An instrument used in navigation to measure the altitude of a celestial body. Originally it had an arc of 60° (one sixth of a circle, hence its name) but modern instruments have various angles. The sextant uses two mirrors: the horizon glass, in which only the lower half is silvered, and the index mirror, which can be rotated about an axis perpendicular to the plane of the instrument. An arm attached to the index glass sweeps round the calibrated arc, from which angles are read. The instrument is aimed at the horizon and the index mirror rotated until the celestial object can also be seen through the telescope. After careful adjustment to make the image of the celestial body just touch the horizon, the angle is read off the graduated scale.
Sextant.
A navigational instrument for measuring the altitude of celestial objects above the horizon. It consists of a graduated 60° arc (one-sixth of a circle) with a movable arm and sighting devices. An index mirror attached to the movable arm reflects the chosen object to another mirror, silvered across half its width, so that its image can be seen alongside the horizon. The object’s altitude can then be read off the scale. A telescope and dark filters are provided to improve accuracy and allow observation of the Sun. Because the objects observed are reflected, the range of angles which can be measured is 120°, double that of the graduated scale.
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