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IERS Message No. 530                                      March 05, 2025
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CCTF-IERS workshop on negative leap second - March 13, 2025


Aim: Understanding the probability of a negative leap second, to support
the decision on Continuous UTC at the next CGPM 2026.

Dear Colleagues, 

The General Conference on Weights and Measures (CGPM) decided in 2022 to
extend the tolerance between UTC and UT1 to avoid the frequent
discontinuities given by the leap seconds. The resolution is available
here: https://www.bipm.org/en/cgpm-2022/resolution-4

The CGPM has to decide which will be the new tolerance and when it
should be implemented "in, or before, 2035". One of the main issues to
anticipate the date with respect to 2035 is the risk of a negative leap
second which has never been applied and raise a considerable risk of
malfunctioning in several applications. This decision will be discussed
at the Consultative Committee for Time and Frequency (CCTF) in September
2025 in view of final adoption by the member states during the CGPM in
October 2026.

Studying the rotation of the Earth needs several expertise such as:
the interaction between core and mantle, the effect of melting ice, the
secular slowing down, ...

To bring together insights from various scientific disciplines, the
CCTF, in collaboration with the IERS, is organizing a dedicated
workshop. The goal is to assess the probability of a negative leap
second within the next decade. In this workshop Earth rotation experts
give a short presentation, from different points of view, on the
variation of the Earth rotation and LOD, and its prediction that could
lead to the need of a negative leap second.

Workshop Details:

Date: Thursday, March 13, 2025
Time: 14:00 - 16:00 UTC
Format: Online (Zoom Webinar)

Agenda:
Each presentation will be 10-15 minutes long:

1. Stamatakos (USNO): IERS Observations of UT1 and LOD
2. McCarthy (IAU): An Astronomer's Overview
3. Mitrovica (Harvard University): Historical Geophysical Effects on
                                   Earth Rotation
4. Agnew (UCSD): The Future of UT1 - A Geophysical Perspective
5. Surendra Adhikari: Climate-Driven LOD Changes in the 20th and
                      21st Centuries
5. Dehant V., Chicot G., Laariara D., Gillet N., Mandea M., Cazenave A.
   (ORB, Belgium): Core Contributions to LOD
7. Giulio Tagliaferro (BIPM): Stochastic Modeling and Prediction

To participate, please register via the following link:

[https://bipm-org.zoom.us/webinar/register/
WN_eK8oWapXR3CMGi3vg_f04g#/registration]

(Please note that due to message formatting constraints, the link may
appear split into two lines. When accessing it, ensure you copy and
paste the entire URL into your browser.)

Best regards,
Noel Dimarcq, President of the CCTF
Patrizia Tavella, executive secretary 
Robert Heinkelmann, IERS Analysis Coordinator


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