Part of Speech: Verb

Wane- (of the moon) have a progressively smaller part of its visible surface illuminated, so that it appears to decrease in size.

Appears in Edna St. Vincent Millay’s “Pity Me Not Because the Light of Day” Line 5

Source: Google Dictionary

This definition adds to the poems imagery of light fading from the moon, and years passing by and the idea of things fading with the change of day.