چاه ِ سرسویی
Fr.: puits 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 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.