Progress on the research work of astronomical constants:IAU 2009 astronomical constant system
The situation of astronomical constant system adopted by the IAU since 1900 has been briefly reviewed.The mathematical relation between some astronomical constants and the main reason for the change of astronomical constant system each time in the past are described.Especially,the work on the IAU astronomical constant system,including the establishment of working groups and adoption of the current best estimates since 1991,is introduced.The background of the IAU 2009 astronomical constant system instead of the IAU 1976 astronomical constant system is given.As for this change the reasons are the appearance of new measurements of astronomical constants with exploration of the Solar system,application of the Barycentric Celestial Reference System and Geocentric Celestial Reference System under the relativistic reference frame after 1991 and the adoption of new models for precession and nutation after 2006.The comparison between the IAU 2009 and 1976 astronomical constant system is given.Finally,the work on astronomical constants in China,such as determination of precession constant from proper motion,the definition of astronomical unit under the relativistic reference frame,determination of nutation constant from globe optical classical observations during 1962.0—1982.0,Corrections to the IAU 1976 precession constant and the coefficients of the IAU nutation series from VLBI data by using various networks during April 1980—December 1995,establishment of a new nutation model of a non-rigid earth with ocean and atmosphere etc.is briefly mentioned.Some suggestions for the further work on astronomical constants are proposed as follows: (1)Shanghai Astronomical Observatory(SHAO)incorporates the international organizations of ILRS,IVS and IGS.The globe solution are given from processing the observational data of SLR、GPS and VLBI routinely.By using these globe data not only the Earth Orientation Parameters,plate motion and regional crustal deformation but also the parameters of the shape and size and the equatorial radius and flattening of the Earth,the Geocentric gravitation constant,dynamical form factor and its long-term variation,which are related to the gravitational field of the Earth can be obtained.In addition,the experiment on time synchronism between remote ground clocks and comparison of their frequency stabilities by using laser telemetry is being carried out recently.The information on the fluid core of the Earth,such as the resonant frequency associated with retrograde free core nutation(RFCN) can be further studied with analyzing the VLBI data. (2)Since 2003 the PMOE planetary/lunar ephemeris has been compiled with fitting various observational data such as radar and astrometric observations for planets and natural satellite by a group at Purple Mountain Observatory.The astronomical constants are the by-products of this work.Especially it is pointed that the work on optical observations of planets,natural satellites and asteroids in China is remarkable.But these data are not collected in this kind of public database.It will be mentioned here that the astronomical unit has not been finally made the decision yet and will be discussed at the 28th IAU General Assembly,Beijing,2012.We should pay attention to it. In addition,the number of observing asteroids in China ranked among the top-ten observational programs in the world so the mass of asteroids and its ephemeris should be compiled by this group. Recently,the exploration of the Solar system,such as lunar exploration project,Chang'E series,Martian exploration project,Yinghuo etc.is being curried out in China.The missions to other planets will be also launched in the near future.The planetary gravitational field and geometrical character of its satellites will be studied from various observational data, including range and velocity data of vehicles from Earth-based radio-tracking data,optical measurements of star-satellite imaging obtained with the imaging system on the vehicle and Earth-based astrometric observations of natural satellites.The physical character of the planetary system will be studied if there are other instruments on the vehicle. (3)The MHB 2000 nutation model was adopted by the IAU in 2003.Meantime under the modified ellipticity profile of second-order accuracy for the non-hydrostatic and anelastic earth,including atmosphere and ocean,a nutation model and series were given by a group at SHAO.The effect of differential rotation between each layer in the earth's interior to nutation is also considered.With the Martian exploration project implemented by space agencies in many countries the Martian nutation has been studied.The contribution on this topic will be also made by this group in the future. In brief,the determination of astronomical constants is a long-term work.This work should be combined with other research work,such as the compilation of planetary ephemeris, determination of earth's gravitational field etc..