a telescope mounted to point only at the zenith, used at some observatories for measuring time by the stars.
Origin of zenith tube
First recorded in 1830–40
Also called zenith telescope.
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British Dictionary definitions for zenith telescope
an instrument used to determine the latitude of stars, similar to the meridian circle but fitted with an extremely sensitive level and a declination micrometer
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012