Good answer, because there's no way Indy could have known the precise month or year the Map Room was built. The sun's angle changes throughout the day, making t

Good answer, because there's no way Indy could have known the precise month or year the Map Room was built. The sun's angle changes throughout the day, making this a very time-sensitive operation. If Indy were off by even a few minutes, his calculations would be inaccurate. Also: lack of scale would come into play. One would need much more precise dimensions of the staff, the height of the structure, and the angle of the sun's rays with much greater accuracy. Indy eyeballing it, as he does in the movie, wouldn't get him close to the location of the chamber. Indy and Sallah and crew would have needed to do a lot more digging and exploration to locate the Ark chamber (but of course, that wouldn't have made for a good movie, which is why such events are time-compressed).