NSFA Bulletin 1
To start off, I would like to once again thank all of you for agreeing to serve on the IAU Working Group (WG) for Numerical Standards of Fundamental Astronomy (NSFA). I am looking forward to working with you.
I have started to work on a web page for the WG. It is still in the formative stages, but I hope to be able to make it more user-friendly in the coming weeks. The web page can be found at
In case you are not familiar with the previous IAU WGs that have been formed to determine the “Current Best Estimates” (CBEs), I would like to provide a brief history on the topic. The first Sub-group on Numerical Standards of the IAU WG on Astronomical Standards was headed by M. Standish and their report (Standish, 1995) established the rules which are still in use today. For instance, they decided on the two-tiered approach to the astronomical constants that we are currently using. They also created the first CBEs for a list of IAU constants.
This work was continued by T. Fukushima and his IAU WG on Astronomical Standards (Fukushima, 2000; Fukushima, 2003). Many of their updates concerned work on constants in a general relativistic framework and improved estimates of the precession constant.
The excellent work of both these WGs has helped to establish precedent and allow us to improve incrementally the values for which there are now better estimates.
To help initiate a discussion on the how our WG should proceed, I have compiled a series of questions for discussion. I believe that these questions will help to lead the WG in a productive direction. The list is not necessarily complete and if you have any other questions that you feel are important to our work, please do not hesitate to share them with the group.
1. Which constants should be considered by the WG?
The following table is taken from the Fukushima (2003) and gives a list of the constants considered by past WGs. For a complete explanation of the table and the references, please see Fukushima (2003).
DEFINING
k1.720 209 895 10-2IAU 1976
c2.997 924 58 108 [ms-1]CODATA 1998
LG6.969 290 134 10-10Petit (2000)
PRIMARY
Lc1.480 826 867 4(14) 10-8Irwin and Fukushima (1999)
p5.028 790(5) 103 [“/cy]Fukushima (2003)
08.438 140 59(3) 104 [“]Fukushima (2003)
A4.990 047 863 9(2) 102 [s]DE405
MM/ME1.230 003 45(5) 10-2DE405
MS/MMe6.023 6(3) 106Anderson et al. (1987)
MS/MV4.085 237 1(6) 105Sjogren et al. (1990)
MS/MMa3.098 708(9) 106Null (1969)
MS/MJ1.047 348 6(8) 103Campbell and Synott (1985)
MS/MSa3.497 898(18) 103Campbell and Anderson (1989)
MS/MU2.290 298(3) 104Jacobson et al. (1992)
MS/MN1.941 224(4) 104Jacobson et al. (1991)
MS/MP1.352 1(15) 108Tholen and Buie (1997)
ae6.378 136 6(1) 106 [m]Groten (2000)
J21.082 635 9(1) 10-3Groten (2000)
GME3.986 004 415(8) 1014 [m3s-2]Fukushima (2003)
W06.263 685 60(5) 107 [m2s-2]Groten (2000)
7.292 115 0(1) 10-11 [rad s-1]Groten (2000)
G6.673(10) 10-11 [m3kg-1s-2]CODATA 1998
While this list indicates the constants that were considered by past WGs, it should not be seen as a constraint. Perhaps a better way to consider this question is ‘What constants should be added/dropped from the list above?’.
2. What values should be adopted for the IAU “Current Best Estimates”?
Once the list of constants to be considered is settled, then the CBEs for those constants need to be determined. The tentative goals of the WG stated that the values should conform, as much as possible, with IAU Resolutions, the IERS Conventions, and the SI.
The IERS Conventions maintains a list of numerical standards that very closely matches the most recent IAU CBEs. The pertinent values for these standards can be found in Chapters 1 and 3. Comparing the two lists of constants, it looks like the differences are that the Conventions includes values for LB, GMS, cA, J2S, , 1/f, ge, and R0 and that it also has different values for p, A (given in TDB units), and GME.
Which of the numerical constants in the IERS Conventions should be considered as replacements for the IAU CBEs? Are other values better than both?
3. What mechanism should be adopted for keeping the Current Best Estimates up to date?
Currently, when the timing is appropriate, the IAU forms a WG to update the CBEs. This method has worked well in the past and could be extended indefinitely into the future.
However, I would like to propose an alternative idea for consideration. The IERS is an IAU service organization and it already has a mechanism in place to maintain the CBEs for use in determination of Earth orientation and reference systems. Since the IAU list of CBEs is a subset of the IERS Conventions list and the IERS works for the IAU (to some extent), would it be reasonable to ask the IERS Conventions to maintain the IAU CBEs in the future?
4. Should the IAU System of Astronomical Constants be updated?
The current IAU System of Astronomical Constants is from 1976 and the CBEs appear to be roughly one order of magnitude more accurate. However, as Standish (1995) put it, “The present [1976] IAU System of Astronomical Constants provides a stable standard for the consistent reduction of observations.” Is now an appropriate time to change the constants to take advantage of greater accuracy or is the need for consistency in the reduction of observations more important? Or, looking at this problem another way, what will happen if the constants are changed versus what will happen if they are not changed?
5. Does anyone want to get together at future meetings?
While I believe that most, if not all, of the work can be accomplished through e-mail, it might be more efficient to get together occasionally to meet face-to-face to discuss the progress of the WG. If there is interest in doing this, I would propose that we try to meet in conjunction with larger meetings such as the upcoming AGU and/or EGU meetings or possibly the Journées meeting that is scheduled for Fall 2007. Are meetings desirable, and if so, which meetings do you plan on attending in the next year?
References
Fukushima, T., 2000, "Report on Astronomical Constants," Proc. IAU Colloquium 180, Johnston, K. J., McCarthy, D. D., Luzum, B. J., Kaplan, G., pp. 417-427.
Fukushima, T., 2003, "Report on Astronomical Constants," Highlights of Astronomy, Vol. 13, International Astronomical Union, 2002, Rickman, H. (ed.), pp. 107-112.
McCarthy, D. D. and Petit, G. (eds.), 2003, IERS Conventions, IERS Technical Note 32. Particularly pp. 9-13 and 19-20.
Standish, E.M., 1995, "Report of the IAU WGAS Sub-group on Numerical Standards," Highlights of Astronomy, Vol. 12, International Astronomical Union, 1994, Appenzeller, I. (ed.), pp. 180-184.