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

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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