Zenith telescope

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A zenith telescope is an instrument adapted for the measurement of small differences of zenith distance, and used in the determination of astronomic latitude. Zenith telescopes are often portable; however the Monument to the Great Fire of London is a notable example of a large, non-portable zenith telescope.

High-precision (and fixed building) zenith telescopes were also used until about early 1980s to track Earth's north pole position e.g. Earth's rotation axis position. Since then radio astronomical quasar measurements have also measured Earth's rotation axis several orders of magnitude more accurately than optical tracking.

Another interesting modern variation of the theme is Large Zenith Telescope.