Zenith sector telescopes. Diagram of three zenith sectors, which are telescopes used to make accurate observations of the position of an object in the sky, such as a star or the Sun. The telescope is pivoted on a scale and can move in either direction (following the meridian). This allows the position of an object to be measured relative to the zenith (which is directly overhead). These zenith sectors were made by leading instrument makers in the early 19th century: (from left to right) Jesse Ramsden, George Graham and Edward Troughton.