ebtain copies of Ptolemy's Geography; and even then, it was with difficulty they could read and clear his manuscripts of some of their grossest errors, It required a still longer time before they could construct proper instruments for determining, with accuracy, the latitudes and longitudes of places. It was not till COPERNICUS, TYCHO BRAHE, of Denmark, born in 1546, KEPLER, of Germany, born in 1571, GALLILEO, of Italy, born in 1564, DESCARTES, of France, born in 1596, CASSINI, of Nice, FLAMSTEAD, HALLEY, and Sir ISAAC NEWTON, of England, had enlightened the world with their discoveries, that Astronomy and Geography were placed on their true foundation, and reduced to a tolerable degree of consistency and accuracy. THE SOLAR SYSTEM. The supposition of a certain disposition of the heavenly bodies, and the planetary orbits, is called a System of the World. The true System, or disposition of the planets, is called the SOLAR SYSTEM. It is also called the Copernican System, from Copernicus, a Prussian. Several other systenis, as the Ptolemaic and Tychonic, have, at different times, been promulgated to the world; but are now universally exploded. The true solar system was taught by Pythagoraş, 500 years before Christ; but the knowledge of it was nearly lost, when Copernicus revived it, in 1530. This system supposes the sun to be in the centre, and the well known planets to revolve round him in the following order: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Herschel. The two first of these are called inferior planets, because they are nearer to the common centre of gravity of the system, than the earth, or are below the earth, with respect to that centre; the other four are called superior, because, with respect to said centre, they are above the earth. Also the inferior planets are called interior, because their orbits are within that of the earth; and the superior are called exterior because their orbits are without the same. These seven are called primary planets, to distinguish them from a number of other bodies, called secondary planets, B moons, or satellites, which revolve about their respectivè primaries, and also accompany them in their revolutions about the sun. The Orbit of a planet is that path which it describes in moving round the sun. The planetary orbits are not perfectly circular, but elliptical or oval; so that all the planets sometimes approach nearer to, and at other times recede farther from the sun, than if they moved round him in perfect circles. This deviation from a circle is called the eccentricity of the orbit. The planets are retained in their orbits by the attraction of the sun; and at the same time, are kept from approaching him, by a constant tendency to fly off from him in straight lines. This attractive power is called the centripetal force; and that which impels them forward in straight lines, the centrifugal force. A body is said to be in conjunction with the sun, when it is seen in the same point of the heavens ; and in opposition, when it is 180 degrees distant: or, in other words, when it is in that part of the heavens directly opposite to the sun. The SUN is the most glorious luminary of the heavens, and the fountain of light and heat to the planets. It is supposed to be an immense globe of fire, whose diameter is 883,246 miles, and whose body is more than 1,380,000 times larger than the earth. It has been discovered that the sun has a revolution on its axis once in 25 days, 14 hours, 8 minutes. It MERCURY is the nearest planet to the sun, and revolves round it in 87 days, 23 hours, at the mean distance of 36,583,835 miles. Its diameter is 3224 miles. moves in its orbit at the rate of 111,000 miles in an hour. Its magnitude is about one twenty seventh part of that of our earth. When visible to the naked eye, this planet emits a very clear, white light; but from its nearness to the sun, it is seldom seen, and then only for a short time. VENUS, the next planet in the system, is placed at the mean distance of 68,368,000 miles from the sun's centre. Its diameter is 7687 miles, and its magnitude about nine tenths of that of the earth. Moving at the rate of 81,000 miles an hour, its annual circuit round the sun is performed in 224 days, 17 hours. When Venus appears west of the sun, it rises before it, and is denominated the morning star; when it appears east from the sun, it is seen above the horizon after sunset, and is then called the evening star. The EARTH is the third planet in the solar system. Its distance from the sun is about 94,507,428 miles; its diameter 7945, and its circumference 25,000 miles. The Earth, as well as all the other planets, has two motions; one round its axis, in 24 hours from west to east, which causes an apparent motion of the heavenly bodies from east to west, and a continual succession of day and night. This is called its diurnal motion; and by this, the inhabitants on the equator are carried 1043 miles every hour. The other is its annual revolution round the sun, which is performed in one year, or 365 days, 5 hours, 48 minutes, 48 seconds. In its annual course, the earth moves at the rate of 75,222 miles an hour; which motion causes the difference in the length of the days and nights, and the agreeable succession of the seasons. The earth has one satellite, the Moon, which revolves in an elliptical orbit round the earth, at the mean distance of 239,029 miles