The natural explanation for the miracle of the sun could be due to unusual local atmospheric conditions. The miraculous aspect being the timing and the location which were predicted beforehand, coupled with the numerous eye witness accounts.
All sorts of people predict all sorts of things all the time. In most cases these predictions do not happen and everyone quietly forgets that the prediction was ever made. By chance there will occasionally be predictions that do come to pass - not because the prediction was actually prescient but because a stopped clock tells the right time twice a day.
How many other predicted miracles have the Vatican quietly forgotten because they didn't come to pass. Let's not forget that in the early years of christianity it was confidently predicted that Jesus would return within their lifetimes. Hmm.