(Yaaseen) 36 : 39
وَالْقَمَرَ قَدَّرْنَاهُ مَنَازِلَ حَتَّىٰ عَادَ كَالْعُرْجُونِ الْقَدِيمِ
And the Moon, We have fixed to its phases till it returns just like the old date stalk.
The new moon is curved like a date stalk. It then passes through different phases, such as quarter moon, half moon, and gradually full moon. Again, by passing through different phases, its shape changes into a curve like the palm stalk. This phenomenon is mentioned in this verse. The purpose of showing the different phases of the moon to mankind is to use it as a calendar. See explanation 2:189; 41:37.