I say with God's guidance: It starts from the sun's zenith until the shadow of everything becomes twice its length, excluding the zenith shadow — which is the shadow that appears when the sun is at its zenith. The way to determine the zenith time and the zenith shadow is to level the ground so that no side is higher or lower, which can be checked by pouring water or using special leveling tools. Then, draw a circle on it — called the Indian circle — and place a vertical measure at its center, ensuring that its distance from three points on the circle's circumference is equal; this confirms that the measure is upright. If its distance from three points on the circumference in three directions is equal, then it is equidistant from all directions, indicating it is upright without inclination. Its height should be a quarter of the circle's diameter, as it should be such that its shadow is shorter than half the circle's diameter to distinguish its entry and exit. The shadow's presence in most regions is only conceivable in this manner. Its shadow's tip in the early morning is outside the circle, but the shadow decreases until it enters the circle. Mark the shadow's entry point on the circle's circumference. Undoubtedly, the shadow decreases to a certain extent, then increases until it reaches the circumference, then exits after midday. Mark the shadow's exit point, bisect the arc between the shadow's entry and exit, and draw a straight line from the midpoint of the arc to the circle's center, extending to the opposite side of the circumference. This line is the midday line. If the measure's shadow is on this line, it is midday, and the shadow at this time is the zenith shadow. When the shadow moves from this line, it is the zenith time, which is the start of Dhuhr. The end is when the measure's shadow becomes twice the measure's length, excluding the zenith shadow. For example, if the zenith shadow is a quarter of the measure, the end of Dhuhr is when its shadow becomes twice the measure plus a quarter. See: Fath Bab al-Inayah 1: 177, and Umda al-Ri'ayah 1: 145, and God knows best.