I don't know how the Germanic tribes would have explained it, but in 100 B.C., the Romans would have known that the eclipse was caused by the moon obstructing the light of the sun. Thanks to the work of Hipparchus (based partly on older, Babylonian records) they would have known enough about Saros cycles as well, that their astronomers / astrologers could have predicted eclipses of the sun and the moon in advance. Also, it was known that lunar eclipses were the shadow of the earth moving across the face of the moon, and the earth was known to be round due (in part) to the shape of the shadow it cast.

In fact, here is a near-contemporary of your 100 B.C. time period (Cicero, De Natura Deorum, ca. 45 B.C.), in his own words:

"Again the moon, which is, as the mathematicians prove, more than half the size of the earth, roams in the same courses as the sun, but at one time converging with the sun and at another diverging from it, both bestows upon the earth the light that it has borrowed from the sun and itself undergoes divers changes of its light, and also at one time is in conjunction and hides the sun, darkening the light of its rays, at another itself comes into the shadow of the earth, being opposite to the sun, and owing to the interposition and interference of the earth is suddenly extinguished."