Historical eclipses and Earth's rotationThe theme of my lecture thus seems apt. For more than three centuries – commencing with a paper by Edmond Halley in 1695 – the study of ancient and medieval observations of eclipses has provided valuable information about the Earth–Moon system. Today, early eclipse observations are the principal source of data for information on long-term variations in the Earth’s rate of rotation... Variations in the MSD are the subject of this lecture. Following decades of speculation, Sir Harold Spencer Jones in 1939 conclusively demonstrated that even the MSD is not an ideal unit; the Earth in its diurnal rotation is a very good timekeeper but is by no means perfect. Astronomical observations reveal fluctuations in the length of the day (LOD) at the millisecond level; using historical observations these fluctuations can be traced back more than 2500 years... An additional long-term mechanism is post-glacial isostatic compensation; the ongoing rise of land that was glaciated during the last ice-age produces a slow diminution in the moment of inertia of the Earth, with consequent decrease in the LOD. Seasonal and annual fluctuations have been mapped in detail only since the introduction of Atomic Time (AT) in 1955. Decadal variations in the LOD can be traced over the last four centuries or so (i.e. the telescopic period), mainly using occultations of stars by the Moon.
F Richard Stephenson
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September 1752
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Long as Mrs. Lonesome doesn't find out, then I really will be Lonesome.