Sextant (astronomical)

Sextants for astronomical observations were devices depicting a sixth of a circle, used primarily for measuring the positions of stars. They are of significant historical importance, but have been replaced over time by transit telescopes, astrometry techniques, and satellites such as Hipparcos. Sextants for astronomical observations were devices depicting a sixth of a circle, used primarily for measuring the positions of stars. They are of significant historical importance, but have been replaced over time by transit telescopes, astrometry techniques, and satellites such as Hipparcos. There are two types of astronomical sextants, mural instruments and frame-based instruments. The first known mural sextant was constructed in Ray, Iran, by Abu-Mahmud al-Khujandi in 994. To measure the obliquity of the ecliptic, al-Khujandī invented a device that he called al-Fakhri sextant (al-suds al Fakhrī), a reference to his patron, Buwayhid ruler, Fakhr al Dawla (976–997). This instrument was a sixty-degree arc on a wall aligned along a meridian arc (north-south line). Al Khujandi's instrument was larger than previous instruments; it had a radius of about twenty meters. The main improvement incorporated in al-Fakhri sextants over earlier instruments was bringing the precision of reading to seconds while older instruments could only be read in degrees and minutes. This was confirmed by al-Birūni, al-Marrākushī and al-Kāshī. Al-Khujandī used his device to measure the sun's angle above the horizon at the summer and winter solstices; these two measurements allow computation of the latitude of the sextant's location and the obliquity of the ecliptic. Ulugh Beg constructed a Fakhri Sextant that had a radius of 40.4 meters, the largest instrument of its type in the 15th century. Housed in the Ulugh Beg Observatory, the sextant had a finely constructed arc with a staircase on either side to provide access for the assistants who performed the measurements.