The Traianeum was a large sacred complex from the 2nd century AD built by the emperor Hadrian in the ancient city of Italica (Santiponce, Seville, Spain), located in the Baetica province. Its current name comes from the hypothesis that this temple was dedicated to the imperial cult of Trajan. This article works with this interpretation. The data on its astronomical alignment show that the building could be oriented towards the winter solstice sunset, although the temple was accessed through the eastern façade. Immediately after this astronomical phenomenon, the Roman world celebrated the Dies Natalis Solis. These data are put into a symbolic relationship with various features of the building and of the emperors Trajan and Hadrian.