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.