Cultural: This verse is a fascinating addition to the text. Mormon is clearly referencing Joshua, but he notes that there is a problem in Joshua. Joshua’s text implies that the sun revolves around the earth. Thus in Joshua it is the sun and moon that stand still, because they are conceived as moving. Mormon understands that this is not correct. He feels the need to correct the statement to be more accurate.
The question is how Mormon knew such information if Joshua and many in the Old World did not. Mormon was living in a Mesoamerican world that was fascinated with astronomical observation. It is plausible that this information was available to him through the New World traditions, and Mormon is using the New World tradition to correct the Old World “error.”