The sextant was and still is a navigational instrument. It evolved from the ancient octant (eigth part of the circumference of a circle) and is used to measure the angular difference between the horizon and the sun. To take a sight, the sun's image is reflected by a moveable mirror. The mirror is rotated, until the sun's image touches the horizon, which is observed directly. The angle can be retrieved from a scale engraved on the moveable arm of the sextant.
Additional optical filters (grey or coloured) are used to avoid damage to the eye by the intense light of the sun and also to reduce disturbances of the reflected image caused by e.g. clouds. One can determine his position on Earth's surface with the aid of navigational charts and a little mathematical aptitude. The sextant in our HistoLab is an original drum gauge naval sextant from the early 20th century.
Jakobsen, Ivan Tafteberg & Matthiasen, Jesper (2013): Geomat.dk: Historical Instruments of Navigation – used as Educational Tools in Mathematics, in: P. Heering, Klassen, S. & Metz, D. (eds.): Enabling Scientific Understanding through Historical Instruments and Experiments in Formal and Non-Formal Learning Environments. Flensburg: Flensburg University Press, 127-142.