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This sextant was used to measure the angle of a heavenly body, such as the sun or a star, above the horizon. The maximum height (zenith) of heavenly bodies changes depending on how far from the equator the observation point is. Therefore, fishermen and whalers could use this measurement to calculate their latitude.

The sextant is very finely made of brass, with glass fittings. There are two mirrors: the index mirror is fixed to a sliding arm (the index). The horizon mirror is half mirrored and half window. There are also shade glasses for use when looking at bright objects, a 120 degree scale, a magnifier and a telescope.

The angle was measured by lining up the telescope so that the horizon could be seen through the window part of the horizon mirror. The index was moved until the reflected light of the sun or star was seen in the mirrored half. The height could then be read from the scale, using the attached magnifier.

The design for this type of sextant was patented in 1788 by Edward Troughton, a scientific instrument maker based in London. This brass example was made around 1820, probably by Troughton.

The 10-inch ‘pillar frame’ sextant has two thin frames of plate brass held together by a series of brass pillars. It has a platinum scale (which would not tarnish), an early example of the recently discovered metal being used in instrument design. The English chemist, William Hyde Wollaston (1766-1828) managed to isolate platinum chemically in 1804. Troughton, who had introduced a new method of engraving sextant scales in 1785, realised that the hardness of this metal made it ideal for scale engraving.

©SCRAN/National Museums of Scotland
Sextant, 1820