Khujandi worked under the patronage of the
Buwayhid Amirs at the observatory near
Ray, Iran. There he is known to have constructed the first huge
mural sextant in 994 AD, intended to determine the Earth's
axial tilt ("obliquity of the ecliptic") to high precision. He determined the axial tilt to be 23°32'19" for the year 994 AD. He noted that measurements by earlier astronomers had found higher values (Indians: 24°;
Ptolemy 23° 51') and thus discovered that the axial tilt is not constant but is in fact (currently) decreasing. His measurement of the
axial tilt was however about 2 minutes too small, probably due to his heavy instrument settling over the course of the observations. ==Mathematics==