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 £45 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.

Before Bradley became Astronomer Royal he worked with Samuel Molyneux investigating parallax, that is, attempting to use trigonometry to discover the size of the solar system by observing just one star from a fixed position to see how it moved. The atmosphere makes the light from stars bend, which makes them appear in a different position to where they actually are. This bending is greatest at the horizon, and non-existent at the zenith, straight overhead. Therefore, to make very accurate observations of a single star, for example for parallax observations, a zenith sector is the best instrument to use. This zenith sector was made by George Graham in 1727 and first used by Bradley at Wanstead, Essex, and when Bradley became Astronomer Royal in 1742 it was brought to Greenwich. It was with this instrument that, in 1729, while trying to detect parallax, Bradley instead discovered the aberration of light & nutation
George Graham

Original size: 3810 mm x 72 mm