the celestial equator. The iconography also contains two torch-bearing twins (Cautes and Cautopates) framing the bull-slaying image – one holding a torch pointing up and the other a torch pointing down. These torch-bearers are sometimes depicted with one of them (torch up) holding or associated with a Bull and a tree with leaves, and the other (torch down) holding or associated with a Scorpion and a tree with fruit. Ulansey interprets these torch-bearers as representing the spring equinox (torch up, tree with leaves, Bull) and the autumn equinox (torch down, tree with fruit, Scorpion) in Taurus and Scorpius respectively, which is where the equinoxes were located during the preceding "Age of Taurus" symbolized in the tauroctony as a whole. From this, Ulansey concludes that Mithraic iconography was an "astronomical code" whose secret was the existence of a new cosmic divinity, unknown to those outside the cult, whose fundamental attribute was his ability to shift the structure of the entire cosmos, and thereby to control the astrological forces believed at that time to determine human existence. That gave him the power to grant his devotees success during life and salvation after death (i.e., a safe journey through the planetary spheres and a subsequent immortal existence in the sphere of the stars). Rate of precession Though the one degree per hundred years calculated for precession of the equinoxes as defined by Hipparchus and promulgated by Ptolemy was too slow, another rate of precession that was too fast also gained popularity in the 1st millennium AD. By the fourth century AD, Theon of Alexandria assumed a changing rate (trepidation) of one degree per 66 years. The tables of the Shah (Zij-i Shah) originate in the sixth century, but are unfortunately lost, but many later Arabic and Persian astronomers and astrologers refer to them and also use this value. These later astronomers-astrologers or sources include: Al-Khwarizmi, Zij al Sindhind or "Star Tables Based on the Indian Calculation Method"(c. 800); "Tabulae probatae" or "az-Zig al-mumtan" (c. 830); Al-Battani, Albategnius, al-Zij (c. 880); and al-Sufi, Azophi (c. 965); Al Biruni (973-1048), "al Canon al Masud" or "The Masʿūdic Canon"; Arabic fixed star catalogue of 1 October 1112 (ed. Paul Kunitzsch); and "Libros del Saber de Astronomía" by Alfonso X of Castile(1252-1284). Anno Domini There exists evidence that the modern calendar developed by Dionysius Exiguus in the 6th century AD commencing with the birth of Jesus Christ