Out of the meridian: John Bird's equatorial sector and the new technology of astronomical measurement

When Thomas Hornsby commissioned John Bird to build a set of five major astronomical instruments for the new Radcliffe Observatory, Oxford, the order included an Equatorial Sector. This was an instrument of experimental design, intended to measure angles out of the meridian. Although in itself a transitional instrument, the Equatorial Sector's capacity for extra-meridional measurement, especially for the observation of comets, encouraged craftsmen to develop the design first into the Equatorial Circle, and then into the large equatorially mounted refracting telescope used in conjunction with micrometers. Although other Equatorial Sectors were made, Bird's instrument for Oxford is the only one that survives intact. The present study examines this unique artefact within the context of changing astronomical demands between the 1770s and the 1820s.