The world heritage site of the hierothesion of Antiochos I, King of Commagene, at Mount Nemrud (Turkey) certainly constitutes one of the most fascinating historical enigmas in human culture. The monument includes the famous lion “horoscope” which has often been used in various attempts to interpret and date the ruins, with controversial results. After performing on-site observations, during the summer solstice of 2009, we propose that Antiochos’s monument reflects the situation of the skies at special moments of the year 49 b.c., when possibly construction of the monument would have been begun. This alternative, more substantiated than the traditional explanation, is formulated in the present paper by considering not only the lion slab but also the orientation of the eastern and western terraces of the hierothesion, and the inscriptions on the monument. The small Kingdom of Commagene nestled between the upper course of the river Euphrates and the mountains of Anti-Taurus in the south-east of Anatolia. Despite its tiny size, Commagene played a significant role in the history of the Middle East during the late Hellenistic and early Roman periods, as a buffer state between the powerful Seleucid (later Roman) and Parthian Empires. The dynasty of Commagene was founded by King Ptolemaios (c. 163 b.c.), a member of the Orontid family of Armenian satraps and kings with Persian roots, and ended when Antiochos IV Epiphanes was deposed by Emperor Vespasian in 72 a.d. Antiochos I Theos (c. 69–36 b.c.) was arguably the most important of its kings, ruling for more than 30 years in one of the most challenging periods in the history of the region. He was contemporaneous with Triganes the Great of Armenia, Mithradates II and Orodes II of Parthia, the last petty Seleucid kings, Herod the Great, and Lucullus, Pompey, Crassus, Caesar and Antonius from the Roman side. Antiochos was the son of Mithradates I Kallinikos, an Orontid, and of Laodice Thea Philadelphos, daughter of the last Seleucid king of any importance, Antiochos VIII Grypus. Consequently, Antiochos was able to claim both an Iranian and a Hellenistic ancestry, which was reflected in his political behaviour, always balanced between East and West, and, most importantly, in the new cult he was going to establish in his kingdom. The most outstanding example of this new cult was Antiochos’s burial monument on the summit of the highest peak of Commagene, Nemrud Dag (see Figure 1) at a height of 2150 m. Earlier local inscriptions in Luwian speak about Kings Suppiluliuma and Hattusili (c. 800 b.c.), lords of Kummaha (Assyrian Kummuh, undoubtedly the original name of Commagene), who rendered worship to a sacred mountain named Hurtula, possibly Nemrud Dag. Mountain cult was very frequent JHA, xli (2010)