Letters of Intent received in 2017

LoI 2019-1995
The Earth's Time Varying Rotation: A Centennial Celebration

Date: 7 October 2019 to 11 October 2019
Location: Brussels, Belgium
Contact: Richard Gross (Richard.Gross@jpl.nasa.gov)
Coordinating division: Division A Fundamental Astronomy
Co-Chairs of SOC: Richard Gross (Jet Propulsion Laboratory)
Chengli Huang (Shanghai Astronomical Observatory)
Daniella Thaller (Bundesamt für Kartographie und Geodäsie)
Chair of LOC: Veronique Dehant (Royal Observatory of Belgium)



(1) Earth rotation observations, theories, models, and analysis
(2) Celestial reference systems and frames
(3) Terrestrial reference systems and frames
(4) New and emerging measurement systems
(5) Fundamental standards



Observations of the Earth's rotation, encompassing both the rate of rotation or, equivalently, the length of the day, as well as the position of the rotation pole with respect to the Earth's crust and mantle (polar motion) and its position in space (precession and nutation), show that it changes on all observable timescales, from sub-daily to decadal and longer. The wide range of timescales on which the Earth's rotation changes reflects the wide variety of processes that are causing it to change, from external tidal forces to surficial processes involving the atmosphere, oceans, and hydrosphere, to internal processes acting both at the core-mantle boundary and within the solid Earth itself.

Besides being used to study these dynamic Earth processes, Earth orientation parameters, consisting of two polar motion parameters, two nutation parameters, and one spin parameter, are also used to transform the positions of objects in the terrestrial reference frame (TRF) to their positions in the celestial reference frame (CRF). These reference frames are fundamental to positioning and navigating objects in space and on the Earth. Celestial reference frames are used to measure the passage of time, for navigation, and for studying the dynamics of the solar system. Most recently, celestial reference frames have become essential for studying the dynamics of more distant objects and for studying geophysical phenomena on the Earth. Terrestrial reference frames provide the fundamental framework and metrological basis for Earth observations. Since terrestrial reference frames are attached to the Earth, transforming the positions of objects between the celestial and terrestrial reference frames requires knowledge of the Earth's changing rotation. Celestial reference frames, terrestrial reference frames, and the Earth's rotation therefore form an interconnected trio.

The Earth's rotation is a core topic of astronomy as are the terrestrial and celestial reference frames that it connects. Studying the Earth's rotation is foundational to the IAU. Standing Committee 19 on Latitude Variations was formed with the IAU in order to study polar motion. The Constitutive Assembly of the International Research Council was held in Brussels during July 18–28, 1919. On the last day, the Assembly adopted a Statute creating the IAU. So, July 28, 1919 has been taken to be the birth date of the IAU (see https://www.iau.org/about/90years/) and July 28, 2019 will be its 100th anniversary. Along with creating the IAU, 32 Standing Committees were also created on July 28, 1919. One of these was Standing Committee 19 on Latitude Variations. In 1922 all of the Standing Committees became Commissions. So, Standing Committee 19 on Latitude Variations became Commission 19 on Variation of Latitude. In 1964 this was renamed Commission 19 on Rotation of the Earth. In 2015 this became Commission A2 on Rotation of the Earth. So, July 28, 2019 also marks the 100th anniversary of Commission A2.

Since 1919, our knowledge of the Earth's rotation and the processes causing it to change have greatly improved. One hundred years ago, latitude observations were acquired by optical astrometric techniques. Today, the established space-geodetic techniques of satellite and lunar laser ranging (SLR and LLR), very long baseline interferometry (VLBI), global navigation satellite systems (GNSS), and Doppler orbitography and radio positioning integrated by satellite (DORIS) are used to study the Earth's rotation. However, with the launch of the GRACE twin gravity satellites in March 2002, the densification of the global GNSS ground tracking network, the development of next generation VLBI and SLR stations, and the ongoing development of ring laser gyroscopes and quantum technologies, new opportunities for studying the Earth's rotation are becoming available. GRACE directly observes surface mass redistribution while the global network of GNSS receivers senses the associated load deformations of the Earth's surface and its effect on the Earth's rotation. While all the above-mentioned techniques can only provide the position of the celestial intermediate pole, the emerging ring laser gyroscopes are directly sensitive to the terrestrial position of the instantaneous rotation pole of the Earth and thus may contribute to better understanding of ultra-high-frequency rotational variations, particularly those occurring on sub-daily to daily time scales.

Yet despite our improved knowledge of the Earth's time varying rotation, much still needs to be done. The terrestrial and celestial reference frames are a factor of 5 to 10 less accurate than needed by the science community, the theory of the Earth's rotation currently used is over 30 years old and is not accurate enough to study small signals that are now observable like the ellipticity of the Chandler wobble, and many of the larger signals are inadequately modeled. Studying the Earth's rotation and improving the associated reference frames remain active areas of research within the IAU as evidenced by the 16 resolutions on these subjects that have been passed by the IAU at its General Assemblies since 2000.

The proposed Symposium on the Earth's Time Varying Rotation is being organized to celebrate the centennial anniversary of Standing Committee 19/Commission 19/Commission A2. It will be a forum for discussing the observations, theory, modeling, and analysis of the Earth's time varying rotation including precession, nutation, wobble, UT1, and length-of-day; for discussing new and emerging technologies to observe the Earth's rotation; for discussing the use of Earth orientation parameters to connect the terrestrial and celestial reference frames to each other; and for discussing the standards that are required to ensure that Earth orientation parameters and terrestrial and celestial reference frames are consistently determined.