They built observatories,--invented instruments for observing and measuring with correctness,--separated the stars into different groups or constellations, for the more easily finding any particular star,--gave particular names to most of the moving stars or planets, and noted the periods which each took to move through its apparent path in the heavens; and in many other ways the ancients helped to lay the foundations of that mass of astronomical knowledge which men of later ages have brought to more maturity. _Constellation_, a cluster of fixed stars; an assemblage of stars. _Observatory_, a place so built as to command a view of the heavens. Who first taught the true system of the Universe? Pythagoras, one of the most distinguished philosophers of antiquity. He is thought to have been a native of Samos, an island in the Archipelago; he flourished about 500 years before Christ, in the time of Tarquin, the last King of Rome. Pythagoras was the first among the Europeans who taught that the Earth and Planets turn round the Sun, which stands immovable in the centre;--that the diurnal motion of the Sun and Fixed Stars is not real, but apparent,--arising from the Earth's motion round its own axis, &c. After the time of Pythagoras, Astronomy sunk into neglect. _Philosopher_, one who studies philosophy. _Philosophy_, all knowledge, whether natural or moral. The term is derived from the Greek, _philos_, lover, and _sophia_, wisdom. By whom was it revived? By the family of the Ptolemies, kings of Egypt, who founded a school of astronomy at Alexandria, which produced several eminent astronomers, particularly one named Hipparchus. The Saracens, on their conquest of Egypt, became possessed of the knowledge of Astronomy, which they carried with them out of Africa into Spain; and thus, after a long exile, it was introduced afresh into Europe. Did not Astronomy from this time make great progress? Yes; it made considerable advances, being cultivated by the greatest geniuses, and patronized by the greatest princes. The system of the Ptolemies, called the Ptolemaic, had hitherto been used, with some slight alterations; but Copernicus, an eminent astronomer, born at Thorn, in Polish Prussia, in 1473, adopted the system which had been taught by Pythagoras in Greece, five or six hundred years before the time of Ptolemy. About the same time with Copernicus flourished Tycho Brahe, born in Denmark, 1546. _Geniuses_, men gifted with superior mental faculties. _Mental_, belonging to the mind. _Faculties_, powers of doing anything, whether menial or bodily; abilities; powers of the mind. What next greatly forwarded this interesting science? The introduction of telescopes by Galileo, who by their means discovered the small stars or satellites which attend the planet Jupiter; the various appearances of Saturn; the mountains in the Moon; the spots on the Sun; and its revolution on its axis.