Instruments · a Light

The largest instrument yet built sank under its own weight

Around 994, near Rayy in northern Iran, al-Khujandi built the Fakhri sextant, named after the ruler who paid for it. It was an arc of sixty degrees set in a north to south trench and roofed over, its radius long enough that a single degree on the scale could be divided into seconds, which no smaller instrument allowed. A small aperture let a disc of sunlight fall on the scale at noon. As far as is known it was the largest astronomical instrument built up to that time. His measurement of the tilt of the earth's axis came out about two minutes of arc short. Al-Biruni, who examined it and wrote about it, concluded that the aperture had sunk as the masonry settled.

Al-Khujandi · Rayy, c. 994 CE