Franz-Ludwig Deubner writes: Applying Kepler's Third Law to the Precambrian and the present period of revolution of the Moon, and considering the angular momentum transfer between the Earth and the Moon inferred from the Earth’s decreasing rotation rate, a very moderate increase of the semi-major axis of the lunar orbit is obtained, which is in good agreement with the value quoted by Williams (1989a) in his account of the secular acceleration of the lunar nodal period.

In a recent paper on the origin of sandstone rhythmites found in the Elatina Formation, South Australia, Williams (1989a) presents a new interpretation of these sediments in terms of distal ebb-tidal deposits. The author compares in detail the spectrum of lamina thickness variations evident in the drill-core material with the dominant tidal cycles observed in modern sea-level records. He demonstrated a correspondence of the early variations with semidiurnal, diurnal, fortnightly, and monthly cycles, and, moreover, characteristic signatures of the lunar apsides and nodal cycles, superimposed on a large amplitude annual period, presumably linked to a seasonal variation in sea level.

Williams’s study presents an impressive array of evidence in favor of the tidal hypothesis which has also interesting implications for the history fo the Earth-Moon system and for the Earth’s palaeorotation. It is the aim of this short communication to strengthen the arguments in favor of the tidal interpretation by verifying with simple assumptions the internal consistency of the observed secular variations of three fundamental periodicities as taken from table 1 of Williams (1989b).

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