© Science Museum / Science & Society Picture Library
A sextant is an optical instrument for measuring the angular distance between two objects, used in navigation for calculating latitude and longitude. The observer can simultaneously view the horizon (through a telescope) and the Sun (through a mirror attached to an arm on a graduated arc). The mirror can be moved over the arc so that the two images line up, allowing the Sun's altitude to be read off the scale. With the use of vernier angles, this sextant (which has an ebony frame and an ivory scale) can be read to 15 seconds of arc. It was made by Casella of London.