conjunction, but when they are in direct oppofite parts of the zodiac, they are faid to be in oppofition. The place that any planet appears to occupy in the celeftial hemifphere, when feen by an obferver fuppofed to be placed in the fun, is called it's beliocentric place, it's heliocentric latitude, &c. The place it occupies when feen from the earth, is called it's geocentric place, &c. A motion in the heavens in the order of the figns, as from Aries to Taurus, &c. is faid to be in confequentia, and fuch are the real motions of all the planets, though their apparent motions are fometimes contrary, and then they are faid to move in antecedentia. This The points where the celeftial equator cuts the ecliptic are found to have a motion in antecedentia of about fifty feconds every year. change of place of the first point of the ecliptic, from whence the figns are counted. occafions a like change in the figns themfelves, which, though fcarce fenfible for a few years, has now become very confiderable. Thus, fince the time that aftronomy was cultivated by the Greeks, that is, about 2000 years ago, the firft point of the ecliptic is removed backward about a whole fign; and though it was then about the middle of the conftellation Aries, it is now about the middle of Pifces. Notwithstanding this alteration, the figns ftill retain their ancient names and marks. The longitude of a phenomenon in the heavens is the number of degrees counted from the first point of Aries on the ecliptic, to the place where a circle of latitude drawn through the phenomenon would cut the ecliptic at right angles. Thus every phenomenon in the heavens is referred to the ecliptic by the circles of latitude, as the the longitudes of terreftrial places are referred to the equator by the meridians; and whatever fign the circle of latitude paffes through, the phenomenon is faid to have it's place in that fign. OF THE FIGURE AND LIGHT OF THE PLANETS. That the fun and planets are Spherical bodies, is evident from all the obfervations that have been made on them; and that the earth is of the fame figure, is not only deducible from analogy, but it is alfo proved by obfervation, as I fhall fhew in the procefs of thefe Lectures. Aftronomers, when they ay that the planets are spherical bodies, do not mean a geometrical fphere, but a figure called an oblate fpheroid, which is fomething like the figure that a flexible fphere would be formed into by gently preffing it at it's poles. Obfervations have determined this in Jupiter, and it is known that the earth is of this figure, both from obfervation and actual menfuration. That the planets are all pake, or dark bodies, and confequently thine only by the light they receive from the fun, is plain, because they are not vifible when they are in fuch parts of their orbits as are between the fun and earth, that is, when their illuminated fide is turned from us. The fun enlightens only half a planet at once; the illuminated hemifphere is always that which is turned towards the fun, the other hemifphere of the planet is dark. To fpeak with accuracy, the fun being larger than any of the planets, will illuminate rather more than half; but this difference, on account of the great ditance of the fun from any of the planets, is fo fmall, that it's light may be confidered as coming to them in lines phyfically parallel. Like other opake bodies, they caft a fhadow behind them, which is always oppofite to the fun. The line in the planet's body, which diftinguishes the lucid from the obfcure part, appears fometimes ftrait, fometimes crooked. The convex part of the curve is fometimes towards the fplendid, and the concave towards that which is obfcure; and vice verfa, according to the fituation of the eye with respect to the planet, and of the fun which enlightens the planet. OF THE SUN. The fun is the center of the fyftem, round which the rest of the planets revolve. It is the first and greatest object of astronomical knowledge, and is alone enough to ftamp a value on the fcience to which the ftudy of it belongs. The fun is the parent of the feafons; day and night, fummer and winter, are among it's furprifing effects. All the vegetable creation are the offspring of it's beams; our own lives are fupported by it's influence. Nature revives, and puts on a new face, when it approaches nearer to us in fpring; and finks into a temporary death at his departure from us in the winter. Hence it was with propriety called by the ancients cor cæli, the heart of heaven; for as the heart is the center of the animal fyftem, fo is the fun the center of our univerfe. As the heart is the fountain of the blood, and the center of heat and motion; fo is the fun the life and heat of the world, and firft mover of the mundane fyftem. When the heart ceafes to beat, the circuit of life is at an end; and if the fun thould ceafe to act, a total ftagnation would take place throughout the whole frame of nature. "By "By his magnetic beam he gently warms The fun is placed near the center of the orbits of all the planets, and turns round his axis in 25 ☀ days. It is inclined to the ecliptic in an angle of eight degrees. His apparent diameter, at a mean distance from the earth, is about thirty-two minutes, twelve feconds. Those who are not accustomed to aftronomical calculation, will be furprized at the real magnitude of this luminary; which, on account of it's distance from us, appears to the eye not much larger than the moon, which is only an attendant on our earth. When looking at the fun, you are viewing a globe, whofe diameter is above 890,000 English miles; whereas the earth is not more in diameter than 7970 miles: fo that the fun is about 1,392,500 times bigger than the earth. As it is the fountain of light and heat to all the planets, fo it also far furpaffes them in it's bulk. In proportion as science has advanced, and more accurate instruments have been made, the magnitude of this luminary has been found to exceed confiderably the limits of former calculations. If the fun were every where equally bright, his rotation on his axis would not be perceptible; but by means of the spots, which are vifible on his pure and lucid furface, we are enabled to discover this motion. When a spherical body is near enough to appear of it's true figure, this appearance is owing to the fhading upon the different parts of it's furface: for as a flat circular piece of board, when it is properly fhaded by painting will look like a spherical body; fo a spherical body appears of it's true VOL. IV. shape, C fhape, for the fame reafon that the plane board, in the prefent inftance, appears fpherical. But if the sphere be at a great diftance, this difference of fhading cannot be difcerned by the eye, and confequently the fphere will no longer appear of it's true fhape, the fhading is then loft, and it feems like a flat circle. It is thus with the fun; it appears to us like a bright flat circle, which flat circle is termed the fun's disk. By the affiftance of telescopes, dark fpots have been obferved on this disk, and found to have a motion from eaft to weft; their velocity is greater when they are at the center, than when they are near the limb. They are feen firft on the eaftern extremity, by degrees they come forwards towards the middle, and fo pafs on till they reach the weftern edge; they then difappear; and after they have lain hid about the fame time that they continued visible, they will appear again as at firft. By this motion we difcover not only the time the fun employs in turning round his axis, but alfo the inclination of it's axis to the plane of the ecliptic.* The page of hiftory informs us, that there have been periods, when the fun has wanted of it's accuftomed brightness, fhone with a dim and obfcure light for the space of a whole year. This obfcurity has been fuppofed to arife from his furface being at thofe times covered with fpots. Spots have been feen that were much larger than the earth. The fun is fuppofed to have an atmosphere round The observer may view the fpots of the fun with a refract. ing telescope of two or three fect, or a reflecting one of 12 inches, 18 inches, or two feet, taking care to guard the eye with a dark glafs, to take off the glaring light; or the image or picture of the fun, with his fpots, may be thrown into a dark room, through a telescope, and received upon a piece of paper placed nearer or further from the glafs at pleasure, |