I say, with God's guidance: The method to determine the time of zenith and the zenith shadow is to level the ground so that none of its sides is elevated or lowered. This is adjusted by pouring water or by setting up special balances for that purpose. A circle is drawn on it, called the Indian circle, and an upright measure is placed at its center, ensuring that the distance from its top to three points on the circle's circumference is equal. This is to confirm that the measure is upright, for if its distance from three points on the circumference at three sides of the circle is equal, then the distance is equal from all directions, indicating it is upright without inclination. Its height should be a quarter of the circle's diameter, as it must be such that its shadow is shorter than half the circle's diameter to distinguish its entry and exit. The presence of the shadow in most regions is only conceivable within it. The top of its shadow in the early morning is outside the circle, but the shadow decreases until it enters the circle. A mark is placed where the shadow enters the circle's circumference. Undoubtedly, the shadow decreases to a certain extent, then increases until it reaches the circle's circumference and then exits it, which is after midday. A mark is placed where the shadow exits. The arc between the shadow's entry and exit is halved, and a straight line is drawn from the midpoint of the arc to the circle's center, extending to the other side of the circumference. This line is the midday line. When 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 time of zenith, which is the beginning of the noon prayer time. Its end is when the measure's shadow becomes twice the measure's length plus the zenith shadow. For example, if the zenith shadow is a quarter of the measure, the end of the noon prayer time is when its shadow becomes twice the measure and a quarter. Refer to: Fath Bab Al-Inaya 1: 177, and Umda Al-Ri'aya 1: 145. And God knows best.