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A photographic zenith tube (PZT) is a type of telescope permanently mounted so as to aim at the zenith. They are/were used for precise measurement of time (particularly differences in celestial time versus time standards) and of minute variations in Earth motion (such as the Chandler wobble). Additional details of their design provide the necessary high precision. They can be used to very-precisely determine latitude. They were state-of-the-art in the mid-20th century and still offer usable data. The USNO formerly operated one.