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cles of this kind, parallel to, the equator, and, 23 -| degrees distant from ^t; one in the northern hemisphere, which is called the tropic of Cancer; .the other in the southern, which is called the tropic of Capricorn. If we conceive the planes of these circles expanded, till they reach the starry heaven, the sun will be seen to move in that circle which corresponds to the tropic of Cancer on the longest summer's day; and in that circle which answers to the tropic of Capricorn on the shortest winter's day.
The polar circles are two lesser circles, conceived to be described at 23 i- degrees distance from each pole.
The axis of the earth is inclined to the plane of the ecliptic, and makes with it an angle of 66^ degrees; therefore, the plane of the earth's equator cannot coincide with the plane of the ecliptic; but these two planes make with one another an angle of 23* degrees.
OF THE ANNUAL MOTION OF THE EARTH.
... The foregoing definitions being understood, we may now proceed in the description of the phenomena of our system. It is owing to the industry of modern astronomy, 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 philosophers, yet they were not able to give their opinions that degree of probability, which is attainable from modern discoveries; mach less the evklencs arising; from those demonstrative proofs, of which we are now in possession. We shall,, therefore, enumerate some of the reason* which induce astronomers to believe that the earth moves round the sun; and then explain further the natnre of this motion, calculated to afford us the useful and delightful variety of the seasons, the mutual allay of immoderate heat and cold, as also for tfie successive growth and recruit of vegetation.
The celestial motions become incomparably more timpTe, and free from these looped contortions which must be snpposerf in the other case; and which are not only extremely improbable, but incompatible with what we know of motion.
This opinion is a'so more reasonable, on account ef the extreme minuteness of the earth, when compared with the immense bulk of the Sun, Jupiter, and Saturn; and there are no known laws of motion, according to which so great a body as the sun can revolve about so small a one as the earth.
The sun is the fountain of light and heat, which it darts through the whole system; it ought, therefore, to be in the centre, that its influence may be regularly diffused through the whole heavens, and communicated in just gradations to the whole system.
When we consider the sun as the centre of the system, we find all the bodies moving round it, agreeable to the universal laws of gravity: but upon any other consideration we are left in the dark. •
The motion of the earth round the sun accords with that general harmony and universal law, which all the other moving bodies in the system observe; namely, that the squares of the periodic times are as the cubes of the distances; but if the sun moves round the earth, that law is destroyed, and the general order of symmetry in nature interrupted.;
It is incontestibly proved by observation, a motion having been discovered in all the fixed stars, which arises from a combination of the motion of light with the motion of the earth in its orbit. .. ; • • •
It will be clearly shewn in its place, that Venus and Mercury move round the sun in orbits that are between it and the earth; that the orbit of the earth is situated between that of Venus and Mars; and that the orbits of Mars, Jupiter, &c. are exterior to, and include the other three.
Ofr The Apparent Motion Of The Sun, Arisins From The Earth's Annual Motion Round It.
As when a person sails along the sea-coast, the shore, the villages, and other remarkable places on land, appear to change their situation, and to pass by him; so it is in the heavens. To a spectator upon the earth, as it moves along its orbit, or sails as it were through celestial space, the sun, the planets, and the fixed stars, appear to change their places.
Apparent change of place is of two sorts; the one is that of bodies at rest, the change of whose place depends solely on that spectator; the other is that of
bodies in motion, whose apparent change of place depends as well on their own motion, as on that of the spectator.
We shall first consider only that apparent change -•which takes place in those which are at rest, and which is owing wholly to the motion of the earth; and shew that the sun, when seen from the earth, will appear to move in the same manner, whether .it re-vdlves round the earth, or whether the earth revolves round the sun. JLet us suppose the earth at rest, without any motion of its own, and let the sun be supposed to revolve round it in the orbit ABCD, plate 4, jig. 1; and let EFGH be a circle in the concave sphere of the starry heavens; as the sun moves in the order of the letters ABCD in its orbit, it will appear to a spectator on the earth to have described
the circle EFHG. When the sun is at A, it will
appear as if it was among the fixed stars that are at E; when it is at B, it will appear among the fixed stars at F; when at C, among those at H ; and when it is at D, it will appear among the fixed stars at G. Indeed, the fixed stars and the sun are not seen at the same time; but we have shewn, that we may tell in what part of the heavens ttye sun is, or what fixed stars it is near, by knowing those which are opposite to it, or come to the south at midnight. Therefore, if we find that any set pf stars, as those at G, for instance, come to the *outh at midnight, we may be sure that they are eipposite to the sun; and, consequently, if we could, see the stars in that
part of the heaven where the sun is,-we should find them to be those at F.
Secondly, let us suppose that S is the sun, having no motion of its own; that it rests within the orbit A'BCD, in which we shall now suppose the earth W move, in the order of the letters ABCD. Upon thiif supposition, when the earth is at A; the sun will ap-' pear in that part of the heavens where the stars, Hj are; when the earth is at B, the sun will appear irt that part of the heavens where the stars, G, are; wheii the earth is at C, the sun will appear in that part of the heavens where the stars, E, are; and as the earth ^ revolves round the sun in the orbit ABCD, the sun will appear to a spectator on the earth to describe the
Thus, whether the earth be at rest, and the sun revolves in the orbit ABCD; or the sun be at rest, and the earth revolves in the same orbit, a spectator on the earth will see the sun describe the same circle EFGH, in.the concave sphere of the heavens.
Hence, if the plane of the earth's orbit be imagined to be extended to the heavens, it would cut the starry firmament in that very circle, in which a spectator in the sun would see the earth revolve every year; while an inhabitant of the earth would observe the sun to go through the same circle, and in the same space of time, that the solar spectator would see the earth describe it.
The inhabitants of all the other planets will observe just such motions in the sun as we do, and for the very same reasons; and the sun will be seea