Qibla observation by shadows
Twice every year, the Sun culminates at the zenith of the Kaaba in Mecca, the holiest site in Islam, at local solar noon, allowing the qibla to be ascertained in other parts of the world by observing the shadows cast by vertical objects. This phenomenon occurs at 12:18 Saudi Arabia Standard Time on 27 or 28 May, and at 12:27 SAST on 15 or 16 July. At these times, the Sun appears in the direction of Mecca, and shadows cast by vertical objects determine the qibla. At two other moments in the year, the Sun passes through the nadir of the Kaaba, casting shadows that point in the opposite direction, and thus also determine the qibla. These occur on 12, 13, or 14 January at 00:30 SAST, and 28 or 29 November at 00:09 SAST.
The qibla determines the direction, from any point in the world to the Kaaba, in the Great Mosque of Mecca.
As a result of the Earth's orbit around the Sun, the Sun's latitude position appears to move seasonally between the Tropic of Cancer and the Tropic of Capricorn.
At the time of rasd al-qibla, the shadow of a vertical object indicates the direction of qibla.
Nasir al-Din al-Tusi (1201—1276), an early astronomer who wrote about qibla observation by shadows.
The zenith is an imaginary point directly "above" a particular location, on the celestial sphere. "Above" means in the vertical direction opposite to the gravity direction at that location (nadir). The zenith is the "highest" point on the celestial sphere.
The shadows of trees are the shortest on Earth when the Sun is directly overhead (at the zenith). This happens only at solar noon on certain days in the tropics, where the trees' latitude and the Sun's declination are equal.