Large Regency period English vernier octant
by Joseph Gaitskill, London

The name 'octant' derives from the Latin 'octans' meaning eighth part of a circle, referring to the angle of 45 degrees between the arms of the frame. The scale is divided into 90 degrees, or one fourth of a circle. John Hadley (1682-1744) described an instrument of this sort to the Royal Society of London in 1731 and obtained a British patent in 1734. Accordingly, octants are sometimes known as Hadley quadrants.