A
sextant is an instrument used to measure the angle between any two visible objects. Its primary use is to determine the angle between a celestial object
and the horizon which is known as the
altitude. Making this measurement is known as
sighting the object,
shooting the object, or
taking a sight and it is an essential part of celestial navigation
. The angle, and the time when it was measured, can be used to calculate a position line
on a nautical or aeronautical chart.