章动序列计算和地球定向参数测定需要一个中间的天球参考极作参照。1984年起，采 用IAU 1980章动理论，选取天球历书极作为参考极。利用改善的岁差章动模型和由天文测 地新技术确定地球定向参数实现的天球历书极，其精度可达0.1mas。随着理论和观测精度 的提高，在微角秒量级下，章动和极移模型中周日和半周日成分应被考虑，地球定向参数的 高频成分已能被测定，因此天球历书极的原先定义不再适用，需要更改。叙述了不同天球参 考极的概念、天球历书极的定义，评述了天球历书极的目前实现及其缺陷，介绍了新的 天球参考极---\hspace*{-1.5mm}---天球中间极的定义及其实现。
The computation of nutation series and the estimation of the earth orientation parameters (EOP) make use of an intermediate pole. Since the adoption of the IAU 1980 theory of nutation, the reference pole is the celestial ephemeris pole (CEP). The CEP is currently realized in the accuracy of 0.1mas by an improved precession-nutation model and the estimation of the EOP from various techniques. In the accuracy of microarcsecond, the diurnal and sub-diurnal components of the pole must be taken into account both in the terrestrial reference system (TRS) and in the celestial reference system (CRS). Moreover, the new observing procedure provides the estimation of the EOP with a few hour resolution. The reference pole has therefore to be redefined. The concepts of the different reference poles and the definition of the CEP are described. The realization of the CEP and its deficiencies are reviewed. A new celestial reference pole, CIP, is introduced.