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Historical eclipses and Earth’s rotation
By F Richard Stephenson
Astronomy and Geophysics, Volume 44:2 (2003)
Abstract: The Earth, in its diurnal rotation, acts as a remarkably accurate timekeeper. However, small variations in the length of the day occur at the millisecond level. Historical eclipse observations, recorded by various ancient and medieval cultures, enable changes in the Earth’s spin rate to be monitored with fair precision as far back as around 700 BC. Although lunar and solar tides are the main causes of long-term changes in the length of the day, the early observations reveal that non-tidal mechanisms are also important. In this paper I review both the historical development of this subject and recent advances.
Excerpt: Few chronicles give careful descriptions of very large partial solar eclipses. However, under the year AD 1330, the annals of the monastery of Aula Regia (at Zbraslav near Prague) describes such an event in adequate detail. The recorded date (equivalent to 15 July) is one day in error; the actual date was 16 July.
1330. In this same year on the Ides of July at the 8th hour of the day, the Sun was so greatly obscured that of its great body only a small extremity like a three-night-old Moon was seen.