L. V. Morrison

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.

Check on JPL DExxx Using Hipparcos and Tycho Observations

Obervations of the positions of Europa (J2) and Titan (S6) by Hipparcos, and Ganymede (J3) and Callisto (J4) by Tycho are analysed to give checks on the latest JPL ephemerides of the planets Jupiter and Saturn. The observed positions of the satellites are compared with DExxx using the G5 theory of the Galilean satellites and D.B. Taylors theory of Titan to calculate their o sets from the barycentres of the two systems. The Hipparcos observations of J2, and Tycho of J3 and J4 put tight constraints on the orbit of Jupiter, and agree closely with the series of ground-based observations made by the Carlsberg Meridian Telescope.

The Solar Diameter Since 1715

The debate, on whether there is any valid evidence for deducing significant variations in the diameter of the Sun, still continues over a century later. Claims for secular decreases, increases and periodicities on a wide range of time scales have all been proposed and, at some later time, they have all been disputed. If significant changes in the size of the Sun were established they would lead to variations in the gravitational potential energy on time scales shorter than evolutionary ones and substantial modifications would need to be made to our current understanding of the solar interior.

The Almagest Greek and Roman Occultations Re-Visited:

The errors in the timings of the Almagest occultations are investigated to ascertain what contribution they made to Fotheringham and Longbottom’s 1915 result for the (tidal) acceleration of the Moon. It is found that their result is quite close to the modern value, once the apparent acceleration of the Moon due to the retardation of the Earth’s spin rate is removed.