Photozenith tube

Photozenith tube

Since 1955 Greenwich Observatory near London has employed the photozenith tube (PZT) for checking its standard clock against the stars. Selected stars which transit near the zenith are automatically photographed on the same plate twice before and twice after crossing the meridian, at times dictated by the standard sidereal clock. The time of transit according to this clock can then be found by interpolation.

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Industry/Domain: Chronometry
Category: Clock
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