Iranian astronomical observer and instrument designer Abu-Mahmud al-Khujandi (circa 940-1000) constructed the first known mural sextant, with a radius of 66 feet (20 m), on an accurate north-south facing wall in Ray, near modern Tehran, Iran. The name "sextant" refers to the fact that the instrument had an angular scale that was 60 degrees in length, one sixth of a circle. (When measuring latitude, one minute is equal to one sixtieth of a degree.)
The instrument was designed to measure the altitude of the sun above the horizon at noon on the days of both the summer and winter solstice, the two dates in the year when this angle has its maximum and minimum value. From the average of these two angles, an observer could determine his or her latitude—the angular distance between the equator and the observation site.
The height of the sun in the sky was measured by looking at the shadow it cast on an accurate scale. The AI-Khujandi scale was so accurate that the latitude that he obtained was correct to a tiny fraction of a degree. Other famous mural sextants followed, including the Fakhri sextant with a radius of approximately 118 feet (36 m) constructed by Iranian Ulugh Beg in Samarkand, Uzbekistan, in around 1420. More modern astronomical sextants are smaller and pivoted at the balance point. They can be moved to measure the angular separations of stars and planets.
Handheld nautical sextants have become common in the last three centuries. They are fitted with adjustable mirrors and are used to measure the altitudes of celestial bodies.
