Talkshop readers will remember that as well as his work on modelling solar activity, Rick Salvador also built a planetary model to predict variation in Earth’s Length of Day (LOD). The model uses 13 frequencies derives from planetary and lunar motion to replicate changes in Earth’s spin rate.

Rick has retired from modelling now, so this is the final update on the model’s performance. The IERS LOD database was changed in early 2020, so the model performance update ends there. Over the last 4 years, Rick found that to keep it on track, he needed to add a -0.0006 second correction in June each year. The necessity for this is as yet unexplained and comments on possible reasons are encouraged.

Here’s a plot showing the original model in red, the model with the June -0.0006″ corrections in blue, and the observed LOD data in green.

I asked Rick if he had included a term for the 1799 year De Ropp cycle of Lunar variation and he hadn’t. He kindly took the trouble to add it in and got this result back to the earliest LOD estimates going back to 1630AD. The fit is quite good, with perhaps the hint of a ~200yr residual.

On behalf of everyone at the Talkshop, I’d like to thank Rick for the superb work he has done to advance the solar-planetary osciallation theory (SPOT). His 2013 solar activity prediction paper stands out as a landmark in the study of solar system dynamics, which is standing the test of time well. We wish him all the best for a long and enjoyable retirement with his family.

via Tallbloke’s Talkshop

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April 11, 2021 at 04:30AM