 deg. 17 min. of Taurus, Venus’s in 13 deg. 59 min. of Gemini II, and Mercury’s in I4 deg. 43 min. of Taurus. Here we conſider the earth’s orbit as the ſtandard, and the orbits of all the other planets as oblique to it.
When we ſpeak of the planets orbits, all that is meant is their paths through the open and unreſiſting ſpace in which they move, and are kept in, by the attractive power of the ſun, and the projectile force impreſſed upon
