"The sun is bigger than the moon."
Translation:Ο ήλιος είναι μεγαλύτερος από το φεγγάρι.
Has Σελήνη fallen into disuse? How did it come to be replaced by φεγγάρι?
No, not at all. Σελήνη is perfectly in use (also one of the accepted translations). Φεγγάρι means "the one that shines" from "φέγγω"="to radiate/shine". Their difference is that φεγγάρι can be used for any moon (even of other planets) whereas Σελήνη is the name of our planet, such as Γανυμήδης is the name of one of Jupiter's moons. The more scientific word is δορυφόρος=satellite.