Reflecting circle, known as Borda circle, made by Francois-Antoine Jecker

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The reflecting circle was used to measure angles necessary for finding longitude. It was originally devised in the 1750s by the German astronomer Tobias Mayer (1723-62). A number of improvements to the instrument were published by the Chevalier de Borda (1733-99) and gave it its alternative name, 'Borda circle.' This brass and silver example was made around 1810 in the Parisian workshop of scientific instrument maker Francois-Antoine Jecker.

The telescope, horizon glass and shades are all on one arm, with a separately pivoted index arm with mirror. A series of observations are made, and the mean value found from them. By using the mirror, the time taken to gain a correct reading is less than with a conventional repeating circle.

The reflecting circle was based on the same principle as the sextant, but the arc is taken to the full circle, and readings can be arranged to remove instrumental error. It was a much more popular instrument with the navies of France and Germany than with British seamen.

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• Online ID: 000-180-000-901-C
• Image Rights Holder: National Museums Scotland
• Project: National Museums Scotland Part 2
  Project description   View all records in project
• Ref: National Museums Scotland  T.1987.149
• Date: Around 1810
  c. 1800
  
• Material: Brass, silver. Inscription: No 41 Jecker a Paris
  Reflecting circle
  
• Dimensions: 310 mm D x 120 mm
  
• What: Reflecting circle
  
• Subject: 9. NAVIGATION (Departmental Classification)
  
• Who: Borda
  Jecker, Paris (Maker)
  The Leith Nautical College Collection
  
• Where: France, Paris
  
• Event:
• Description: Borda-type reflecting circle in brass, by Jecker of Paris, c. 1800
  
• References:
  • Daumas, M., Scientific Instruments of the 17th and 18th Centuries and their Makers. London: 1972, pp 203, 279-80, 320-1, 336 
  • For the 'Borda circle', see A.J. Turner. From Pleasure and Profit to Science and Security: Etienne Lenoir and the Transformation of Precision Instrument-Making in France 1760-1830. Cambridge, 1989, pp 51-60 
  • See article 'Circle', ch. 1 'On the Reflecting Circle', in the Edinburgh Encyclopaedia, edited by David Brewster, Edinburgh, 1830, vol VI, pp 486-492. 
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