Mariner's quadrant

A polished brass quarter circle with two sights on the right-hand edge. It has a pair of sights, a pinhole sight and a cross-hair sight. The shadow square is used for taking altitudes in surveying. The back of the instrument is blank. Engraved scale along the circular edge of the instrument, from 0° to 90° by 30 arcminutess; the shadow square runs from 0° to 50° to 0° by 1 arcminute. This can be used for making basic surveying measurements. Each scale has a decorative border of overlapping leaves. The instrument has a fitted mahogany box and the sights and plumb bob are probably later replacements.

The mariner's quadrant is a simple instrument that enabled navigators to determine their latitude (position north-south). To use the instrument, the sun or the pole star is observed through the sights and point where the line with the plumb bob crosses the curved scale indicates the altitude of the celestial body above the horizon in degrees. The instrument was developed from the astronomical quadrant, which was known to ancient Greek astronomers and was used in navigation by the mid-15th century.