Zenith telescope

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A zenith telescope is designed to point straight up at the sky. It helps measure the positions of stars very precisely and can be easier to build. The classic type, called a zenith sector, sits on a sturdy mount that rotates and is fitted with leveling screws. It uses a very accurate level and an eyepiece with a micrometer to read tiny changes in a star’s height, which helps determine astronomical latitude. Other examples include the Monument to the Great Fire of London, which has a central shaft used as a zenith telescope. Fixed, high-precision zenith telescopes were used until the early 1980s to track the Earth's north-south axis motion (polar motion). Since then, measurements of distant quasars with VLBI have given far more accurate results. Modern zenith telescopes include the NASA Orbital Debris Observatory (3 m) and the Large Zenith Telescope (6 m); both use liquid mirrors and can only point straight up.