Lunar tidal acceleration determined from laser range measures

Lunar laser range measures covering the period 1969 to 1976 have been used to determine the anomalous secular acceleration in the mean longitude of the moon, commonly attributed to the effect of tidal friction in the earth. The acceleration determined is -24.6 +/- 1.6 arc seconds per century squared, against an atomic time scale, where the uncertainty is the formal standard deviation of the solution. The realistic uncertainty is surely larger, as evidenced by the ensemble of solutions performed with various models and observation sets. The determined value is in good agreement with the conventional value and with several recent determinations by other methods. An attempt to determine the rate of change of the mean distance, essential for separating the tidal effect from a time variation of the gravitational constant, yielded no significant result, because the observations still span too short a time.