A paper by L.V. Morrison, F.R. Stephenson, C.Y. Hohenkerk and M. Zawilski "Addendum 2020 to 'Measurement of the Earth's rotation: 720 BC to AD 2015'" in the Proceedings of the Royal Society A, February 2021, 478, adds new data from eclipse observations made in medieval Europe to the determination of Earth Rotation (ΔT) presented in Proc. R. Soc. A 2016, 472.
Today, ΔT is the difference between Terrestrial Time (TT) and Universal Time (UT1) i.e. ΔT = TT − UT1. It is a measure of the difference between a time scale based on the rotation of the Earth (UT1) and an idealised uniform timescale at the surface of the Earth (TT). TT is currently realised from TAI, International Atomic Time, where TT = TAI + 32.184 seconds. Before atomic time, Ephemeris Time (ET) was used as the uniform timescale based on Newcombe's Tables of the Sun. In order to predict the circumstances of an event on the surface of the Earth, such as the circumstances of a solar eclipse, a prediction of ΔT must be made for the instant of observation.
Annual values of ΔT and length of day (lod) using the results from the 2021 paper are available.
The Supplement to the latest (2021) paper includes plots of sixteen solar eclipse tracks over medieval Europe between 1133 and 1598. For each of these eclipses an excerpt giving the date, where the eclipse was observed, the author, the name of the document and a description of the event. This canon of eclipses was collated by M. Zawilski.
FAQ's on the rotation of the Earth are available to download.
The two images, which are on the respective covers from the Royal Society Proceedings, show a Babylonian clay tablet with a record of an eclipse observation and a painting from the 1570's of astronomers in medieval times observing an eclipse of the Sun, illustrate the different sources of information.
The image of a clay tablet written in cuneiform script contains an accurate account of the occurrence of a total solar eclipse witnessed at Babylon in 136 BC. The narrowness of the belt of totality crossing Babylon fixes the rotational phase of the Earth at that epoch. This, and other extant records of eclipses — both solar and lunar — from ancient Babylon, China, Greece, the Arab Dominions and medieval Europe, are used to trace the history of the Earth's rotation over the past two and a half millennia.