Letter to the Editor

The IAU 2006 precession quantities with an improved Earth’s J₂ long-term variation

¹ School of Astronomy and Space Science, Key Laboratory of Modern Astronomy and Astrophysics (Ministry of Education), Nanjing University, 163 XianLin Avenue, Nanjing 210023, China
² Shanghai Astronomical Observatory, Chinese Academy of Sciences, 80 Nandan Road, Shanghai 200030, China
³ School of Astronomy and Space Science, University of Chinese Academy of Sciences, 19A Yuquan Road, Beijing 100049, China

^⋆ Corresponding author: jcliu@nju.edu.cn

Received: 13 October 2025
Accepted: 3 November 2025

Abstract

Context. In 2006, the IAU adopted a new precession theory, called the IAU 2006. The time variation of the Earth’s dynamical flattening J₂ was considered in this model as an important contribution to the precession rate in longitude. However, a linear J₂ trend, which was valid at that time, is no longer a good approximation and may limit the accuracy of the theory.

Aims. We investigated the contribution of latest nonlinear J₂ variation in developing the precession quantities of the equator.

Methods. Using the most recent satellite laser ranging data, we modeled the Earth’s J₂ long-term variation using a parabola. It was implemented in calculating the polynomial expressions for precession quantities with a method similar to the IAU 2006 approach.

Results. The updated precession solution is clearly more consistent with VLBI observations and can reduce most of the curvature signals in the CPO series. The validity of using a parabolic J₂ variation in precession development is confirmed.

Conclusions. The new precession can be regarded as an update of the IAU 2006 model, and thus we named it IAU 2006_J2. Since the improvement shown by the tests with VLBI observations is quite significant, we propose that a serious discussion for updating the IAU precession be carried out by the IAU/IAG Joint Working Group: Consistent Improvement of the Earth’s Rotation Theory (CIERT). It could also be considered for the next update of the IERS Conventions which took effect more than 15 years ago.