Backstaff

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A backstaff is an instrument with two graduated arcs, formerly used in navigation. This wooden example was made in England in 1726 by Benjamin Macy, a scientific instrument maker based at Hermitage Bridge at Wapping in London between 1713 and 1731.

The backstaff works by the user sighting the horizon through the sighting vane and the slit in the push-in vane, and adjusting the upper shadow vane so that its shadow also falls on the slit. The sum of the angles on the two arcs gives the sun's altitude.

John Davis introduced the back-staff in 1594. It soon became an indispensable navigational tool, remaining popular until it was supplanted by the radically new design of the reflecting quadrant or octant in the latter half of the 18th century.

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• Online ID: 000-190-001-013-C
• Image Rights Holder: National Museums Scotland
• Project: National Museums Scotland
  Project description   View all records in project
• Ref: National Museums Scotland  T.1891.380
• Date: 1726
  
• Material: Wood. Inscription: Made by Benjamin Macy for Captain William Cochren, 1726
  Navigation instrument
  
• Dimensions: 25.25" L x 14.50" W
  
• What: Navigation instrument
  
• Subject: 3. ASTRONOMY, Quadrants (Departmental Classification)
  
• Who: Benjamin Macy, England (Maker)
  William Cochren (Owner)
  
• Where: England
  ENGLAND
  
• Event:
• Description: Wooden navigation instrument or backstaff with two graduated arcs, inscribed "Made by Benjamin Macy for Captain William Cochren, 1726", English made
  
• References:
  • For Macy, see Clifton, Gloria, Directory of British Scientific Instrument Makers 1550-1851. London: 1995, p 176 
• Translations:
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