Lunar orbital evolution: A synthesis of recent results

The present rate of tidal dissipation in the Earth-Moon system is known to be anomalously high, in the sense that the implied age of the lunar orbit is only 1.5 x 10(9) years, though other evidence suggests an age closer to 4 x 10(9) years. To assess how long the anomalous dissipation has persisted, we use published estimates of lunar orbital configurations derived from (a) fine grained sediments containing tidal laminations and (b) numerical ocean models averaged over varying ocean geometries. The implied histories of the lunar semimajor axis are surprisingly consistent over the past 10(9) years. The ocean models imply, on average, reduced dissipation in the past because of a spatial mismatch between tidal forcing and oceanic normal modes of higher frequencies. Webb's ocean model suggests that the "anomalous" oceanic dissipation began about 10(9) years ago and has been increasing since then.