the NORTHERN, the other the SOUTHERN HEMI

SPHERE.

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, fometimes 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 fo little to the east or weft 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 fun appears to describe every year, in the concave fphere of the heavens, is called the ECLIPTIC. It is thus denominated, because in all eclipfes the moon is either in or

near

near the plane of it. But as the earth moves round the fun, in the plane of the ecliptic, it is likewife the plane of the earth's orbit.

If we conceive a zone, or belt, about fixteen degrees broad in the concave sphere of the heaven, with the ecliptic paffing through the middle of it, this zone is called the ZODIAC.

The ftars in the zodiac are divided into TWELVE SIGNS, Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricornus, Aquarius, Pifces.

We may imagine as many circles as we please drawn on the globe, parallel to the equator, and thefe will decrease in their diameter, as they approach nearer the poles.

The TROPICS are two leffer circles of this kind, parallel to the equator, and 23 degrees diftant from it; one in the northern hemifphere, which is called the TROPIC OF CANCER; the other in the fouthern, which is called the TROPIC OF CAPRICORN. If we conceive the planes of these circles expanded, till they reach the starry heaven, the fun will be seen to move in that circle which correfponds to the tropic of cancer on the longest fummer's

fummer's day, and in that circle which answers to the tropic of capricorn on the shortest winter's day.

The polar circles are two leffer circles, conceived to be defcribed at 23 degrees diftance 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 phænomena of our fyftem. It is owing to the industry of modern aftronomers, that the annual motion of the earth has been fully evinced; for though it's 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

G

proofs of which we are now in poffeffion. We fhall, therefore, enumerate fome of the reasons which induce aftronomers to believe that the earth moves round the fun.

The celeftial motions become incomparably more fimple, and free from those 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 also 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 it darts through, the whole system; it ought, therefore, to be in the center, that it's influence may be regularly diffufed through the whole heavens, and communicated in just gradations to the whole system.

When we confider the fun as the center of the fyftem, we find all the bodies moving round it, agreeable

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 observe, namely, that the squares of the periodic times are as the cubes of the distances; but if the fun moves round the earth, that law is destroyed, and the general order of fymmetry in nature interrupted.

It is inconteftibly proved by obfervation, a motion having been discovered in all the fixed ftars, which arises from a combination of the mo tion 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 Venus and Mars; and that the orbits of Mars, Jupiter, &c. are exterior to, and include the other three.