352 ACROPHYSICS. 



Almagest, (p. 161), famous from the Arabic translation of it, was in 

 other respects a valuable compendium of this science. 



On the revival of science, in modern Europe, Alfonso, king of 

 Castile, in the year 1252, corrected the astronomical tables of Ptole- 

 my, and caused those to be published which have since borne his 

 own name. Copernicus, of Thorn, next revived the true solar sys- 

 tem, about 1530 ; and he shares with Pythagoras the honor of giving 

 it his name. It was opposed by Tycho Brahe ; and in part by Lon- 

 gomontanus ; but their systems have been long since rejected. The 

 Tychonic system agreed with the Ptolemaic in supposing the earth to 

 be stationary, and the sun and moon to revolve around it; but it dif- 

 fered in regard to the planets, which it supposed to revolve primarily 

 around the sun. Kepler first analyzed the motions of the planets, 

 and discovered those laws on which rests the theory of universal 

 gravitation. Galileo advocated the Copernican system; and by the 

 aid of one of the first telescopes, discovered Jupiter's satellites. 

 Huyghens discovered Saturn's ring, and fourth satellite ; and four 

 others were soon after noticed by Cassini. 



Newton, in 1686, published his Principia, explaining the theory 

 of universal gravitation, and thereby establishing the Copernican 

 system on an immovable basis. This theory has been farther deve- 

 loped, particularly by Euler, D'Alembert, and La Place. Dr. Brad- 

 ley discovered the aberration of the fixed stars ; of which catalogues 

 have been made by Bayer, Flamstead, Halley, and Herschel : but 

 the best are those of Lacaille, Bradley, Mayer, and Maskelyne. The 

 transits of Mercury in 1631, and of Venus, in 1639, were predicted 

 by Kepler : Gassendi observed the former, and Horrox, the latter. 

 To Halley, who predicted it, and Maskelyne, who observed it, we 

 are most indebted for the application of the transit of Venus, in 

 1761, to discover the true distance of the sun; which was farther 

 corrected by that of 1769. Dr. Herschel discovered the planet 

 Uranus, in 1781 ; and its satellites, in 1822. Ceres was discovered 

 by Piazzi, in 1801 ; Pallas, by Dr. Gibers, in 1802; Juno, by Hard- 

 ing, in 1804; and Vesta, by Dr. Olbers, in 1807. Thus far, only 

 have we room to speak of the history of Astronomy. 



We proceed now to give some ideas of this science, under the four 

 divisions of Descriptive, Siderial, Physical, and Practical Astronomy. 



1. Under the head of Descriptive Astronomy, we would include 

 the study of the magnitudes, distances, motions, and phenomena, of 

 the bodies which compose the Solar System. The sun, which we 

 have already mentioned as placed at the centre of this system, is a 

 globe, 885,000 miles in diameter ; which turns on its axis once in 

 25 days, as ascertained by the spots on its surface. The planets, 

 of which the earth is one, revolve around the sun in orbits, which 

 are ellipses, having the sun in one of their foci. The point at which 

 a planet is nearest the sun, is called its perihelion ; and the opposite 

 or farthest pJint of each orbit is called the aphelion. These points, 

 in connection, are called the apsides , and the line joining them is 

 the line of the apsides. The orbits of the other planets are slightly 

 inclined to that of the earth ; and cut its plane, each in two opposite 

 points, which are called the nodes: that one at which the planet 



