Octants are navigational instruments that use a small mirror to bring two images together, i.e. of the sun and the horizon. For example, an octant can be used to determine the latitude at sea by observing the altitude of celestial bodies. An octant has an arc of 450 or more, which can measure angles of 900 or more. Sextant, a modification of octant is still an essential navigation instrument.

Beautiful and unique, octants are solid brass and hardwood reproduction of an early 1800's measuring instruments. The meaning of the term octant is 1/8 of a circle, or 45 degrees. Earlier octants have mahogany frames and boxwood scales that are read by diagonals. Octants made after around 1800 have brass index arms, ebony frames and ivory scales read by verniers. Due to the arrangements of the mirrors, octants can measure twice the angle of the scale. Although a reading of 90 degrees was possible for octants, 80 degrees was considered a practical limit.

Replicas of the original, the octants manufactured today are generally in perfect working condition. They are usually made of brass and can give accurate measurements. They make wonderful decorative for studies and libraries.