A sextant is an instrument used to measure the angle between any two visible objects.
Its primary use is to determine the angle between a celestial object and the horizon.
Knowing this angle, together with the time at which it was measured, enables the user to calculate a position on a nautical chart
This beautifully made example belonged to Lt. Edward Cecil R.N. (1834-1862), son of Brownlow, 2nd Marquess of Exeter (1795-1867), who briefly commanded the gunboat Dapper, which was part of the Baltic Fleet in the Crimean War.
