SEMO SANCUS, an Italian divinity worshipped by the Sabines, Umbrians and Romans, also called Dius Fidius and (perhaps wrongly) identified with the Italian Hercules. His dual nature, as a god of light and good faith, is indicated by the names Dius Fidius. Sancus is obviously from sancire, meaning one who hallows the acts in which he takes part. Semo has been variously explained as: (i) one who presides over seed-time and harvest (serere, cf. the female Semonia); (2) a being apart from and superior to man (se-homo) ; (3) a demi-god (semis). The priests called bidentales, whose existence is attested by inscriptions, were specially connected with his worship, since lightning which fell from heaven during the day was looked upon as sent by Dius Fidius, and a special class of birds (sanquales) was under his protection. As the god of oaths, he protected the sanctity of the marriage tie, the rights of hospitality, international treaties and alliances. In his sanctuary on the Quirinal, the foundation of which was celebrated on the 5th of June, there were shown the distaff and spindle of Tanaquil, the wife of Tarquinius Priscus, and in the eyes of Roman matrons the embodiment of all wifely virtues. Dionysius of Halicarnassus (iv. 58) states that the treaty concluded between Tarquinius Superbus and the town of Gabii was deposited in the same temple of Sancus, whose name he translates by Zew iriortos. He could only be invoked under the open sky, as partaking of the nature of a god of light and day; hence a round opening was made in the roof of his temple through which prayers might ascend to heaven. If he was invoked in a private house, those who called upon his name stood beneath the opening in the roof called complumum. The bronze orbs mentioned by Livy (viii. 20. 8) as having been set up in his temple are also supposed to have some connexion with this, although they may be merely symbols of the eternal power of Rome. There was a second chapel of Semo Sancus on the island in the Tiber with an altar, the inscription on which led Christian writers (Justin Martyr, Tertullian, Eusebius) to confuse him with Simon Magus, and to infer that the latter was worshipped at Rome as a god. The cult of Semo Sancus never possessed very great importance at Rome; authorities differ as to whether it was of Sabine origin or not. The plural Semones was used of a class of supernatural beings, a kind of tutelary deities of the state.
See Preller, Romische Mythologie; article " Dius Fidius," by Wissowa, in Roscher's Lexikon der Mythologie, and his Religion und Kultus der Romer (1902), who rejects the identity of Semo Sancus Dius Fidius with Hercules; W. W. Fowler, The Roman Festivals (1899); E. Jannettaz, tude sur Semo Sancus Fidius (Paris, 1885), according to whom he was a Sabine fire god.
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)