An Etymological Dictionary of Astronomy and Astrophysics
English-French-Persian

A colorful halo centered on the zenith, appearing when the solar elevation above the horizon is not too high (< 32°).

→ circum-; → zenithal; → arc.

Of or relating to the → zenith; located at or near the zenith.

→ zenith; → -al.

1) A well used in Antiquity from bottom of which the sky could be observed during the day with a better contrast. The aperture of the well reduced the light diffused by the sky.
2) A vertical tunnel in → Paris Observatory (built in 1667), from the top roof to the underground vaults, 55m deep. This well had the purpose of observing the stars near the → zenith and measure their → parallaxes resulting from the Earth motion around Sun. A long plumb line was attached to the top of the well. Astronomers thought they could measure the stellar shifts with respect to the plumb line. The problem was, however, the lack of stability of the images, because the well acted in fact as a chimney generating turbulence. So that the zenithal well was hardly used. See also: → zenith telescope.

→ zenithal; → well.