Borda circle
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The repeating circle , also Repetitionskreis was from Paris mathematician Jean Charles de Borda developed (1733-1799) and is an instrument for measuring of angular degrees . With two telescopes that can be moved against each other , two targets can be sighted at the same time and the angle between them read off as minutes of arc .
history
The instrument is a further development of Tobias Mayer's repeater circle from 1750. The brass instruments were made by the instrument maker Étienne Lenoir (1744–1832). The measuring circle was used for the triangulation of the meridian arc by Pierre Méchain and Jean-Baptiste Joseph Delambre (from 1792 to 1799). The measurement accuracy is approximately one arc minute.
description
The telescopes of the Borda circle are arranged one above the other on the plane of the partial circle. One of the telescopes is firmly connected below the pitch circle, the other moves during the measurement with the reading points that can be rotated coaxially above the pitch circle. The measurement of horizontal angles is possible without any problems when the Borda circle is leveled.
The measurement of vertical angles with horizontal tilting axis is not possible directly, since the telescopes for sighting two points cannot both be tilted at different elevation angles.
In triangulations with target ranges of over 20 km, sighting is usually possible due to the low elevation.
An original copy is in the possession of a museum in Paris.
literature
- J. Loridan: Voyages des Astronomes français à la recherche de la figure de la terre et de ses dimensions. Verlag Desclée, France, 1890, 330 pp. (Reprint 2010, Verlag Kessinger Publishing Company. ISBN 978-1-160-93874-7 ).
- Ken Alder : The measure of the world. The search for the original meter. Bertelsmann, Munich 2003, ISBN 3-570-00545-3 (also: Goldmann, Munich 2005, ISBN 3-442-15335-2 ).
- Maurice Daumas: Scientific Instruments of the Seventeenth and Eighteenth Centuries and Their Makers. Portman Books, London 1989, ISBN 0-7134-0727-1 (English).
Web links
- Triangulation. ingenieurgeograph.de
- 1.3 Borda circles. In: Instruments for field use . (PDF) history-of-geodesy.ch
Individual evidence
- ^ Heinz Haferkorn: Lexicon of optics . Verner Dausien, Hanau 1990, ISBN 3-7684-6858-5