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226 earth was the central body, around which the concave sphere of the starry heavens revolved; while, in the space between, the Moon, Venus, Mercury, the Sun, Mars, Jupiter, and Saturn moved with retrograde courses, in the order here mentioned. In fact, this system was the one which was afterwards adopted, elaborated, and zealously maintained by the famous Ptolemy of Alexandria, and which has ever since borne his name. To Ptolemy, then, who flourished about the commencement of the second century, the world is indebted for the first complete system of astronomy that secured the approbation of all the learned. This it was enabled to do by the ingenious, although not perfect, explanation which it gave of the planetary movements, by supposing these bodies to move in circles whose centres had an easterly motion along an imaginary circle. Thus these epicycles, as the circles were called, moving along the imaginary circle, or deferent, cause the planets to have, at times, an apparent easterly direction, at other times a westerly one, and at other times, again, to appear stationary. Thus recommended, the Ptolemaic system continued to gain adherents, until the irruptions of the Huns under Alaric and Attila, and the destruction of the celebrated library at Alexandria by the fanatical and turbulent Christians of that city, laid waste the fair domains of science. Being thus driven from the places where Learning had fixed her favorite seats, it took refuge with the Arabs, who preserved it with watchful care, until happier times restored it to Europe. It returned with the conquering Moors who established themselves in Spain, was brought again under the notice of the Christian states in the thirteenth century, through the patronage