What does it mean when R' Eliezer says that there is a mitzva of kidush shanim (for yovel) and not kidush hachodesh? The only answer that I found was R' Chananel who says that the mitzva of kidush hachodesh is derabanan, as opposed to kidush shanim which is deoraita. Where does it come from that kidush hachodesh is derabanan? I couldn't find any answers in the standard rishonim or in the Rambam.
Chana Levin, Neve Daniel, Israel
The Gemara learns from a Pasuk that Kidush ha'Chodesh is not Min ha'Torah. This means that the Sanhedrin do not need to say the word "Mekudash" in order to pronounce the new month. However, they would still need to make the calculation as to when the new month will fall, and the new month will be according to the time they fix. See Ramban Sefer ha'Mitzvos Asei 152 and Minchas Chinuch 4.
Rabeinu Chananel writes that Rebbi Eliezer still agrees that Kidush ha'Chodesh is a Mitzvah d'Rabanan. This would appear to be because Rabeinu Chananel is of the opinion that Rebbi Eliezer does not argue with the Mishnah in Rosh ha'Shanah that the Beis Din used to say Mekudash.