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of an observer remain fixed in the heavens, so long as he continues in the same place; but he no sooner changes his position, than the horizon touches the earth in another point, and his zenith answers to a different point in the heavens.

The aris of the earth is an imaginary line, conceived to be drawn through the centre of the earth, upon which line its revolutions are made.

The poles of the earth are the extremities of its axis, or those two points on its surface, where its axis terminates; one of these is called the north, and the other the south pole. The poles of the heavens, or of the world, are those two points in the heavens, where the axis of the earth, if produced, would terminate; so that the north pole of the heavens is exactly over the north pole of the earth, and the south pole of the heavens is directly over the south pole of the earth.

The equator is an imaginary circle, which is supposed to be drawn round the earth's surface, in the middle between the two poles. It divides the earth into two equal parts; one of which is called the northern, the other the southern hemisphere.

If we suppose the plane of the earth's equator to be extended all ways as far as the heavens, it will mark there a circle, that will divide the heavens into two equal parts; this circle is called sometimes the equinoctial, sometimes the celestial equator.

The meridian of any place is a circle supposed to pass through that place and the poles of the earth; we may therefore imagine as many meridians as there

are places upon the earth, because any place that is ever so little to the east or west of another place, has a different meridian.*

By the foregoing definition, we see that the meridian of any place is immoveably fixed to that place, and carried round along with it by the rotation of the earth. The meridian marks upon the plane of the horizon the north and south points.

The circle which the sun appears to describe every year, in the concave sphere of the heavens, is called the ecliptic. It is thus denominated, because, in all eclipses, the moon is either in or near the plane of it. But as the earth moves round the sun, in the plane of the ecliptic, it is likewise the plane of the earth's orbit.

If we conceive a zone, or belt, about sixteen degrees broad, in the concave sphere of the heaven, with the ecliptic passing through the middle of it, this zone is called the zodiac. The stars in the zodiae were divided by the ancients into twelve equal parts or signs, to correspond with the months of the year; and because the number twelve with them was always expressive of fulness or completion, it is used in that sense in sacred writ. The signs are named, Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricornus, Aquarius, Pisces.

We may imagine as many circles as we please drawn on a globe, parallel to the equator, and these will decrease in their diameter as they approach nearer the poles. The tropics are two lesser cir

cles of this kind, parallel to the equator, and 23} degrees distant from it; one in the northern hemi sphere, 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 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; much less the evidencs arising from those demonstrative proofs, of which we are now in possession. We shall, therefore, enumerate some of the reasons which induce astronomers to believe that the earth moves round the sun; and then explain further the nature 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 the successive growth and recruit of vegetation.

The celestial motions become incomparably more simple, and free from these looped contortions which must be supposed in the other case; and which are not only extremely improbable, but incompatible with what we know of motion.

This opinion is also more reasonable, on account of 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 gene ral 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.

OF THE APPARENT MOTION OF THE SUN, ARISING 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

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