Item History: The era of modern navigational instruments was ushered in by the concurrent invention of a doubly reflecting instrument by Thomas Godfrey (1704-1749) of Philadelphia and John Hadley (1782-1744) of London in the early 1730s. Originally this instrument was referred to as "Hadley's quadrant," after the English inventor. These days it is known as an octant, the name given it by Godfrey.
The instrument proposed by Hadley and Godfrey resembled the quadrant in shape but contained an arc of only forty-five degrees, one eighth of a circle. In addition, it had two-mirrors which reflected the light of the sun or a star and enabled the observer to measure altitudes (the angle between the sun, the moon or a star or planet above the horizon) up to ninety degrees. This ability to measure large angles made the instrument especially useful for celestial navigation at sea.
Unlike the cross-staff and backstaff the octants compact size and use of mirrors reduced errors caused by handling and thereby increased the accuracy of the observation. The octant survived through the end of the eighteenth century but it was eventually replaced by the sextant which could measure larger angles, up to 120 degrees.
This octant, like many others, is made of ebony, a material used because of its resistance to dimensional change within the wide range of temperatures and humidity that instruments are exposed to at sea. Mariners used octants like this to find latitude at sea. Bone or ivory was typically used for the inlaid scale because it was easy to engrave, easy to read and resistant to corrosion in the salty sea air.