Astrology in modern times is not seen as being a very credible science. At one time astrology was one of the most credible sciences, just as astrophysics or chemistry are today. Early astronomers such as Galileo and Copernicus were known to be practicing astrologers. The astronomer Claudius Ptolemy of whom very little is known, was seen as a leading intellectual of his day, Ptolemy's study and approval of astrology made astrology a hugely respected subject within academic circles. By giving astrology credibility as a science and also as an art, he ensured its practice would not be hindered during the medieval period. It was at this time that many other studies and occult practices were persecuted on religious grounds. He was deemed to be an authority on astrology and established the Tetrabiblos, (this is Greek which means four books) this became the definitive reference for all astrological students. Arabic scholars used the Tetrabiblos quite extensively, and they regarded Ptolemy to be the absolute expert on the subject. Later European scholars began to use the Tetrabiblos and translated it back into Latin in the 12th century.