The phenomenon "black drop" and reduction of
            the Mercury transits observations.

                     A.M.Sveshnikov
             (St.Petersburg State University)
                     M.L.Sveshnikov
         (Institut of Theoretical Astronomy, RAS)


    The mechanism of image formation of Mercury  is discussed  for visual
observations of contacts, and, first of all, the phenomenon of the "black
drop"  (dark bar  between  the solar  and mercurian limbs) is  described.
This effect had been observed  before  the internal contacts of the solar
and Mercury disks, and it leads to the displacement of the apparent limbs
contacts moments  with regard to  the true (geometrical)  limbs  contacts
moments.  The appearance  of  the black  drop  has been  investigated  by
analytically  and numerical  simulation under  following conditions:  the
solar limb  darkening,  image blurring  and  contrasting function. It was
shown the black drop and the dark filament have the common origin, namely,
the bad conditions of observations. The numerical values of the corrections,
which  could be used  for reducing  apparent contacts to geometrical ones,
were obtained. Application of reducing relation for some sets of transits
(1878,  1953 and others years)  had been shown  that observed moments  of
contacts with drop may be improved by it significantly.