means A navigational instrument that measures the angle between a celestial body and the horizon, letting sailors calculate their latitude and position at sea.

from From the Latin 'sextans,' meaning a sixth part — because the instrument's arc spans 60 degrees, one-sixth of a full circle. It's a cousin of the older 'octant' (an eighth, 45 degrees) and shares its bones with 'sextet' and 'sextuplet.' The name simply describes the geometry: a wedge of brass cut to one-sixth of the heavens.