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Report on Astronomical Constants

Recent progress in the determinations of astronomical constants is reviewed. First is the latest estimation of the general relativistic scale constants, L C , L G , and L B (Irwin and Fukushima, 1999). By reestimating the uncertainty, the value of the first constant is given as L C = 1.480 826 867 4 × 10 –8 ± 1.4 × 10 –17 . Also noted is the rigorous relation among these three, L B = L C + L G – L C L G . Based on the latest determination of the geoidal potential W 0 in the IAG 1999 Best Estimate of Geodetic Parmeters (Groten, 1999), L G and L B were reevaluated as L G = 6.969 290 09 × 10 –10 ± 6 × 10 –18 and L B = 1.550 519 767 3 × 10 –8 ± 2.0 × 10 –17 . Since L G is roughly related to W 0 , a proposal to fix its numerical value is presented in order to remove the geophysical ambiguity in its evaluation in the future. In that case, L G becomes a defining constant for the scale difference between the geocentric and terrestrial coordinate systems. While L C and L B remain as a primary and derived constant, respectively. Next is the correction to the current precession constant, Δ p . The recent estimates of Δ p based on Very Long Baseline Interferometry (VLBI) observation seem to converge to a value close to –0.30″/cy (Mathews et al. , 2000; Petrov, 2000; Shirai and Fukushima, 2000; Vondrak and Ron, 2000). Unfortunately this is significantly different from –0.34″/cy, the latest value determined from the Lunar Laser Ranging (LLR) data (Chapront et al. , 1999). The difference is roughly ten times larger than the sum of their formal uncertainties. Since the cause of this difference is not clear, we first arranged the best estimates based on VLBI and LLR techniques, respectively, then took a simple mean of these two best estimates, and recommend it as the current best estimate. The value derived is p = 5 028.78 ± 0.03 ″/cy. Similar estimates were given for some other quantities related to the precession formula; namely the correction to the obliquity rate of the IAU 1976 precession formula (Lieske et al. , 1977), Δ e 1 = (–0.024 5 ± 0.002 5) ″/cy, and the offsets of the Celestial Ephemeris Pole of the International Celestial Reference System, Δ ψ 0 sin e 0 = (–17.5 ± 0.8) mas and Δ e 0 = (–5.2 ± 0.4) mas. As a result, the obliquity of the ecliptic at the epoch J2000.0 was estimated as e 0 = 23°26′21.″405 6 ± 0.″000 5. The draft IAU 2000 File of Current Best Estimates of astronomical constants, that is to replace the 1994 version (Standish, 1995) or maybe even the formal IAU 1976 System of Astronomical Constants (Duncombe et al. , 1977), after discussion at the 24th General Assembly of the IAU is presented.