Maeglin: I admit that I don’t really understand how the selection of day names and astrology really work, but I don’t see why the connection between Wednesday and mercredi even has to exist at all. Can it not be merely coincidence that in the northern tradition Wednesday is named after Wotan and in the southern the day is named for Mercury?
Probably not. As for the weekday-naming thing: We know (cf. O. Neugebauer, History of Ancient Mathematical Astronomy) that the Greeks (following the practice of the Babylonians, but using different deities) named the five star-planets after gods: Ares, Aphrodite, Hermes, Kronos, Zeus (“aster Aphrodites” = Venus, etc.). As far back as Cicero, we see attested in ancient works the standard Hellenistic order of the seven planets (including the sun and moon) in descending order of apparent angular velocity, and by hypothesis, of geocentric distance:
Saturn Jupiter Mars Sun Venus Mercury Moon
(Kronos) (Zeus) (Ares) (Helios) (Aphrodite) (Hermes) (Selene)
The arrangement of the weekdays (standard everywhere that Greek astrology had influence) uses the same seven bodies but in a different order:
Sun Moon Mars Mercury Jupiter Venus Saturn
The question is, how was that “mixed-up” (i.e., not obviously related to any astronomical quantity) order derived? Cassius Dio in about 200 CE suggested two derivations (implying that even back then the question had become kind of mysterious). One was based on harmonic intervals, and the other depended on the astrological system of the rulers of the hours. By that system, each of the 24 hours of a day has one of the seven planets as its ruler, and the lord of the first hour of a weekday is the lord of the day itself.
So assuming that the ruler of the first hour of the first day ought to be the Sun, as the most powerful, we start counting in sequence down the standard Hellenistic order from the Sun:
1-Sun, 2-Venus, 3-Mercury, 4-Moon, 5-Saturn, 6-Jupiter, 7-Mars, 8-Sun, 9-Venus, etc. etc. When we get to 24, we’re on Mercury, so the ruler of the last hour of the first day is Mercury. So continuing the sequence, the ruler of the first hour of the second day is the next planet, i.e. the Moon. So the Moon is lord of the second day of the week. And so it goes and so it goes, down through Saturday. (Since 24 and 7 are relatively prime, we don’t get any repeats in the first-lord position till we’ve completed the whole week, which is nice.)
The Romans borrowed this system but, of course, stuck in the corresponding Roman deities:
Sol Luna Mars Mercury Jupiter Venus Saturn
Again, in the Norse system we get the same arrangement, but somewhat different (although corresponding) deities:
Sun Moon Tyr Wotan Thor Freya (Saturn?)
Considering the basic similarity of order, and the resemblances between, say, Jupiter and Thor and Venus and Freya, I think it’s very unlikely that this is coincidental. Although I confess I don’t know much about how Graeco-Roman astrology was transmitted to the northern barbarians…was it really as early as Caesar’s Germania?