Theodolite by Ertel,
precision instrument built in 1855 by Ertel & Sohn, München (Inv ID 065)
This instrument of superb quality was part of the second series of precision instruments ordered by Adolphe Quetelet in the period 1853-1857. Quetelet was the first director of the Observatory of Brussels, which became the Royal Observatory of Belgium in 1891.
It was used more for geodetic than for astronomical observations: especially to obtain the relative position of one location relative to other locations. The distance between the observation points could be several kilometres. The students of the Royal Military Academy, who did their internship at the Observatory, practiced with this instrument under supervision of Quetelet, who was also professor at that same school.
The rifle scope has an ocular with a prism and a wire micrometre. The objective has a diameter of 40cm and a focal length of 50cm. The azimuthal circle has a diameter of 35cm. The silver edge is divided every three arcminutes. Four vernier scales make it possible to read the value with an accuracy of 2 arcseconds. Two micrometric microscopes make it possible to read up to the arcsecond. The vertical circles have scales up to 3 minutes on the silver edge. A vernier scale makes it possible to assure the accuracy up to an arcminute. Two micrometric microscopes ensure up to an arcsecond.
Descendants of the theodolites, the GNSS (Global Navigation Satellite Systems) receivers are not as elegant, but easier to use and also more efficient than their ancestors! These small devices pick up the signal of GNSS satellites (such as GPS, GLONASS and Galileo) allowing obtaining a position with centimetre accuracy.
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