The words of a waxing half moon and a waning half moon, used to show phases of the moon such as full moon => waning half moon => new moon => waxing half moon, compare the moon to a bow.
満月→下弦の月→新月→上弦の月という、月の満ち欠けの周期の状態を表す言葉の、上弦・下弦とは弓に例えた言葉である。