Francis Richard Stephenson

Long-Term Fluctuations in the Earth's Rotation: 700 BC to AD 1990

Records of solar and lunar eclipses in the period 700 BC to AD 1600, originating from the ancient and medieval civilizations of Babylon, China, Europe and the Arab world, are amassed and critically appraised for their usefulness in answering questions about the long-term variability of the Earth’s rate of rotation. Results from previous analyses of lunar occupations in the period AD 1600-1955.5, and from high-precision data in AD 1955.5-1990, are included in the dataset considered in this paper.

Long-term changes in the rotation of the Earth : 700 B.C. to A.D. 1980

Occultations of stars by the Moon, and solar and lunar eclipses are analysed for variations in the Earth’s rotation over the past 2700 years. Although tidal braking provides the dominant, long-term torque, it is found that the rate of rotation does not decrease uniformly as would be expected if tidal friction were the only mechanism affecting the Earth’s rotation. There are also non-tidal changes present that vary on timescales ranging from decades to millennia. The magnitudinal and temporal behaviour of these non-tidal variations are evaluated in this paper.

Historical eclipses and the variability of the Earth's rotation

Abstract Ancient Babylonian clay tablets buried for centuries beneath the sands of the desert are part of an extensive historical archive which contains vital information about the rotation of the Earth. Many eclipse records are preserved from a variety of cultures, and using these seemingly crude ancient and medieval observations, variations in the Earths rotation can be traced back over the past 2500 years. The tidal torque exerted by the Moon (and to a lesser extent, the Sun) is the predominant mechanism in reducing the Earths rate of spin. It is known that by this mechanism, the length of the day is increasing by 2.3 ms per century. From analyses of the discordance between observations and calculations of eclipses, the average measured increase in the length of the day is 1.8 ms per century, from which it is concluded that besides the tidal contribution there is another long-term component acting to decrease the length of the day by 0.5 ms per century. This component, which is thought to result from the decrease of the Earths oblateness following the last ice age, is consistent with recent measurements made by artificial satellites. Besides these secular trends, there is clear evidence of fluctuations of the length of the day on a timescale of centuries, with an amplitude of 4 ms which is similar to that of the decade fluctuations.

Solar and lunar eclipse measurements by medieval muslim astronomers, II: Observations

Les eclipses solaires et lunaires ont ete observees et mesurees par des astronomes arabes du Moyen Age. Leurs documents et leurs calculs ont ete traduits

Historical evidence concerning the Sun: interpretation of sunspot records during the telescopic and pretelescopic eras

The value of sunspot observations in investigating solar activity trends - mainly on the centennial to millennial timescale — is considered in some detail. It is shown that although observations made since the mid-eighteenth century are in general very reliable indicators of solar activity, older data are of dubious quality and utility. The sunspot record in both the pretelescopic and early telescopic periods appears to be confused by serious data artefacts.

Historical eclipses and the rotation of the earth

This paper summarises recent research on long-term variations in the Earths past rotation. A variety of ancient and medieval observations of both solar and lunar eclipses are investigated to determine changes in the length of the day (LOD) between 700 BC and AD 1600. These data originate from Babylon, China, Europe and the Arab world. In all, some 400 individual observations - both timed and untimed - are analysed. The results indicate an average rate of increase in the LOD by 1.7 milliseconds per century (ms/cy), considerably less than that calculated on the basis of lunar and solar tidal friction alone (i.e. 2.3 ± 0.1 ms/cy). The residual mean non-tidal decrease in the LOD by about 0.6 ms/cy is in good accord with that expected on the basis of post-glacial isostatic compensation. However, there is also some indication of long-term fluctuations in the LOD of amplitude some 4 ms on a time-scale of around 1500 years. Possible causes are core-mantle coupling and variations in global sea-level associated with climatic fluctuations.

The Variation of UT from 700 BC to the Present

Records of solar and lunar eclipses in the period 700 BC to AD 1600 originating from the ancient and medieval civilisations of Babylon, China, Europe and the Arab world are amassed and critically appraised for their usefulness in answering questions about the long-term variability of the Earth’s rate of rotation. Results from previous analyses of lunar occultations in the period AD 1600-1955.5 and from high-precision data in AD 1955.5-1990 are included in the dataset considered in this paper.