Bonaventure — on Rev 6:12 (Collations on the Hexaemeron, Collation 13)

“The sun signifies spiritual combat. In this regard, "the sun will be darkened, and the moon will not give her light." When the fight will be between Christ and Antichrist, between the teaching of truth and that of falsity, then "the sun" will become "black as sackcloth of hair," and a certain teacher of truth or superior in truth shall be obscured by errors, while others will stand firm, although they may seem to be darkened in their reputation.”