Question: Having recently been inspired by the Siyum HaShas, I am attempting to learn Daf Yomi. I have two questions on Daf Gimmel. The first is: Rebbi Eliezer's opinion is obviously one that was offered at a time when the Beis HaMikdash was standing. Yet, the proof as to the existence of the Ashmuros is taken from a possuk in Yirmiahu, (written after the Churban HaBayis). What is the reason the Gemorah had to bring this proof?
The second question regards the end of daf Gimmel. We see that Dovid suggested that Israel go to was in order to feed the poor of the nation. Before acting on Dovid's advice K'lal Yisroel asked Eitza and went to the Sanhedrin. Rashi explains that the Sanhedrin was asked to pray for permission and to pray for the success of the project. Afterwards they went to the Urim v'Tumim to find out if their endwavor would be successful. Why did they need to do both? Why not just go directly to the Urim v'Tumim?
(a) The Gemara cites a Beraisa in which Rebbi Eliezer himself explicitly states that there are three Ashmuros in the night, and during each Ashmurah, Hash-m, as it were, roars like a lion. These Ashmuros are heavenly watches, as Rashi points out (DH Iy Kasaver). There is no reason to assume that the heavenly watches are different when the Beis ha'Mikdash is standing than when it is not. Of course, there are differences in the heavenly spheres (see, for example, Chagigah 13b), but the number of Ashmuros is not one of them.
(b) Rashi says that they went to the Sanhedrin to ask permission, so that the Sanhedrin would pray for their success. Asking permission was a formal requirement of national etiquette. Once the people showed their reverence to the spiritual leaders of the nation, those leaders would then pray for their success. The prayer, however, was not a prophecy of their success, but was certainly necessary.
Next, they inquired of the Urim v'Tumim if they would be successful. This was a prophecy and not a prayer. Why didn't they go to the Urim v'Tumim first to find out if they would be successful, and then go to the Sanhedrin to pray for them?
The Urim v'Tumim was to be used only after all other measures of preparation had been taken. It was a drastic measure to ask for a prophecy, and therefore it was done only after everything else had been done. We might also suggest that if they went to the Urim v'Tumim first and were informed that they would be successful, they would be so confident that they would fail to ask the Sanhedrin permission so that the Sanhedrin would pray for them. They would thus be lacking the critical component of Bitachon in Hash-m for their success.