An octant produced by Thomas Hemsley in London, circa 1820 - 1840. The octant features an ebony frame with an ivory index features a brass index bar - which has a radius of 29.2mm - and a raised centre. The index includes vernier brass clamping and tangent screws. At the top of the octant is a mirror and lens with three glass index shades - in red, green and purple/black - positioned in between.
The octant is stored in its original case (00027256).
SignificanceThe magnetic compass, the chart and the astrolabe were the first real milestones in precise ocean navigation.
All these improved during the 16th and 17th centuries and gave way to the back-staff, the quadrant, octant, and
sextant and the chronometer.The octant went out of extensive use in the19th century, replaced by the sextant as the standard navigational instrument for the measurement of altitudes.
HistoryEarly long distant voyages were made hugging the coastline, but to venture out of sight of land, navigators had to locate themselves in relation to celestial bodies such as the sun and stars. To use these bodies, mariners needed an instrument that could measure their angular height above the horizon.
The back-staff used small sliding vanes attached to two arcs to determine the height of the sun above the horizon. Although invented in the late 16th century, it was popular with mariners into the 18th century due mainly to its simplicity, and because navigators could measure the height of the sun without actually staring at it. On the down side, the back-staff could only read angles up to 65 degrees, a significant limitation that precluded its use for a noon-day sight, when the sun is often much higher in the sky. For that the navigator needed a more sophisticated instrument: the octant.
Invented in the early 18th century, and incorporating mirrors, the octant allowed a new level of accuracy. Using it, a navigator could view a reflection of the sun or star alongside the horizon and read the angle off on a precisely engraved scale. Octants were made from wood, metal and often ivory, and are beautiful examples of the instrument maker’s craft. They remained popular until displaced by the even greater accuracy of the sextant, made of metal and fitted with telescopic sights. Sextants owed much to the ingenuity of the Industrial Revolution and new machines that could create highly accurate engraved arcs. With their improved optics and micrometre adjustments, sextants were a quantum leap in navigation science, resolving the last problems associated with taking accurate sights and defining latitude.