This paper deals with the use of lunar imagery in the book of Ben Sira. The scribe of Jerusalem refers to the moon on four occasions: Sir 27,11; 39,12; 43,6-8 and 50,6. With the exception of Sir 43,6-8, in the other texts he refers to it metaphorically. While in Sir 39,12 and 50,6 he shows a positive image of the moon, in Sir 27,11 it serves a negative one. From the analysis of these texts the author draws the following conclusion: lunar imagery, which at first sight might seem a mere ornamental embellishment of the text, constitutes a powerful tool used by the sage to transmit his teaching convincingly and to enhance his authority as a teacher of wisdom.