Sextant

The sextant has a polished brass straight-bar pattern pillar frame, with twenty-four pillars, and a wooden handle with a brass-lined threaded hole for a second handle or a stand. The tangent screw and clamping screw are located on the back of the index arm. The lower part of the index glass is silvered, and can be covered by the Maskelyne flap, whereas the upper part is blackened. The sextant has three shades, two red and one green, and three horizon shades, two red and one green. Index-glass adjustment is made by a screw and on the horizon glass by screws and a thumbscrew. Attached to the sextant is a threaded telescope bracket in two parts, fitted for correcting collimation error. It has perpendicular adjustment by a rising-piece and a milled knob. The telescope is 184 mm in length with an inverted image, two parallel cross wires, and a shaded red eyepiece. The sight-tube is 117 mm in length and has a red shaded eyepiece. An extra drawtube, magnifying glass and an unidentified part are missing. The sextant is contained in a mahogany keystone box.

The instrument has a polished brass scale from -5° to 153° by 20 arcminutes, measuring to 126°. The sextant has a brass vernier measuring to 30 arcseconds, with zero at the right.

Edward Troughton patented the pillar and plate frame (no. 1644 of 1788).

Object Details

ID: NAV1226
Collection: Astronomical and navigational instruments
Type: Sextant
Display location: Not on display
Creator: Troughton
Date made: circa 1790
Credit: National Maritime Museum, Greenwich, London
Measurements: Overall: 120 x 350 x 350 mm; Radius: 254 mm
Parts: Sextant
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