Brant A. Gardner discusses Helaman 12:15 and suggests it is a modern expansion by JS.
Brant A. Gardner, Second Witness: Analytical and Contextual Commentary on the Book of Mormon, six vols. (Salt Lake City: Kofford Books, 2007), 5:169
Culture: This verse is a fascinating addition to the text. Mormon clearly notes the problem in Joshua's text that implies the sun revolves around the earth and, hence, for this miracle, stands still. Mormon understands that this description is incorrect and explains the accurate status. How did Mormon know such information? Mesoamerican cultures were great sky-watchers, so this might have been information available to Mormon. However, I am unaware of any indication that Mesoamericans believed anything other than that the sun moved around the earth. Their typical conception of the sun was that it passed through the underworld after descending in the west so that it could arise in the east. It would seem more likely that Joseph Smith made this emendation.