We are told by Diodorus Siculus, that the philofophers of Babylon, at the time of Alexander's entry into that city, reckoned four hundred and three thousand years from the beginning of their aftronomical obferva tions. And upon a fuppofition that the ecliptic was firft perpendicular to the equator, and afterwards began to approach towards it, according to the rate above-mentioned, this period nearly agrees with the diminu tion of the angle, which in that time, had taken place, and reduced the obliquity to twenty-three degrees and a half. Some, however, are of a contrary opinion; and are disposed to believe, that the obliquity of the ecliptic has been always the fame. But in this they are certainly miftaken; for befides the apparent decrease of this angle, which has been obferved by moft aftronomers fince the time of Hipparchus, the variation of latitude in the fixed ftars is fuch as could arife from no other cause. Ptolemy tells us exprefsly that he determined the obliquity, for several years together, to be twenty-three degrees fifty-one minutes, and it is now known to be twenty-three degrees twenty-eight minutes; the attention which has been bestowed upon this fubject for nearly a century past, has enabled us to decide with certainty that the diminution is real, and according to a certain law, Hipparchus, in comparing his obfervations with thofe of Timocharis, which had been made at Alexandria about a century before, firft perceived that the ftars changed their pofitions, and appeared to have a flow motion from weft to eaft with regard to the equinoctial points. This change of the ftars in longitude, which has now become fufficiently apparent, is owing to a fmall retrograde motion of the equinoctial points, of about fifty feconds in a year, which is occafioned by the attraction of the fun and moon upon the protuberant matter about the equator. The fame cause alfo occafions à small deviation in the parallelifm of the earth's axes, by which it is continually directed towards different points in the heavens, and makes a complete revolution round the axes of the ecliptic in about twenty-five thousand nine hundred and twenty years. In confequence of this shifting of the equinoctial points, an alteration has taken place in the figns of the ecliptic; thofe ftars which in the infancy of aftronomy were in Aries, being now got into Taurus; those af Taurus into Gemini ; &c. The The Copernican, or folar fyftem. This fyftem of the world, as defcribed on Plate 2, is not a late invention, ut was known and taught by the wife Samian Pythagoras, and others mong the ancients; which, in after-times, was loft; till, in the 15th entury, it was again revived by the famous Polish philofopher, Nicholas Copernicus, who was born at Thorn, in the year 1473. In this he was Followed by the greateft mathematicians and philofophers that have fince ived, as KEPLER, GALILEO, DESCARTES, GESSENDUS, and Sir ISAAC NEWTON, who have eftablished this fyftem on fuch an everlasting founda. Eion of mathematical and phyfical demonstration, that the gates of ignor. ance fhall never prevail against it. The most famous of the antiquated fyftems are two, viz. one taught by PTOLEMY, the EGYPTIAN, aftronomer, faid to have lived 138 years before Chrift; the other by the noble Dane, TYCHO BRABE, born in Schonen, A. D. 1546. The PTOLEMEAN SYSTEM fuppofed the earth immoveably fixed in the center of the world, about which moved feven planets, viz. the Moon, Mercury, Venus, the Sun, Mars, Jupiter and Saturn; above thefe is placed the firmament of the fixed ftars, then the two crystalline spheres ; all which were included in, and received motion from, the primum mobile; which constantly revolved about the earth in 24 hours from east to west. The TYCHONIAN SYSTEM fucceeded the PTOLEMEAN, but was never fo univerfal. This fuppofed the earth in the center of the world, or firmament of fixed ftars; as alfo of the two luminaries, the Moon and Sun: but then he supposes the Sun the center of the planetary motions, viz. of Mercury, Venus, Mars, Jupiter, and Saturn; thefe, with the Sun, all revolved about the earth in the space of a year, to account for the annual motion; and the earth he made to revolve about the axes every 24 hours, from west to eat. This hypothefis being part falfe, was embraced by few, and foon gave way to the only true and rational solar fyftem, restored by COPERNICUS as aforefaid. A BRIEF DESCRIPTION OF THE SOLAR SYSTEM: The fun, à ftupendous body of fire, is placed in the centre of the fyftem round whofe orb, the planets, fatellites, and comets, perform their revolutions, with an order and regularity which muft fill our minds with the most exalted conceptions of their divine Original. Mercury, the nearest planet to the fun, goes round him in about 87 days and 23 hours, or a little lefs than three months; which is the length of his year. But being feldom feen, on account of his nearness to the fun, and no fpots appearing on his furface, or difk, the time of his rota tion upon his axis, or the length of his days and nights, is not yet determined. His distance from the fun is computed to be about thirtyfeven millions of miles, and his diameter three thousand two hundred ; and and in his courfe round the fun, he moves at the rate of a hundred and five thousand miles an hour. Venus, the next planet above Mercury, is computed to be fixty-eight millions of miles from the fun, and by moving at the rate of feventy-fix thousand miles an hour, fhe completes her annual revolution in two hundred and twenty-four days and feventeen hours, or about feven months and a half. Her diameter is seven thousand seven hundred miles, and her diurnal rotation on her axis, is performed in twenty-three hours and twenty-two minutes. When this planet appears to the weft of the fun, the rifes before him in the morning, and is called the Morning Star; and when the appears to the east of the fun, fhe fhines in the evening, after he fets, and is then called the Evening Star; being in each situation, alternately, for about two hundred and ninety days. The next planet above Venus, in our fyftem, is the Earth. Its diftance from the fun is ninety-five millions of miles, and by travelling at the rate of fifty-eight thousand miles an hour, its annual revolution is performed in three hundred and fixty-five days, five hours, and forty. nine minutes, or the fpace of a year; which motion, though one hundred and twenty times swifter than that of a cannon ball, is but little less than half the velocity of Mercury in his orbit. The earth's diameter is seven thoufand nine hundred miles; and as it turns round its axis every twentyfour hours, from weft to eaft, it occafions an apparent motion of all the heavenly bodies, from eaft to weft, in the fame time. Next above the earth's orbit, is Mars, whose distance from the Sun is computed to be about one hundred and forty-four millions of miles. He moves at the rate of fifty-five thousand miles an hour, and completes his revolution round the Sun in a period little less than two of our years. His diameter is four thousand two hundred miles; and his diurnal rotation upon his axis is performed in about twenty-four hours and thirty-nine minutes. Jupiter, the largest of all the planets, is still higher in the fystem than Mars. He is reckoned to be about four hundred and ninety millions of miles from the Sun, and by going at the rate of twenty-nine thoufand miles an hour, completes his annual revolution in fomething less than twelve of our years. His diameter is computed to be eighty-nine thousand miles, and, by a prodigioufly rapid motion upon his axis, he performs his diurnal rotation in nine hours and fifty-fix minutes, Saturn, the next planet in the fystem above Jupiter, is about nine hundred millions of miles from the Sun; and by travelling at the rate of twenty-two thousand miles an hour, he performs his annual circuit round that luminary in about twenty-nine and an half of our years. His dia. meter is computed to be feventy-nine thousand miles; but, on account of his immenfe distance, and the deficiency of light occafioned by fuch a remote fituation, the time of his diurnal rotation upon his axis has not. yet been afcertained. There will be a Tranfit of this beautiful Planet over the Sun's disk, December 6, 1882. For an Explanation, and Orthographical Projection of this Phenomenon, fee Frontispiece to this work. |