zenith-linehttp://www.oed.com/dictionary/zenith-line_n%3Ftab%3Dmeaning_and_usezenith line, n.
Revised 2018

zenith linenoun

Now rare.
  1. 1585–
    a. An arc from the zenith to the horizon, which it intersects at right angles (= azimuth n. 1), esp. to the point due east on the horizon; b. a straight vertical line between the zenith point at a given location and the centre of the earth.
    1. 1585
      The 15. azimuth reckoned from the East or zenith line.
      J. Blagrave, Math. Iewel 39
    2. a1652
      How to draw and divide the Ellipsis upon any Plain, to an Index that stands upright..to the Zenith line of the place, or perpendicular to the plain of the Horizon.
      S. Foster, Elliptical Horologiography (1654) 69 (heading)
    3. 1778
      The imaginary Lines then, perpendicular to the Zenith Line from the different Elevations..are to be understood as the respective true Horizons.
      N. D. Falck, Ready Observator (ed. 2) 80
    4. 1871
      The white streamers [of the aurora] extended almost to the eastern horizon, and some were to the south of the zenith line.
      Exchange & Mart 19 April 480/1
    5. 1987
      Your latitude is the angle formed at the Earth's centre between the equator and your zenith-line.
      J. Raban in Granta No. 10. 29

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

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