The Nuttall Encyclopedia
Name of Arab origin given to the point of the heaven directly overhead, being as it were the pole of the horizon, the opposite point directly under foot being called the Nadir, a word of similar origin; the imaginary line connecting the two passes through the centre of the earth.
Wood, James, ed. Entry for 'Zenith'. The Nuttall Encyclopedia. https://www.studylight.org/encyclopedias/eng/nut/z/zenith.html. Frederick Warne & Co Ltd. London. 1900.