Images(2)

Octant case

Maker Thomas Hemsley

Date1820 - 1840

Object number00027256

NameCase

MediumMahogany, metal

DimensionsOverall: 322 x 325 x 98 mm, 1.15 kg

ClassificationsTools and equipment

Credit LineANMM Collection Gift from Charles Badham

Terms

London

DescriptionA mahogany octant case produced by Thomas Hemsley, circa 1820 - 1840. It was used to hold the octant 00027255. The stepped lid was specially designed to accommodate the index mirror of the octant and is fabricated from six pieces of mahogany that have also been nailed together. The exterior of the case is lacquered and the wooden interior remains raw and untreated. A label attched to the inside of the box reads 'Thomas [Hemsley] 11 King St. Tower Hill, London. Inventor of the Improved Storm & Steerage Compass, Manufacturer of telescopes, sextants, quadrants, and all other nautical instruments...'. 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.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.