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 asthe object's
altitude. Using 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. Common uses of the sextant include sighting the
sun at
solar noon and sighting
Polaris at night (in the Northern Hemisphere), to find one's
latitude. Sighting the height of a landmark can give a measure of
distance off and, held horizontally, a sextant can measure angles between objects for a
position on a chart.
[1] A sextant can also be used to measure the
lunar distance between the moon and another celestial object (e.g., star, planet) in order to determine
Greenwich time which is important because it can then be used to determine the
longitude.
The scale of a
sextant has a length of
⅙ of a
turn (60°); hence the sextant's name (
sextāns, -antis is the
Latin word for "one sixth"). An
octant is a similar device with a shorter scale (
⅛ turn, or 45°), where as a
quintant (
⅕ turn, or 72°) and a
quadrant (
¼ turn, or 90°) have longer scales.
