SEXTANT

A sextant is a measuring instrument generally used to measure the angle of elevation of a celestial object above the horizon. Making this measurement is known as sighting the object, shooting the object, or taking a sight. The angle, and the time when it was measured, can be used to calculate a position line on a nautical chart. A common use of the sextant is to sight the sun at noon to find latitude. Held horizontally, the sextant can be used to measure the angle between any two objects, such as between two lighthouses, which will, similarly, allow for calculation of a line of position on a chart.

The sextant allows celestial objects to be measured relative to the horizon, rather than relative to the instrument. This allows excellent precision. Unlike the backstaff, the sextant allows direct observations of stars. This permits the use of the sextant at night when a backstaff is difficult to use. For solar observations, filters allow direct observation of the sun.
Since the measurement is relative to the horizon, the measuring pointer is a beam of light that reaches to the horizon. The measurement is limited by the angular accuracy of the instrument and not the sine error of the length of an alidade, as it is in a mariner's astrolabe or similar older instrument.

The horizon and celestial object remain steady when viewed through a sextant, even when the user is on a moving ship. This occurs because the sextant views the unmoving horizon directly, and views the celestial object through two opposed mirrors that subtract the motion of the sextant from the reflection.
The sextant is not dependent upon electricity (unlike many forms of modern navigation) or anything human-controlled (like GPS satellites). For these reasons, it is considered an practical back-up navigation tool for ships. a full circle (60°), hence the sextant's name