Parshat Emor - Appointing a cohen gadol

The Rambam says in Hilchos Klei Hamikdash (4:15):

אין מעמידין כ"ג אלא ב"ד של שבעים ואחד

“Only the beis din hagadol in Yerusholayim can appoint a cohen gadol.”

However, when the Rambam enumerates the things that require a beis din of 71 at the beginning of the 5^th perek of Hilchos Sanhedrin, although he mentions appointing a king, he does not mention appointing a kohen gadol. Why is this?

Dayanim today

The gemara in Bava Kamma (88) explains that we do not have semicha in an unbroken line from Moshe Rabbeinu. Therefore, we are not qualified to judge any case where the Torah requires dayanim. Nevertheless, the chachamim said that in order to maintain law and order we still judge any type of monetary  case which is frequent and which involves a monetary loss . The gemara explains that we can do this becuase שליחותייהו עבדינן  - beis din today are the sheluchim of the dayanim who did have semichah in an unbroken line from Moshe Rabbeinu.

The Rashba in Bava Kamma asks, how can we be mekabel gerim, this also requires a beis din? The Rashba answers (Bava Kamma 88a) that for kaballas gerim we also say that we are the sheluchim of previous batei din. However the Chiddushei Harim asks that the Rashba in Yevamos (46b) says that the reason that we can accept gerim today is because you do not need expert judges, and not because we are sheluchim?

Rb Naftoli Trop z”l (Sanhedrin siman 195) explains as follows;

Whenever a מומחה  (expert judge) is required to judge a case, the judge has to have two qualities. He needs to beגמיר וסביר  (learned and be able to reason in halacha) and he also needs to be סמוך (have semicha in a line from Moshe). Rb Naftoli Trop z"l explains that this is becuase a dayan needs to have both the knowledge (גמיר וסביר) and also the authority (נקיט רשותא) to judge.

Rb Naftoli Trop z"l explains that the halacha of שליחותייהו קעבדינן (we are sheluchim of the original batei din) does not remove the requirement for a dayan to be a mumcheh (for cases where this is required). Rather, שליחותייהו קעבדינן only addresses the requirement for a dayan to be נקיט רשותא - authorised to judge. Therefore, in order to explain how beis din can be mekabel gerim today, the Rashba has to cover both the requirement for a dayan mumcheh to be gamir vesavir and also the requirement for a dayan to be nakit reshusah. Regarding the requirement to be gamir vesavir, the Rashba in Yevamos explains that dayanim required for a beis din for kabbalas gerim do not need to be mumchin (experts). However, they are still a beis din and need reshus to judge, whch is why the Rashba in Bava Kamma adds that regarding giyur they are the sheluchim of previous batei din.

Using this explanation, we can answer why the Rambam mentions the requirement for a beis din of 71 to appoint a cohen in Hilchos Beis Habechirah but not in Hilchos Sanhedrin. This is becuase there is no pesak halacha (halachic decision) that is made when beis din appoint a cohen gadol. The reason that the beis din in Yerusholayim is required to appoint a cohen gadol is because the beis din hagadol has the authority to appoint a cohen gadol on behalf of klal yisroel. In other words, this is only a din that requires reshus and is not a din that requires beis din to excercise their expertise of gamir vesavir. The Rambam in Hilchos Sanhedrin, however, only enumerates cases where the beis din need both authority and also have to make a pesak din, and therefore he does not mention there appointing a cohen gadol.