"For the skin of his face shone"

Twice in our Parshat Ki-Tisa, Moshe descends from Mount Sinai: he brings down the first set of tablets prior to the sin of the Golden Calf and then the second set following the sin. The two descents differ in many respects, one of the most prominent differences being that on his second descent, the text notes that Moshe's face shone. Unquestionably, this is a dramatic change for Moshe's image and his standing in the eyes of the nation. From this point onwards, whatever Moshe conveyed to the nation in God's Name would be obeyed, in fear of and in deference to the sight of his shining face. But why does this change take place specifically upon his second descent from the mountain? Through a close examination of the text, we can understand that the radiance of Moshe's face was actually a result of the sin, and that had the sin not taken place, this phenomenon would not have occurred at all.