A fact, however explained.  Plutarch (Sympos. Prob. iii. 10) discusses the question Διὰ τί τὰ κρέα σήπεται μᾶλλον ὑπὸ τὴν σελήνην ἢ τὸν ἥλιον, and refers the decomposition to the moistening influence of the moon.  “Air, moisture, and a certain degree of warmth, are necessary to the decay of animal bodies…where moisture continues present—even though warmth and air be in a great measure excluded—decay still slowly takes place.”  J. F. W. Johnston, Chemistry of Common Life, ii. 273.