On 2020-08-08 10:46, John Sauter via LEAPSECS wrote:
UT2 captures the seasonal change in the length of day, so it can be
ignored for long-term estimates. The important number, therefore, is
-0.00010, which I will call the UT1 slope.
Perhaps "slope of UT2 - UTC (as predicted by the IERS)" would be a
better
name, as the formula of Bulletin A implies (by taking the derivative
after adding the implied units):
d(UT2)/d(UTC) = 1 - 0.10 ms/d.
But anyway, UT2 still contains known short period (<= 35 d)
fluctuations,
and currently, the combined amplitude of these fluctuations exceeds
by far the indicated secular trend of UT2 during their periods.
Another observation, namely the increase in the uncertainty of UT1
- TAI
as given in the long term EOP parameters (published by the IERS in
[https://datacenter.iers.org/data/latestVersion/38_EOP_C01.1900-NOW_V2013_0138.txt])
from 4 µs to 30 µs since J2020.40 = 2020-05-26.6 may be a related
effect.
Michael Deckers.
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