division of the zodiac into 12 signs of 30 equal degrees each is the Anaphoricus of Hypsicles of Alexandria (fl.190BC). Particularly important in the development of Western horoscopic astrology was the astrologer and astronomer Ptolemy, whose work Tetrabiblos laid the basis of the Western astrological tradition. Under the Greeks, and Ptolemy in particular, the planets, Houses, and signs of the zodiac were rationalized and their function set down in a way that has changed little to the present day. Ptolemy lived in the 2nd century AD, three centuries after the discovery of the precession of the equinoxes by Hipparchus around 130 BC. Hipparchus's lost work on precession never circulated very widely until it was brought to prominence by Ptolemy, and there are few explanations of precession outside the work of Ptolemy until late Antiquity, by which time Ptolemy's influence was widely established. Ptolemy clearly explained the theoretical basis of the western zodiac as being a tropical coordinate system, by which the zodiac is aligned to the equinoxes and solstices, rather than the visible constellations that bear the same names as the zodiac signs. Hindu zodiac According to mathematician-historian Montucla, the Hindu zodiac was adopted from the Greek zodiac through communications between ancient India and the Greek empire of Bactria. The Hindu zodiac uses the sidereal coordinate system, which makes reference to the fixed stars. The tropical zodiac (of Mesopotamian origin) is divided by the intersections of the ecliptic and equator, which shifts in relation to the backdrop of fixed stars at a rate of 1° every 72 years, creating the phenomenon known as precession of the equinoxes. The Hindu zodiac, being sidereal, does not maintain this seasonal alignment, but there are still similarities between the two systems. The Hindu zodiac signs and corresponding Greek signs sound very different, being in Sanskrit and Greek respectively, but their symbols are nearly identical. For example, dhanu means "bow" and corresponds to Sagittarius, the "archer", and kumbha means "water-pitcher" and corresponds to Aquarius, the "water-carrier". Middle Ages During the Abbasid era, Greek reference books were systematically translated into Arabic, and Islamic astronomers then did their own observations, correcting Ptolemy's Almagest. One such book was Al-Sufi's Book of Fixed Stars, which has pictorial depictions of 48 constellations. The book was divided into three sections: constellations of the zodiac, constellations north of the zodiac, and southern constellations. When Al-Sufi's book, and other works, were translated in