Borda

Reflecting Circle

1807 ca. - Giuseppe Stefani, Padova / Brass with wooden box / Diameter: 10 inches (26 cm).

Reflecting circles were a development of the octant and sextant, and were used to measure any angular distance.

This circle has a brass frame and a diameter of 10 inches (about 26 cm). Two movable arms, called ‘alidades’, run along its limb. The first alidade, embracing the whole circle, carries the telescope and the smaller mirror; the second one pivots on the centre of the instrument and carries the larger mirror. It is equipped with an ebony handle, screwing under the frame, four different coloured and transparent filters for the larger mirror and five other filters for the smaller mirror. On the limb is engraved: “GIUSEPPE STEFANI FECE”.

The circle was made by Observatory’s mechanic, Giuseppe Stefani, as a copy of the prototype by the French inventor Jean-Charles Borda. It was purchaed by the Observatory in 1807.