zenith-tubehttp://www.oed.com/dictionary/zenith-tube_n%3Ftab%3Dmeaning_and_usezenith tube, n.
Revised 2018

zenith tubenoun

Now rare.
  1. 1825–
    A form of zenith telescope; (also) a fixed open tube aligned to the zenith, for projecting the image of the sun when overhead.
    1. 1825
      The first zenith tube was I believe constructed for Dr. Tiarks.
      Philosophical Transactions (Royal Society) vol. 115 174
    2. 1911
      The series of observations with the Greenwich Reflex Zenith-Tube, starting in 1856 and continued to 1899.
      Monthly Notices Royal Astron. Society vol. 71 541
    3. 2005
      The first appearance of the noonday sun in the Xochicalco zenith tube occurs exactly fifty-two days before the June solstice.
      C. Ruggles, Ancient Astronomy 188

Originally published as part of the entry for zenith, n. & adj.

zenith, n. & adj. was revised in June 2018.