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.