μια πιθανή εξήγηση, που σύμφωνα με το διαδίκτυο περιέχεται στο βιβλίο
"Dictionary of Subject and Symbols in Art" του James Hall, είναι οτι αντιπροσωπεύουν τον ήλιο και το φεγγάρι ως μάρτυρες της σταύρωσης.
“ The sun and moon, one on each side of the cross, are a regular feature of Medieval crucifixions. They survived into the early Renaissance but are seldom seen after the 15th century. Their origin is very ancient. It was the custom to represent the sun and moon in images of the pagan sun gods of Persia and Greece, a practice that was carried over into Roman times on coins depicting the emperors.”
(...) The sun is[sometimes represented as simply a man’s bust with a radiant halo, the moon as a woman’s with the crescent of Diana. Later they are reduced to two plain disks, the moon having a crescent within the circle, may be borne by angels. The sun appears on Christ's right, the moon on his left.”
δες εδώ για περισσότερα και άλλες φωτογραφίες, στο κομμάτι Crucifixion:
http://www.sprezzatura.it/Arte/Arte_UFO_2_eng.htm