It is owing to the industry of modern aftronomers, that the annual motion of the earth has been fully evinced; for though this motion had been known to, and adopted by many among the ancient philofophers, yet they were not able to give their opinions that degree of probability, which is attainable from modern difcoveries, much lefs the evidence arifing from thofe demonftrative proofs, of which we are now in poffeffion. I fhall, therefore, enumerate fome of the reafons which induce aftronomers to believe that the earth moves round the fun, and then explain further the nature of this motion, which is calculated to afford us the useful and delightful variety of the feafons, the mutual allay of immoderate heat and cold, and the fucceffive growth and recruit of vegetation.

On the fuppofition of the earth's motion, the celeftial motions become incomparably more fimple, and free from thofe looped contortions which must be fuppofed in the other cafe, and which are not only extremely improbable, but incompatible with what we know of motion.

This opinion is alfo more reasonable, on account of the extreme minuteness of the earth, when compared with the immenfe bulk of the fun, Jupiter, and Saturn; and there are no known laws of motion, according to which fo great a body as the fun can revolve about fo fmall a one as the earth.

The fun is the fountain of light and heat, which

* The whole of what is faid under this head, may be beautifully illuftrated by the armillary fphere.

which it darts through the whole fyftem; it ought, therefore, to be in the center, that it's influence may be regularly diffufed through the whole heavens, and communicated in juft gradations to the whole fyftem.

When we confider the fun as the center of the fyftem, we find all the bodies moving round it, agreeable to the univerfal laws of gravity; but upon any other confideration we are left in the dark.

The motion of the earth round the fun accords with that general harmony, and univerfal law, which all the other moving bodies in the fyftem obferve, namely, that the fquares of the periodic times are as the cubes of the diftances; but if the fun moves round the carth, that law is deftroyed, and the general order of fymmetry in nature interrupted.

The annual motion of the earth is inconteftibly proved by obfervation, a motion haying been difcovered in all the fixed ftars, which arifes from a combination of the motion of light with the motion of the earth in it's orbit.

It will be clearly fhewn in it's place, that Venus and Mercury move round the fun in orbits that are between it and the earth; that the orbit of the earth is fituated between that of Veaus and Mars; and that the orbits of Mars, Jupiter, &c. are exterior to, and include the other three.

OF THE APPARENT MOTION OF THE SUN, AKISING FROM THE EARTH'S ANNUAL MOTION ROUND IT.

As when a perfon fails along the fea coaft, the fhore, the villages, and other remarkable places on land, appear to change their fituation, and to pass by him; do it is in the heavens. To a fpectator upon the earth, as it moves along it's orbit, or fails as it were through celeftial fpace, the fire

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nets, and the fixed ftars, appear to change their places.

Apparent change of place is of two forts; the one is that of bodies at reft, the change of whose place depends folely on that of the fpectator; the other is that of bodies in motion, whofe apparent change of place depends as well on their own motion, as on that of the fpectator.

I fhall firft confider only that apparent change which takes place in those which are at reft, and which is owing wholly to the motion of the earth; and fhew that the fun, when feen from the earth. will appear to move in the fame manner, whether it revolves round the earth, or whether the earth revolves round the fun.

Let us fuppofe the earth at reft, without any motion of it's own, and let the fun be fuppofed to revolve round it in the orbit ABCD, fig. 1, pl. 4. and let EFGH be a circle in the concave sphere of the ftarry heavens; as the fun moves in the order of the letters ABCD in it's orbit, it will appear to a fpectator on the earth to have defcribed the circle EFGH. When the fun is at A, it will appear as if it was among the fixed ftars that are at É; when it is at B, it will appear among the fixed ftars at F; when at G, among thofe at H; and when it is at D, it will appear among the fixed ftars at G. Indeed, the fixed ftars and the fun are not seen at the fame time; but I have fhewn, that we may tell in what part of the heavens the fun is, or what fixed ftars it is near, by knowing thofe which are. oppofite to it, or come to the fouth at midnight. Therefore, if we find that any fet of ftars, as those at G for inftance, come to the fouth at midnight, we may be fure that they are oppofite to the fun, and confequently, if we could fee the ftars in that part of the heaven where the fun is, we fhould find them to be thofe at F.

Secondly,

Secondly, let us fuppofe that S is the fun, that it has no motion of it's own, that it refts within the orbit A B CD, in which we fhall now fuppofe the earth to move, in the order of the letters A B CD. Upon this fuppofition, when the earth is at A, the fun will appear in that part of the heavens where the ftars Hare; when the earth is at B, the fun will appear in that part of the heavens where the ftars Gare; when the earth is at C, the fun will appear in that part of the heavens where the ftars E are; and as the earth revolves round the fun, in the orbit A B CD, the fun will appear to a fpectator on the earth to defcribe the circle G H EF.

Thus whether the earth be at rest, and the fun revolves in the orbit A B CD; or the fun be at rejt, and the earth revolves in the fame orbit, a fpectator on the earth will fee the fun defcribe the fame circle EFGH, in the concave fphere of the heavens.*

Hence if the plane of the earth's orbit be imagined to be extended to the heavens, it would cut the ftarry firmament in that very circle, in which a fpectator in the fun would fee the earth revolve every year: while an inhabitant of the earth would obferve the fun to go through the fame circle, and in the fame fpace of time that the folar fpectator would fee the earth defcribe it.

The inhabitants of all the other planets will obferve jutt fuch motions in the fun as we do, and for the very fame reafons; and the fun will be feen from every planet to defcribe the fame circle, and in the fame fpace of time, that a spectator in the fun would obferve the planet to do. For example, an inhabitant of Jupiter would think that the fun revolved

*This is pleasingly illuftrated by the armillary fphere, where the fphere may be moved independent of the earth, and the earth of the sphere.

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revolved round him, defcribing a circle in the heavens in the space of twelve years: this circle would not be the fame with our ecliptic, nor would the fun appear to pafs through the fame ftars which he does to us. On the fame account, the fun, feen from Saturn, will appear to move in another circle, diftinct from either of the former; and will not seem to finish his period in lefs time than thirty years. Now as it is impoffible that the fun can have all these motions really in itfelf, we may safely affirm, that none of them are real, but that they are all apparent, and arife from the motions of the refpective planets.

One phenomenon arifing from the annual motion of the earth, which has already been flightly touched upon, may now be more fully explained; for as from this motion, the fun appears to move from west to caft in the heavens, if a ftar rifes or fets along with the fun at any time, it will in the course of a few days rife or fet before it, because the fun's apparent place in the heavens will be removed to the eastward of that ftar. Hence those ftars which at one time of the year fet with the fun, and therefore do not appear at all, fhall at another time of the year rife when the fun fets, and fhine all the night. And as any one ftar fhifts it's place with refpect to the fun, and in confequence of that with refpect to the hour of the night, fo do all the reft. Hence it is that all those stars, which at one time of the year appear on any one fide of the pole ftar in the evening, thall in half a year appear on the contrary fide thereof.

From what has been faid it follows in general, 1. That, in whatever regards the fun's place, only with respect to the point in the heavens in which it appears, it may be fuppofed to move in an infinitely great circle, called the ecliptic, whofe center is the obferver's eye.

VOL. III.

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2. That