Earth Rotation | NJU Astrometry Group

Astrometry, Astronomical Reference Systems and Earth Rotation: New Challenges in the Microarcsecond Era

Sun, 23 Aug 2026 00:00:00 +0800

Our group organized the summer school and workshop “Astrometry, Astronomical Reference Systems and Earth Rotation: New Challenges in the Microarcsecond Era”, held in Nanjing on 23–29 August 2026.

Hosted by the School of Astronomy and Space Science, Nanjing University, with support from the Astrometry Committee of the Chinese Astronomical Society and the Jiangsu Astronomical Society, the event brought together more than 100 researchers, early-career scientists, and postgraduate students from universities and research institutes across China.

Scientific scope

Advances in Gaia space astrometry, very long baseline interferometry (VLBI), and laser ranging are placing increasingly stringent demands on astronomical reference systems, theoretical models, and data analysis. The event combined foundational training with discussions of current research and the challenges of microarcsecond astrometry.

The scientific scope included:

High-precision astrometry.
Celestial reference systems and frames.
Earth-rotation models and Earth orientation parameters.
Time scales.
Solar-system ephemerides.
Applications of fundamental astronomy.

Summer school

The three-day summer school, held on 24–26 August, introduced the fundamental theories, observational techniques, and data-processing methods used in astrometry, astronomical reference systems, and Earth-rotation research.

Specialist lectures were accompanied by practical exercises, connecting theoretical models with observations, computation, and scientific applications. The programme provided students and early-career researchers with opportunities to discuss methods and research questions directly with the lecturers.

Research workshop

The workshop opened on the morning of 27 August, with Jia-Cheng Liu chairing the opening session. Xiangdong Li, Dean of the School of Astronomy and Space Science at Nanjing University, Dali Kong, Director of Shanghai Astronomical Observatory, and Yuqiang Li, Deputy Director of Yunnan Observatories, delivered opening remarks.

Participants also honoured Prof. Zi Zhu for his longstanding contributions to astrometry research, education, and academic exchange.

The two-day scientific programme focused on three themes:

The Earth–Moon system and space dynamics.
VLBI, radio observations, and celestial reference frames.
Space exploration, catalogues, and high-precision astrometry.

By combining lectures, practical training, and research presentations, the event supported exchanges between institutions and provided a forum for developing future collaborations.

Meeting resources

This overview draws on the conference introduction and the news report by Lin Xu.

Astronomical determination of the Earth's dynamical flattening using VLBI observations and IAU 2006/2000 Precession-Nutation Model

Thu, 21 May 2026 00:00:00 +0000

Background and method

The Earth’s dynamical flattening, $H_{\mathrm d}$, describes the distribution of its principal moments of inertia and is a fundamental parameter in precession-nutation theory:

$$ H_{\mathrm d}=1-\frac{A+B}{2C}, $$

where $A$ and $B$ are the equatorial principal moments of inertia and $C$ is the axial moment.

We estimate this parameter directly from celestial intermediate pole (CIP) coordinates obtained by combining the IAU 2006/2000 precession-nutation model with VLBI celestial pole offsets. The analysis considers four individual analysis-centre series and the IERS C04 combined series, with free core nutation removed before fitting.

The method explicitly accounts for the dependence of several precession-rate contributions and the main nutation term on $H_{\mathrm d}$, while incorporating additional theoretical corrections. This provides a consistent determination of dynamical flattening within the precession-nutation framework.

Main results

The estimated dynamical flattening is $H_{\mathrm d}=0.00327380936\pm5\times10^{-11}$, with the quoted uncertainty being formal. This differs from the IAU 2006 adopted value by approximately 4.54 ppm.
The simultaneously estimated frame-bias components are $\delta X=-16603\pm8,\mu\mathrm{as}$ and $\delta Y=-7033\pm8,\mu\mathrm{as}$, in good agreement with the IAU model.
A separate analysis using 15-year sliding windows with a 2-year step estimates long-term changes in $H_{\mathrm d}$ and converts them to changes in the Earth’s second zonal gravity coefficient, $J_2$, under the adopted relationship.
The VLBI-derived $J_2$ variations show a broadly parabolic long-term trend, generally consistent with smoothed satellite laser ranging results, although differences remain for some windows.

These results demonstrate the value of VLBI precession-nutation observations as an independent probe of long-term changes in the Earth’s dynamical figure and provide observational constraints for further refinement of Earth-rotation models.

The IAU 2006 precession quantities with an improved Earth's $J_{2}$ long-term variation

Sat, 01 Nov 2025 00:00:00 +0000

Updating the long-term variation of Earth’s oblateness

The IAU 2006 precession model incorporates a linear trend in Earth’s gravitational coefficient $J_{2}$, which characterizes its oblateness. However, satellite laser ranging observations extending from 1976 to 2025 show that the long-term evolution of $J_{2}$ is better represented by a parabola.

We use these observations to determine an updated empirical model of $J_{2}$ and incorporate it into the integration of the precession equations. The resulting solution, named IAU $2006_{J_{2}}$, follows the framework of the IAU 2006 model while updating the polynomial expressions for the precession quantities.

Main results

A better-supported nonlinear trend: Nearly 50 years of satellite laser ranging data provide stronger evidence for a parabolic variation of $J_{2}$ than was available in earlier studies.
Revised precession quantities: The updated $J_{2}$ model mainly changes the quadratic and cubic terms of the precession in longitude, while the precession in obliquity remains close to the IAU 2006 solution.
Improved agreement with VLBI: Comparisons with the C04 and OPA2025a celestial pole offset series show that the updated model removes most of the residual curvature in $\mathrm{d}X$.
Smaller residuals before empirical fitting: The weighted root-mean-square values decrease from 179 to 130 microarcseconds for C04 and from 193 to 147 microarcseconds for OPA2025a.

Implications

The results provide observational support for updating the precession model within the IAU 2006 framework. The paper proposes the solution for consideration by the IAU/IAG CIERT working group and in a future revision of the IERS Conventions.

Further work on the corresponding nutation corrections is needed to ensure consistency between precession and nutation when the time variation of Earth’s dynamical flattening is taken into account.