http://www.fordham.edu/halsall/source/decameronintro.html
Boccaccio: THE DECAMERON , “INTRODUCTION”
Thirteen hundred and forty-eight years had passed since the fruitful Incarnation of the Son of God, when there came into the noble city of Florence, the most beautiful of all Italian cities, a deadly pestilence, which, either because of the operations of the heavenly bodies, or because of the just wrath of God...
Just guessing, but it might depend upon the origin of the reports, Earth was the centre of the Universe...and ‘rocks’ didn’t fall from the sky. Boccaccio was very brave to allude to ‘the operations of the heavenly bodies’ methinks.
Heavenly bodies operate, whether we’re spinning and moving, or we’re stationary and they’re swirling around us and occasionally falling down out of the firmament and smiting us. Just a matter of perspective. The end result is the same.