"Hadley’s quadrant"
by cm
Hadley s Quadrant
Public DomainHadley's Quadrant
A device to measure the angular distance between two celestial objects, the ancestor of the modern sextant was also known as the octant or reflecting quadrant.   It incorporated mirrors, shades to allow observation of bright objects, a graduated scale and a removable telescope.   The device was one eighth of a circle (hence “octant”) but the mirrors doubled the angle covered to a quarter circle.   Crucially, the observer could see both objects being viewed at the same time (one as a reflection), reducing error.   Octants were mass produced in wood and ivory.  

The octant could only measure angles of 90º or less, so was eventually replaced by the sextant, which is more accurate and measures up to 120º.   Sextants are still considered an important back-up to satellite and radio navigation systems.

Sextant animation by Joaquim Alves Gaspar, Wikimedia