HIGH-ACCURATE TIMING OF GEOMETRIC OCCULTATIONS OF
               STARS BY THE MOON IN ANALYSIS OF PHOTOELECTRIC
                            DIFFRACTION CURVES.

			   EUGENE TRUNKOVSKY

       Sternberg Astronomical Institute, Moscow State University
	  Universitetskii prospect 13, Moscow 119899, Russia.
                    e-mail:  TEM@SAI.MSU.SU

			       ABSTRACT

	It is well-known that high-accurate recording of the times of
occultations of stars by the Moon provide a valuable information on the
lunar motion, relief of the lunar surface, accurate coordinates of the
observation locations on the Earth, and accurate coordinates of stars.
	When we record a diffraction curve resulting from the star's
occultation by the Moon, we have a possibility to determine the moment
of geometric occultation of the stellar disk centre with an accuracy of
the order of 1 msec. For this purpose it should be taken into account
that the moment of occultation of a point source corresponds to a point
on the diffraction curve where light flux is equal to L2 +0.25*(L1-L2),
L1 and L2 being flux levels before and after occultation, respectively.
However, if the angular size of a star is non-negligible then the
moment of geometric occultation of the center of the stellar disk would
be shifted towards larger flux values. This moment can be found as one
of the parameters when an occultation trace is reduced by fitting an
optimum diffraction model. A good accuracy can be reached in the
determining of the occultation moment if the cessation time of data
recording in the computer is determined with an accuracy not worse than
1 msec when radio signals giving the exact time are used.
	In processing of a number of photoelectric occultation curves
the high-accurate values of these times were obtained.