When Nephi (son of Helaman) prophesied of the murder of the Chief Judge, it was a prophecy that was immediately testable. It was true. Samuel’s prophecy was equally testable, but only after five years. The problem with prophesies like that is that it is hard to know just how much time five years would be. Was it to be five years precisely, or no longer than five years, or perhaps somewhere within the fifth year?
That meant that while it was specific, it was difficult to time. As with other prophecies, it was really only understood once it was fulfilled. The signs that were to accompany the birth would have been unmistakable. Just as important as the fact of the signs was the nature of the signs. There would be light for a day and a night and a day. There were to be “many signs and wonders in heaven.” These signs are all celestial, or heavenly. By their nature they do not belong to earth, and therefore highlight an event that does not belong to the earth, but to heaven. They are fitting for when God comes to earth.