COMMISSION 27 OF THE I.A.U.
                 INFORMATION BULLETIN ON VARIABLE STARS
                              Number 2983

                                                         Konkoly Observatory
                                                         Budapest
                                                         11 February 1987

                                                         HU ISSN 0374 - 0676


      ADOPTION OF THE BARYCENTER OF THE SOLAR SYSTEM AS THE
  REFERENCE POINT FOR THE TIMING OF VARIABLE STAR OBSERVATIONS.

A number of considerations lead to a strong recommendation
that observers of variable stars express times in Barycentric
Times:
1. The advent of radio time signals giving Coordinated Universal
   Time, and the use of microcomputers for timekeeping, makes
   it possible to determine current Terrestrial Dynamic Time (TDT)
   with considerable precision.
2. Heliocentric times referred to the center of the Sun are
   subject to a periodic component caused by the motion of the
   Sun around the Barycenter. The principal term has a period of
   about 12 years (Jupiter) and causes a variation of about
   +/-0.00004 day.

The recent changes in the format of the Astronomical Almanac
and the development of the Floppy Almanac permit convenient
calculation of the Barycentric Equatorial Rectangular Coordinates 
of the Earth, J2000, for any TDT.

   An algorithm for determining the light time correction to
the TDT of an observation follows:

                       [FIGURE 1]

      r_1 = unit vector in the direction of a star.
      R   = radius vector from the Barycenter to the center
            of the Earth.
      r   = R * r_1 = R cos ( R, r_1).
          = X cos delta cos alpha + Y cos delta sin alpha + Z sin delta .
Light Time Correction = r x 499.004782 seconds
X,Y,Z = Barycentric equatorial rectangular coordinates
       of the Earth, J2000, for time TDB (differs
       from TDT by no more than 2 milliseconds).
alpha, delta = Mean Right Ascension and Declination of the
              star, Epoch J2000.
499.004782 = I.A.U. primary constant; light time for a unit
             distance in the Solar System.

   The light time correction is added algebraically to TDT to
get Barycentric Time. The maximum monthly variation of this
correction, caused by the motion of the center of the Earth
around the center of mass of the Earth-Moon system, is about
+/- 15 milliseconds. The maximum possible daily variation,
caused by the rotation of the Earth, is about 21 milliseconds.

  If adopted, this convention should remove the principal
timing error caused by the observer's motion and eventually
increase the accuracy of period variation studies with a
long time base.


  WILLIAM BLITZSTEIN

  Dept. of Astronomy and Astrophysics
  University of Pennsylvania
  Philadelphia, PA 19104-6394



Editors' note: Dr. P. Ahnert (Sonneberg Observatory, G.D.R.) published
a Table in "Mitteilungen über Veranderlichen Sterne, Suppl. 1. (1961)"
for the barycentric light time corrections for the years 1850 - 2051 taking
into account the effect of the five outer planets.