idea that this choice constituted a deliberate Christianization of a festival of the Birthday of the Unconquered Sun. For example, Michael Alan Anderson writes: The same point is made by Hijmans: He also states that: A study of Augustine of Hippo remarks that his exhortation in a Christmas sermon, "let us celebrate this day as a feast not for the sake of this sun, which is beheld by believers as much as by ourselves, but for the sake of him who created the sun", shows that he was aware of the coincidence of the celebration of Christmas and the Birthday of the Unconquered Sun, although this pagan festival was celebrated at only a few places and was originally a peculiarity of the Roman city calendar. It adds: "He also believes, however, that there is a reliable tradition which gives 25 December as the actual date of the birth of our Lord." In the 5th century, Pope Leo I (the Great) spoke in several sermons on the Feast of the Nativity of how the celebration of Christ's birth coincided with the increase of the Sun's position in the sky. An example is: "But this Nativity which is to be adored in heaven and on earth is suggested to us by no day more than this when, with the early light still shedding its rays on nature, there is borne in upon our senses the brightness of this wondrous mystery. Christian iconography The charioteer in the mosaic of Mausoleum M has been interpreted by some as Christ by those who argue that Christians adopted the image of the Sun (Helios or Sol Invictus) to represent Christ. In this portrayal, he is a beardless figure with a flowing cloak in a chariot drawn by four white horses, as in the mosaic in Mausoleum M discovered under Saint Peter's Basilica and in an early-4th-century catacomb fresco. The nimbus of the figure under Saint Peter's Basilica is rayed, as in traditional pre-Christian representations. Clement of Alexandria had spoken of Christ driving his chariot across the sky. This interpretation is doubted by others: "Only the cross-shaped nimbus makes the Christian significance apparent". and the figure is seen by some simply as a representation of the sun with no explicit religious reference whatever, pagan or Christian. Life of Christ and astrological comparisons Another speculation connects the biblical elements of Christ's life to those of a sun god.