HILAL

Hilal (Arabic: هلال‎) is an Arabic term, meaning crescent moon, first developed in pre-IslamicArabia. The very slight crescent moon that is first visible after a new moon. Muslims look for the hilal when determining the beginning and end of Islamic months, but they don't worship it. The Quran says: "And from among His Signs are the night and the day, and the sun and the moon. Prostrate not to the sun nor to the moon, but prostrate to Allah Who created them, if you (really) worship Him." (41:37). The need to determine the precise appearance of the hilal was one of the inducements for Muslim scholars to study astronomy.

(from wikipedia)