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Reflecting Circle

The reflecting circle–essentially an octant extended to a full circle–was designed for the purpose of determining longitude by measuring the distance between the moon and a nearby star (the so-called lunar distance method). Johann Tobias Mayer, the German astronomer who introduced the form, also developed the tables of the moon’s motions that were needed for longitude determinations. Tests of Mayer’s instrument in England in the 1750s led to the invention of the sextant. The French physicist Jean-Charles Borda described an improved reflecting circle in 1787, and instruments of this type remained in use into the nineteenth century. They were used for latitude as well as longitude, and on land as well as at sea.

Ref: J. A. Bennett, The Divided Circle. A History of Instruments for Astronomy, Navigation and Surveying (Oxford, 1987), pp.134-136.

Jean-Charles Borda, Description et Usage du Cercle de Réflexion (Paris, 1787).

Collection:

Dollond Reflecting Circle
Hardy Reflecting Circle
Jones Reflecting Circle
Lenoir Reflecting Circle
Troughton & Simms Reflecting Circle (#240)
Troughton Reflecting Circle