It was common practice for the Romans--who, like any empire, saw themselves as the pinacle of civilization--to identify "barbarian" gods with their own pantheon. This practice is not unique to the Romans, of course; many times my history teachers would compare historical figures to familiar American figures--"X's George Washington".
This practice was not only born out of a sense of superiority or cultural insensitivity (such terms and ideas were non-existant in those days, of course), but out of the simple fact that the Roman pantheon was, from an early date, identified with the older Greek pantheon, due to the close proximity of the cultures and the Greek colonies on the Italian peninsula during the early days of the Roman Republic. The Romans quickly sought to identify certain gods with the Greek pantheon--Minerva with Athena, Diana with Artemis, Mars with Ares, Saturn with Cronos--even though they were not an exact match1. Other gods names were derived from a common source, and thus made an even easier identification--Zeus and Iuppiter (Zeus Pater?)2. When there was no equivalent, the Romans simply addopted the god as a whole--Apollo in particular springs to mind, as well as the many mystery cults devoted to Eastern deities, such as Isis or Attis.
The term interpretatio romana derives from Tacitus' Germania, ch. 43, wherein he describes two German gods worshipped as brothers and youths--twins--as being like Castor and Pollux, "according to the Roman interpretation." The practice of i.r. is especially well known through Caesar's De Bello Gallica, where he describes the gods of the Gauls:
"Among the gods, they most worship Mercury. There are numerous images of him; they declare him the inventor of all arts, the guide for every road and journey, and they deem him to have the greatest influence for all money-making and traffic. After him they set Apollo, Mars, Jupiter, and Minerva. Of all these deities they have almost the same idea as all other natioins: Apollo drives away diseases, Minerva supplies the first principles of arts and crafts, Jupiter hods the empire of heaven, Mars controls wars. ...The Gauls affirm that they are all descended form a common father, Dis, and say that this is the tradition of the Druids" (Caesar 6.17-18)
Scholars have long sought to identify these Roman gods with their Gallic counterparts. Mercury is usually identified with Lugos/Lugh, Minerva with Brigantia/Brigit, based on the above descriptions. The other gods are more difficult, for unlike Mercury and Minerva, there are numerous gods who are given dual names at various locations throughout the empire: Mars Alator, Mars Albioris, Mars Barrex... Mars Nodens... Mars Segomo, Mars Smertrios, and Mars Teutates -- at least thirty-five gods are identified with the Roman Mars. Of Apollo there are ten, of Mercury there are nine, of Minerva three, and of Jupiter there is only only semi-positive identification, Taranis. As for Dis Pater, there is no positive identification (except for the Berne scholia's identification with Taranis), but conjectures have lead to the Gallic Sucellus, Smertios, and the Irish god Donn. However, if he is father of the people, he may be Teutates, whose name possibly means "fater of the tribe".
Also significant where the Celts are concerned is Lucan's Pharsalia and the commentary of the Berne scholia:
And those who pacify with blood accursed
Savage Teutates, Hesus' horrid shrines,
And Taranis' altars cruel as were those
Loved by Diana, goddess of the north
The Berne scholia--a collection of glosses on Lucan's poem, originally written in the 4th c. CE, but only surviving in a single medieval manuscript, identfies Teutates with either Mercury or Mars, Esus with either Mars or Mercury, and Taranis with either Dis Pater or Jupiter. Inscriptions only support the identification of Teutates with Mars and Taranis with Jupiter--fitting, as Taranis means "Thunderer."
And there are other examples; Hercules is given bynames like Andossus, Graius, and Ilunnus in inscriptions in Gaul; in Lukian of Samosata's Herakles (2nd c CE) with Ogmios, god of eloquence. Diana is identified in dedications with Abnoba and Arduinna, both continental mountain goddesses.
