The Collaborative International Dictionary of English v.0.48:
Zenith \Ze"nith\ (?; 277), n. [OE. senyth, OF. cenith, F.
z['e]nith, Sp. zenit, cenit, abbrev. fr. Ar. samt-urras way
of the head, vertical place; samt way, path + al the + ras
head. Cf. Azimuth.]
1. That point in the visible celestial hemisphere which is
vertical to the spectator; the point of the heavens
directly overhead; -- opposed to nadir.
To noon he fell, from noon to dewy eve,
A summer's day; and with the setting sun
Dropped from the zenith, like a falling star.
2. hence, figuratively, the point of culmination; the
greatest height; the height of success or prosperity.
I find my zenith doth depend upon
A most auspicious star. --Shak.
This dead of midnight is the noon of thought,
And wisdom mounts her zenith with the stars. --Mrs.
It was during those civil troubles . . . this
aspiring family reached the zenith. --Macaulay.
Zenith distance. (Astron.) See under Distance.
Zenith sector. (Astron.) See Sector, 3.
Zenith telescope (Geodesy), a telescope specially designed
for determining the latitude by means of any two stars
which pass the meridian about the same time, and at nearly
equal distances from the zenith, but on opposite sides of
it. It turns both on a vertical and a horizontal axis, is
provided with a graduated vertical semicircle, and a level
for setting it to a given zenith distance, and with a
micrometer for measuring the difference of the zenith
distances of the two stars.