Octant

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An octant is a navigational instrument used for measuring angles necessary for determining a ship's position at sea. This wooden example was made in England around 1750. It is unsigned.

The octant has diagonal scales and is fitted for taking backsights (angles of more than 90 degrees) as well as fore-sights (angles up to 90 degrees). There is an ivory plate inscribed 'G.E. ABS., [the initials of a later owner] 1796'.

An octant is so-called because the scale is one-eighth of a circle, although it can measure 90 degrees using the mirror on the top of the radial arm. This example has a second pinhole sight and second horizon glass, allowing measurements greater than 90 degrees.

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• Online ID: 000-100-104-273-C
• Image Rights Holder: National Museums Scotland
• Project: National Museums Scotland
  Project description   View all records in project
• Ref: National Museums Scotland  T.1981.21
• Date: Around 1750
  c. 1750
  
• Material: Mahogany, boxwood. Inscription: G T ABS 1796
  
• Dimensions:
• What:
• Subject:
• Who: G.T. Abs (Inscribed on the object)
  
• Where:
• Event:
• Description: Mahogany and boxwood 18" radius octant, inscribed 'G.E. Abs, 1796', English, c. 1750
  
• References:
  • Bennett, J.A. The Divided Circle: A history of instruments for astronomy, navigation and surveying. Oxford: 1987, pp 132-4 
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