چاه ِ سرسویی
Fr.: puit zénithal
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.
The problem was the lack of stability of the images so that
the zenithal well was hardly used.
See also: → zenith telescope.
→ zenithal; → well.