This is because the crescent is something observable, visible to the eyes. What is seen with the eyes is among the most certain things we know. Hence it is called hilāl (crescent), for this root conveys the meaning of becoming apparent and clear, whether to the ear or to the eye. Thus one says ahalla bi-l-‘umrah (he raised his voice proclaiming ‘umrah) and ahalla bi-l-dhabīhati li-ghayr Allāh (he raised his voice invoking someone other than Allah over the sacrificial animal). The sound of falling rain is called halal. One also says istahalla al-janīn when a newborn emerges crying, and tahallala wajhuhu when someone’s face becomes bright and radiant.

It has also been said that the root originally meant raising the voice. Since people would raise their voices upon sighting the crescent, they called it hilāl. Hence the verse:

Its riders raise their voices at al-Farqad,
As does the mounted pilgrim performing ‘umrah.

The expression tahallul al-wajh (the brightening of the face) is derived from the crescent’s radiance.

The point is that the appointed times have been determined by something manifest and clear, equally accessible to everyone. Nothing else shares this quality with the crescent. The conjunction of the sun and moon—their alignment before the crescent appears—is a hidden occurrence, known only through calculations mastered by certain people alone. These involve toil, the loss of much time, and distraction from what matters to people and from their indispensable needs. They may also be subject to error and disagreement.