You wouldn’t expect a Norse myth to be a parable on economic exploitation, and how bad deeds rebound on the doer. Grottasöngr, a story of how king Frodi forced two giantesses to work a mill without cease, and thus wrought his own destruction, is an unusual myth.
We are used to thinking of the moon and sun as opposites, a fallacy that even pervades astronomy, as Space.com points out. For a single instant in a lunar month (29.5 days) the sun is opposite the moon in the sky. This is the exact moment of the full moon. Apart from this, the two may be anything from 180 to 0 degrees apart, and are often seen in the sky together.
This may explain why Völuspá refers to the moon-god Mani as the sun’s companion or escort. (Depending on translation.) Also, when the two appear in the sky for the first time, their routes are not yet established, so it makes sense that they would be together.
We all know that Yggdrasil is the World Tree of Norse myth, and that it holds together the nine worlds. However, the Norse, like the Finns and the Hindus of India, seem to have had some notion of a world mill (or churn) as well, which turned the heavens and could grind out various products. This cosmic mill shows through the confused myths about the sun and moon.