Title: Sextant, John Sewill, Liverpool, 1871
Accession #: NHHC 1967-415-AA
Size: 13 x 14 x 6"
Medium: Metal, Wood, Glass
Location: Headquarters Artifact Collection, Naval History and Heritage Command
The sextant is comprised of a telescope mounted to a wedge-shaped frame with a scale along the arc of the frame. A series of mirrors mounted to the frame allows the user to take measurements of the angle between two objects. The copper alloy frame of this sextant is painted black. The arc is polished brass. The arc scale measures degrees from 0 to 130. Below the scale, the arc is marked // J. Sewill 61 South Castle St. Liverpool & 30 Cornhill London / Maker to the Admiralty // Next to the maker’s information is an engraving // U.S. Navy 12451 //
The sextant is housed in a wedge-shaped wood case with a hinged lid. There is a metal plate on the exterior of the lid engraved // Thomas Strong / 1871 // The interior of the case is lined in blue velvet.
The basic design of the sextant has been unchanged since they first came into use in the early 18th century. The name is derived from the shape- the arc is one sixth of a circle. Aboard ship, they are used to measure the angle between an astronomical object and the horizon. The measured angle and the time the measurement was taken can be used to calculate a ship’s position on a nautical chart.