Neither the epithet "indiges" (which fell out of use sometime after Caesar) nor the epithet "invictus" are used with any consistency however, making it impossible to differentiate between the two. "Sol Invictus" (English translated as "Unconquered Sun") was long thought to have been a foreign state-supported sun god introduced from either Emesa or Palmyra in Syria by the emperor Aurelian in 274 and overshadowing other Eastern cults in importance, until the abolition of classical Roman religion under Theodosius I. However the evidence for this is meager at best, and the notion that Aurelian introduced a new cult of the sun ignores the abundant evidence on coins, in images, in inscriptions, and in other sources for a strong presence of the sun god in Rome throughout the imperial period. Tertullian (died 220 CE) writes that the Circus Maximus was dedicated primarily to Sol. During the reign of Aurelian, a new college of pontiffs for Sol was established. There is some debate over the significance of the date December 21 for the cult of Sol. According to a single, late source, the Romans held a festival on December 21 of "Dies Natalis Invicti", "the birthday of the unconquered one." Most scholars assume Sol Invictus was meant, although our source for this festival does not state so explicitly. December 25 was commonly indicated as the date of the winter solstice, with the first detectable lengthening of daylight hours. There were also festivals on other days in December, including the 11th (mentioned above), as well as August. Gordon points out that none of these other festivals are linked to astronomical events. When the festival on December 25 was instituted is not clear, which makes it hard to assess what impact (if any) it had on the establishment of Christmas. Throughout the 4th century the cult of Sol continued to be maintained by high-ranking pontiffs, including the renowned Vettius Agorius Praetextatus. The Greek assimilation of Apollo and Helios was already established in Rome by the end of the republic. Various Roman philosophers speculated on the nature of the sun, without arriving at any consensus. A typical example is Nigidius, a scholar of the 1st century BCE. His works have not survived, but writing five centuries later, Macrobius reports that Nigidius argued that Sol was to be identified with Janus and that he had a counterpart, Jana, who was Luna.