SIMBAD references

The rediscussion of the precession constant was performed by optical observations of the counterparts of extragalactic radio sources with longer observational intervals. After a reduction of the optical catalogue systems, the fictitious `proper motions' of extragalactic radio sources were obtained in the FK5 system. On the assumption that these fictitious `proper motions' of extragalactic sources are the reflection of an incorrcet value of the precession constant, then the correction of precession constant can be estimated. The corrections are -2.60±1.35mas/yr for 43 sources and -3.35±1.62mas/yr for 24 sources, respectively. If the improvement of the 18.6-year nutation term is considered in the data processing, the corrections of the precession constant are -2.91±1.38mas/yr and -3.52±1.68mas/yr, respectively. The results were also compared with those from VLBI.