In the Islamic world, the study of astronomy was closely linked to religious practices. The faithful had to determine the times of the five prayers prescribed by the Koran. They also had to find the direction of Mecca in whatever region of Islam they happened to be. These calculations could not be performed accurately without applying spherical trigonometry and studying the Ptolemaic celestial model. To read the heavens, the Arabs compiled astronomical tables, built major observatories, and invented instruments that remained in use until the eighteenth century, such as the astrolabe, the quadrant, the equatorium, and the torquetum. Islamic astronomy spread to Christian Europe thanks to the Tables prepared in Toledo by the astronomer al-Zarqâlî in the eleventh century. Since then, many Arabic terms such as zenith, azimuth, nadir, and almucantars have entered into the astronomical lexicon of the western world.