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 ebony example was made in England in 1755. It is unsigned.

The octant has backsights (for measuring angles of more than 90 degrees) and diagonal scales. There is an ivory plate inscribed with the date, 1755, and the owner's name, 'George Harris'. The name has been obliterated, probably when the octant changed hands.

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: 000-100-104-177-C
• Image Rights Holder: National Museums Scotland
• Project: National Museums Scotland
  Project description   View all records in project
• Ref: National Museums Scotland  T.1876.11
• Date: 1755
  
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• Who: George Harris ? (Maker)
  
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• Description: Octant, an instrument for measuring the angular distances of objects by reflection, dated 1755
  
• 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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