NavList:

   

Reply

A 2016 Royal Society paper discussed historic changes in Earth rotation.

"The current value of the tidal acceleration of the Earth’s spin has
been measured reliably from the perturbations by lunar and solar tides
on near-Earth satellites, together with the requirement that angular
momentum be conserved in the Earth–Moon system... The satellite and
lunar laser ranging measurements were obtained from data collected over
about 50 years or so. However, they can be applied to the past few
millennia because it has been demonstrated satisfactorily that the
mechanism of tidal friction has not changed significantly during this
period.

"While the tidal component of the Earth’s acceleration can be derived
from recent high precision observations, the actual long term
acceleration, which is the sum of the tidal and other components, cannot
be measured directly from modern data because it is masked by the
relatively large decade fluctuations... This non-tidal acceleration is
probably in part associated with the rate of change in the Earth’s
oblateness attributed to viscous rebound of the solid Earth from the
decrease in load on the polar caps following the last deglaciation.
However, by itself, this mechanism cannot account completely for the
non-tidal acceleration, and some additional correction for core–mantle
coupling is required."

Measurement of the Earth's rotation: 720 BC to AD 2015, F. R.
Stephenson, L. V. Morrison and C. Y. Hohenkerk, 2016.

https://royalsocietypublishing.org/doi/10.1098/rspa.2016.0404

Apparently we are seeing one of those "decade fluctuations." The authors
give a formula for lod (length of day) which does not include decade
fluctuation but does include a 1500 year oscillation. If t is centuries
since 1825 (currently 1.97),

lod = 1.78 t − 4.0 sin 2π (t / 15)

if the sine argument is radians. To work the formula on a calculator
with sines in degrees,

lod = 1.78 t - 4 sin (24 t)

In either case, lod is the amount (milliseconds) by which one Earth
rotation exceeds one day of atomic time. The formula predicts +0.57 ms
at present, whereas the true value is negative.

IERS Bulletin A says UT1-UTC = -0.03 s now, and predicts +0.08 s a year
from now. So Earth should gain about a tenth second on atomic time in
the next 365 days, which equals an average lod of -0.3 ms. We shall see.

The lod formula is actually not too bad. My Lunar 4.4 program uses the
integral of the formula for years after 2015. At present the accumulated
error is only 0.16 s.

   

Reply