from the earth's centre. The Moon has also a rotation on its axis, the time of which is equal to the time of its revolution round the earth, that is, 27 days, 7 hours, 43 minutes. The time of its synodic revolution, or that which elapses between two successive changes, is 29 days, 12 hours, 44 minutes. The diameter of the Moon is 2180 miles. The Moon is an opaque body, like all the planets, and shines only by reflecting the light of the sun. Hence the incessant variation of its appearance; for, when it is in conjunction with the sun, its whole surface turned toward the earth being dark, it is invisible to us. As it advances forward, we see its enlightened side, like a radiant crescent this is called the new moon; which increases more and more to our view, till it is in opposition to the sun. Its whole enlightened side being then turned toward the earth, it appears as a round illuminated orb, which is called the full moon. From the full moon it appears to decrease gradually, till its next conjunction with the sun, when it disappears as before. MARS, the next planet above the orbit of the earth, is 144,000,000 miles distant from the sun. It appears of a fiery reddish hue, and is supposed to be encompassed with a thick cloudy atmosphere. Its diameter is about 4,189 miles, and its magnitude, as 7 to 24 to that of the earth. It moves round the sun with a velocity of 50,000 miles an hour, and completes its year in 687 days, of our time; which makes only 6674 of its own days, its diurnal rotation being performed in 24 hours, 40 minutes. To a spectator on this planet, Mercury would be seldom, if ever, visible; Venus would appear about the same distance from the sun, as Mercury does to us; our earth would appear about the size of Venus; and would be, as Venus is to us, alternately a morning and an evening star. JUPITER, the largest of all the planets, is the next in order, and is 491,702,301 miles from the sun. Its diameter is about 89,170 miles; and its magnitude nearly 1400 times greater than that of our earth. It completes its anQual revolution in 4322 days of our time. Its rotation about its axis is performed in 9 hours, 55 minutes, so that its year consists of 11 years 315 days of our time; and the inhabitants on its equator are carried 30,000 miles an hour, besides their motion round the sun. Jupiter is surrounded by faint substances, called Belts, in which so many variations appear, that they are generally ascribed to clouds. This planet has also four Moons, or Satellites, revolving round it at various distances and periods. These satellites must afford a pleasing spectacle to the inhabitants of Jupiter; they sometimes rise altogether, and at other times are altogether on the meridian, ranged under one another. That which is most distant from the planet will appear as large as our moon does to us; and from the various revolutions of the four, they produce four different kinds of months. SATURN is still higher than Jupiter, and, until the year 1781, was supposed to be the most remote planet in our system. This planet is placed at the distance of 901,668,908 miles from the sun's centre; and travelling 22,100 miles an hour, finishes its annual circuit in 10,759 days of our time. Its diameter is 79,042 miles; and, con, sequently, this planet is near 1000 times as large as that which we inhabit. Saturn is surrounded by a thin broad Ring, calculated to be 21,000 miles in breadth, and about that distance from its body on one side. This ring appears like a luminous arch in the heavens. Dr. Herschel lately discovered that it has a rotation on its axis, the time of which is about 10 hours, 32 minutes. Besides the light which Saturn receives from the sun, and the reflection of the ring, it has the light of seven satellites; which revolve round it, at different distances, HERSCHEL. On the evening of the 30th of March, 1781, Dr. Herschel discovered in the vast regions of space, and far beyond the orbit of Saturn, a planet, revolving about the sun, which had probably never before been seen by mortal eyes. On account of its immense distance, it is but just visible to the naked eye. It is computed to be 1,803,534,392 miles from the sun; and although it travels at the rate of 15,000 miles an hour, it takes up 83 years, 150 days, 18 hours of our time, to complete its annual revolution. Its diameter is 35,112 miles; consequently, it is about 90 times larger than our earth. It has a rotation on its axis, the time of which is not known. In England, this planet has been called GEOrgium SIDUS, in honor of the king in whose reign it was discovered; but in the rest of Europe, Uranius, and in america, it is more justly denominated HERSCHEL, in honor of its learned discoverer. Six satellites have been discovered, revolving about Herschel; and it is not improbable that there are others, which have not yet been observed. Besides the seven well known planets, above described, since the commencement of the present century, four others have been discovered, denominated Asteroids; two by M. Piozzi and Dr. Olbers, named CERES and PALLAS;; one by Mr. Harding, near Bremen, which he called JUNO; the other by Olbers, named VESTA. The orbits of the two former are between those of Mars and Jupiter, and their annual revolutions about the sun are performed in a-bout 4 years, 7 months, and 10 days. Juno is represented as similar to Ceres in height and apparent magnitude. |