1911 Encyclopædia Britannica/Summanus
SUMMANUS, according to some, an old Sabine or Etruscan deity; the name, however, is Latin, formed by assimilation from sub-mānus (cf. mane, Matuta), signifying the god of the time “ before the morning.” His sphere of influence was the nocturnal heavens, thunderstorms at night being attributed to him, those by day to Jupiter. Summanus had a temple at Rome near the Circus Maximus, dedicated at the time of the invasion of Italy by Pyrrhus, king of Epirus (278), when a terra-cotta image of the god (or of Jupiter himself) on the pediment of the Capitoline temple was struck by lightning and hurled into the river Tiber. Here sacrifice was offered every year to Summanus on the 20th of June, together with cakes called summanalia baked in the form of a wheel, supposed to be symbolical of the car of the god of the thunderbolt. In Plautus (Bacchides iv. 8, 54) Summanus and the verb sum man are are used for the god of thieves and the act of stealing, with obvious reference to Summanus as a god of night, a time favourable to thieves and their business. The later explanation that Summanus is a contraction from Summus Manium (the greatest of the Manes), and that he is to be identified with Dis Pater, is now generally rejected.
See Augustine, De civitate dei, iv. 25; Ovid. Fasti, vi. 729; Festus, s.v. Provorsum fulgor; G. Wissowa, Religion und Kultus der Römer (1902); W. W. Fowler, The Roman Festivals (1899).