On 2024-04-03 22:51, Steve Allen via tz wrote:
Ironically, as soon as that agreement went into effect the place over which the mean sun stands at 12:00 Universal Time began an increasing deviation eastward from Greenwich.
Yes, and it's also ironic that although the ITRF2020 definition of the prime meridian has reached new levels of accuracy (about 1 mm at the equator![1]), that meridian currently passes about 100 meters east of the Airy Transit Circle at Greenwich. Although there are good reasons[2] for the 100 meter discrepancy, this can all be pretty confusing to amateurs such as myself. [1]: Altamimi Z, Rebischung P, Collilieux X, Métivier L, Chanard K. ITRF2020: an augmented reference frame refining the modeling of nonlinear station motions. J Geod 97, 47 (2023). https://doi.org/10.1007/s00190-023-01738-w [2]: Malys S, Seago JH, Pavlis NK, Seidelmann PK, Kaplan GH. Why the Greenwich meridian moved. J Geod 89, 1263–1272 (2015). https://doi.org/10.1007/s00190-015-0844-y