THE MODERN PRECEPTOR. CHAPTER VII. ASTRONOMY. THE object of Astronomy is to explain the nature and motions of the heavenly bodies, and the laws by which these motions are governed, agreeably to the import of the two Greek words composing the name, viz. astron, a star, and nomos, a law. Astronomy has engaged the attention of mankind of all ages, and in every stage of rudeness or refinement; various theories having at different periods been formed and promulgated by the learned; that, however, which is now universally adopted by men of science is the theory of Nicolaus Copernicus, born at Thorn, in Prussia, in 1472, who, in 1543, a little before his death, produced the system known by his name, to the formation of which he had been led partly by considering the absurdities of the prevailing theories of his day, and partly by a few hints scattered in antient authors of the opinions of Pythagoras and other eminent men of the earliest times. Of the Copernican system, the following are the principal articles. The sun, the great source of light and heat to the earth we inhabit, is fixed in the centre of a system of other bodies, revolving round him at different distances and in different different periods of time, of which this earth is one. These bodies, arranged in the order of their proximity to the sun, are Mercury, Venus, the Earth, Mars, Jupiter, Saturn, all visible to the naked eye, together with three others, only discoverable by means of glasses, viz. Ceres, Pallas, Herschel, otherwise called the Georgian planet, or Uranus; Ceres and Pallas describing their course between Mars and Jupiter, while Herschel is removed to a great distance beyond Saturn, and probably towards the utmost verge of the Solar system. Of the magnitudes of these bodies, and of their several distances from the sun, the periods of their revolutions round his body and round their own axes, and some other particulars, a competent notion may be obtained from the following table. " In this table the first column contains the names of the sun and the planets composing his system; the second shews the diameter of each body in English miles, on the supposition of its being a perfect sphere; the third, the distance of each planet from the sun, on the supposition of its moving round him in a circle; the fourth, the angle of inclination formed by the plane of the orbit of each planet with that of the orbit of the earth or the ecliptic; the fifth, the angle formed by the direction of the axis of each planet with the plane of its own orbit; sixth, the time in which a planet performs one revolution round its axis, or the length of the day; seventh, the time in which a planet performs its revolution round the sun from any given point of its orbit to its return to the same point, or the length of its tropical year; and the eighth indicates the time required by a planet in passing from a conjunction with any fixed star, as observed from the sun, to its return to the same star, after a complete revolution round the sun, or the length of its sidereal year. Thus, for example, the second column points out, that if the sun were a perfect sphere, its diameter would be 883,246 English miles; the fifth column, that the axis on which he turns is inclined in an angle of 82 deg. 44 min. to the plane of the ecliptic, or the orbit of the earth; the sixth column, that his revolution round his axis is performed in a period of 25 of our days, 14 hours, and 8 minutes: the other columns corresponding to the sun are void, because the matters contained in them are inapplicable to that luminary, who remains unchangeably fixed in the centre of the revolutions of the other bodies named in the table. Again, the table shews that the diameter of the earth, were it a perfect sphere, would, according to the calculation upon which the table is founded, be nearly 7,912 English miles. (See Geography, vol. ii. p. 7.) that, were the orbit or path described by the earth, in its annual revolution round |