How is the commencement of Dhuhr time (Zawāl al-Shams) determined?
Chapter on Prayer Times
Al-Mughni
Book of Prayer
Primary text
The passing of the sun from the zenith (Zawāl al-Shams) is determined by its deviation from the middle of the sky. This is ascertainable by observing the shadow of a person after it has reached its shortest length. If a person measures their shadow, waits a short period, and the shadow increases, then the sun has passed its zenith. If it decreases, it has not yet passed. Determining this precisely by fixed measurements (like paces) varies according to the seasons and geographical locations, as the shadow shortens when the day lengthens and vice versa. Approximations for the latitude of Iraq, Syria, and similar regions are provided based on astronomical calculations, indicating the shortest zenith deviation occurs near mid-June (one and one-third of a pace) and the longest deviation occurs near mid-December (ten and one-sixth paces).
Supporting text
To practically measure the zenith passage, one stands on level ground, marks the end of the shortest shadow, then places the right foot in front of the left, aligning the heel with the big toe. The distance spanned by this foot placement that the shadow exceeds after reaching its minimum length indicates the time the sun has deviated and Dhuhr becomes obligatory.