[11] The objection which has been urged against astrology, that the signs are continually moving from their positions, cannot invalidate this conclusion. That objection has, in fact, no real existence; for Ptolemy seems to have been aware of this motion of the signs, and has fully provided for it in the 25th Chapter of the 1st Book of the Tetrabiblos. From that chapter it is clear that the respective influences he ascribes to the twelve signs (or divisions of the zodiac) were considered by him as appurtenant to the _places_ they occupied, and not to the _stars_ of which they were composed. He has expressly and repeatedly declared that the point of the vernal equinox is ever the beginning of the zodiac, and that the 30 degrees following it ever retain the same virtue as that which he has in this work attributed to Aries, although the stars forming Aries may have quitted those degrees: the next 30 degrees are still be accounted as Taurus, and so of the rest. There is abundant proof throughout the Tetrabiblos, that Ptolemy considered the virtues of the _constellations_ of the zodiac distinctly from those of the _spaces_ they occupied. In the year 827[12] the “Great Construction” was translated by the Arabians into their own language, and by them communicated to Europe. It is through them that it has been usually known by the name of the Almagest. In the 13th century, the Emperor Frederic II caused it to be translated from the Arabic into Latin, and Sacrobosco[13] was consequently enabled to write his famous work upon the sphere. It was not, however, until about the end of the 15th century that the “Great Construction” was translated into Latin from the _original_ text; and this important service was rendered to science by Purbach, a professor of philosophy at Vienna, who learned the Greek tongue at the instigation of Cardinal Bessarion. By means of this translation, the Ephemerides of George Müller, surnamed Regiomontanus, a disciple of Purbach’s, were first composed. The Greek text of the Almagest, or Great Construction, was first published at Basle, by Simon Grynæus, in 1538; and it was again printed at the same place in 1551, with certain other works of Ptolemy.[14] The rest of Ptolemy’s works connected with astronomy, and now extant, are the Tetrabiblos, or Four Books of the Influence of the Stars[15] (now translated); the Centiloquy, or Fruit of his Four Books, being a kind of supplement to the former; and the Significations of the Fixed Stars. The last is merely a daily calendar, showing the risings and settings of the stars, and the nature of the weather thereby produced. There are likewise extant his geographical work (which has rendered important service to modern geographers), and also his celebrated book on Harmonics, or the Theory of Sound. [12] The French say 813, but 827 is the date given by English chronologists.