Historical eclipses and Earth's rotation

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.