Zenith sector

This zenith sector telescope was constructed by George Graham in 1727 for Bradley's personal use in studying the parallax of the star Gamma Draconis following work he had begun on the detection of parallax at Kew with Samuel Molyneux. When he was appointed third Astronomer Royal in 1742, Bradley only agreed to bring this instrument to Greenwich after the Government paid him the princely sum of Õú45 for it. With it he discovered two major phenomena: the aberration of light and the nutation (wobbling) of the Earth's axis. It was used at Greenwich until 1837. It was then used at the Observatory at the Cape of Good Hope between 1837 and 1839.

A zenith sector is a telescope that points straight up, to the zenith. This means that it can only see the stars directly above and are usually designed with the observation of a particular star in mind. This telescope for example was designed to allow observers to study the position of the star Gamma Draconis (the third brightest star in the constellation Draco) very accurately over the course of the year. A major reason for doing this is to allow astronomers to account for the error introduced by atmospheric refraction. At any angle besides straight up the atmosphere will refract the light coming in from a far away star making it appear in a slightly different location to its actual position. The amount the atmosphere refracts the light will vary depending on the weather. If observations are compared with those made with the zenith sector however, this error can be accounted for.
George Graham