sextant

The geometry of the sextant. When the light from a star can be seen at the same time as light from the horizon, the angle A can be read from the position of the index arm on the angle scale.

An 18th-century sextant (West Australia Museum, Australia). The sextant is used to determine the observer's position on the Earth's surface by measuring angles between the horizon and celestial bodies. The sextant has been a basic navigational tool for more than two centuries.

Navigational instrument for determining latitude by measuring the angle between some heavenly body and the horizon. It was invented in 1730 by John Hadley (1682-1744) and can be used only in clear weather.

When the horizon is viewed through the right-hand side horizon glass, which is partly clear and partly mirrored, the light from a star can be seen at the same time in the mirrored left-hand side by adjusting an index mirror. The angle of the star to the horizon can then be read on a calibrated scale.