Addendum 2020 to ‘Measurement of the Earth’s rotation: 720 BC to AD 2015’

Historical reports of solar eclipses are added to our previous dataset (Stephenson et al. 2016 Proc. R. Soc. A 472 , 20160404 ( doi:10.1098/rspa.2016.0404 )) in order to refine our determination of centennial and longer-term changes since 720 BC in the rate of rotation of the Earth. The revised observed deceleration is −4.59 ± 0.08 × 10 −22 rad s −2 . By comparison the predicted tidal deceleration based on the conservation of angular momentum in the Sun–Earth–Moon system is −6.39 ± 0.03 × 10 −22 rad s −2 . These signify a mean accelerative component of +1.8 ± 0.1 × 10 −22 rad s −2 . There is also evidence of an oscillatory variation in the rate with a period of about 14 centuries.