Noun
An instrument formerly used for taking the altitude of the heavenly bodies, but now superseded by the quadrant and sextant; -- so called because the observer turned his back to the body observed.
Source: Webster's dictionaryAdvantages Like the Davis quadrant (also called backstaff), the sextant allows celestial objects to be measured relative to the horizon, rather than relative to the instrument. Source: Internet
However, unlike the backstaff, the sextant allows direct observations of stars. Source: Internet
This permits the use of the sextant at night when a backstaff is difficult to use. Source: Internet