Tacitus, not surprisingly relied on this practice in Germania: like Caesar's comment on the Gauls, Tacitus tells us that "they worship Mercury above all, whom they consider it right on specific days to propitate with human as well as other sacrifices" (Tacitus 9.1)3. He also mentions worship of Hercules and Mars, and that the Suebi--a tribe living between the Elbe and the Oder--also worshipped Isis, symbolized by a ship; later he says they worship Nerthus, "Mother Earth"--who was possibly the Isis he had in mind. Mercury is agreed to be Odin/Woden, and Mars Tyr/Tiwaz. Hercules, it is thought, was likely Thor/Donar.
The practice of i.r.--and its Anglo Saxon reverse--can be seen in the naming of the days of the week:
ANCIENT GREEK ROMAN FRENCH ENGLISH
hemera heliu dies Solis dimanche Sunday
hemera selenes dies Lunae lundi Monday
hemera Areos dies Martis mardi Tuesday (Tiw's Day)
hemera Hermu dies Mercurii mercredi Wednesday (Woden's Day)
hemera Dios dies Jovis jeudi Thursday (Thor's Day)
hemera Aphrodites dies Veneris vendredi Friday (Frigg's Day)
hemera Kronu dies Saturni samedi Saturday4
It should be noted that the Greeks borrowed this system from either the Babylonians or the Egyptians (depending on who you ask), and that they substituted the seven planets of the Babylonians (or Egyptians) with their Greek equivalents--interpretatio Graeci, I suppose. I suppose the English names were a sort of reversal of i.r., taking the original Roman identifications of Germanic gods, and turning them to the days of the week.
Caesar, Julius Gaius. The Gallic War. ed. & trans. H.J. Edwards. Loeb Classical Library. Cambridge, MA: Harvard University Press, 1986.
Lucan. Pharsalia. Book I, l.498-502. See http://sunsite.berkeley.edu/OMACL/Pharsalia/book1.html
Maier, Berhard. Dictionary of Celtic Religion and Culture. trans. Cyril Edwards. Rochester, NY: Boydell, 1997.
Tacitus. Germany. trans. Herbert W. Benario. Warminster, England: Aris & Phillips, LTD., 1999.
- This is especially evident the the cases of Mars-Ares and Saturn-Cronos. Mars, while a war god, was not usually seen as the nearly-psychotic Ares, but more as a glorious warrior and god of husbandry--if anything, his role was progenitor of the Romans (being the father of Romulus and Remus) and protector of the people's livelihood, whether as a warrior or god of agriculture. The same can be said of Saturn, who was depicted not as the cannibalistic Cronus, but as the ruler of the Golden Age of the past, and also a god of agriculture, represented with the scythe. Of course, there is also the problem with the confusion between Khronos (Time, and hense our image of Father Time) and Kronos (the Titan); whether that identification originated with the Romans or with Anglophones, I'm not sure. The origins of Kronos and Khronos are different--one is simply the word for time, the other is the name of the Titan, father of Zeus and his siblings; their identification is much later, and a mistake. The etymology of Kronos (the Titan) is unknown. Gritchka pointed out to me that it may be a coincidence that has been conflated into a single figure of differing origins. In the end, no one is really sure.
- Both names ultimately derive, like the word deus, from the Indo-European dyeu-, meaning "to shine". Jupiter, Zeus, Deus, Tiw, Diana, Asmodeus, Dagda, Devi, day, and so on, ultimately are thought to derive from dyeu-.
- Even the wording is nearly identical:
Caesar: "Deum maxime Mercurium colunt"
Tacitus "Deourm maxime Mercurium colunt"
- Why Saturday retained the Roman-influenced name is still unclear; some, like Jacob Grimm, argued that the god should be Loki.
Celtic Examples of Interpretatio Romana
Dis Pater (Identified with Taranis in the Berne Scholia)
Jupiter (Indentified with Taranis in the Berne Scholia)
Iupiter Taranucus (Both probably Taranis)
Mars (Identified with Teutates and Esus in the Berne scholia)
Mars Iovantucarus (also identified with Mercury)
Mercury (Identified with Esus and Teutates in the Berne scholia)
Mercury Iovantucarus (also identified with Mars)
Victoria Brigantia (also identified with Minerva)