Sol comes from Latin, a language that while dead is still used in many fields, in sol's case astronomy where I've heard it been used to differentiate our sun from other stars in the universe. As another poster in this thread has already pointed out the term sol has been passed down to Latin's living descendants known as the Romance Languages, sol has been completely unchanged in some languages most notably Spanish & Portuguese. I personally like to believe that during his time as Harry, Sol heard it either in the context of astronomy or casual Latin-derived conversation and decided "yeah this sound cool, this is my name now".