Rashi explains that Hashem showed Moshe an image of exactly how the moon should appear -- "hachodesh hazeh" -- to declare Rosh Chodesh. The Brisker Rav (stencil al haTorah) questions why Moshe needed a visual image of the moon to understand this halacha, as there is no shiur to the size of the new moon's appearance required to declare Rosh Chodesh.
If last week's post on Yerushalmi didn't whet your appetite for it enough, let me try to add some fuel to the fire. The Mishna in Rosh HaShana tells us that Rabban Gamliel would show images of the moon to witnesses to test if they were telling the truth. The Yerushalmi (13a in the Vilna ed., perek 3 halacha 4) on that Mishna has the following line:
כמה היה רחב כשעורה ויותר מכשעורה
The Pnei Moshe comments that the gemara is explaining Rabban Gamliel's test. Rabban Gamliel would ask the witnesses whether the width of the moon visible to them matched the image he showed them, whether it was the same shiur, or whether it was wider, in which case he knew they were lying.
The first thing that should grab your attention is simple spelling. The word "shiur" in Aramaic is שיעורא , the word that appears in the text of the Yerushalmi is שעורה . Maybe you can chalk that up to textual corruption and girsa, which is always problematic in the Yerushalmi, but there are logical problems with this pshat as well. The nature of Rabban Gamliel's test is a bit puzzling -- why was he concerned about the width of the moon more than its elevation or any other feature? And what does this gemara add to the Mishna, which already tells us that Rabban Gamliel had the witnesses compare their observations with images?
The Tziyun Yerushalayim therefore learns a different pshat here. The text means what it says -- שעורה is a piece of barley. The gemara is telling us information not found in the Mishna: even a sliver of a moon that appears no wider than a piece of barely is sufficient to declare kiddush hachodesh.
Now we can answer the question of the Brisker Rav: Moshe had to be shown the moon because there is a shiur for kiddush hachodesh! -- the moon must be at least the width of a barley grain.
I admit I did not remember this Yerushalmi off the top of my head before seeing it quoted, but I also know I would have ignored the mareh makom completely had I not been at least partially tuned into the idea that the Yerushalmi is understandable and not a resource to be overlooked. I wonder how the Brisker Rav learned this Yerushalmi, or did it just not enter the equation because the Rambam does not quote it.