The gemorah says that a traifah who kills in front of bais din is killed, since bais din directly witnesses the act, but if witnesses bring this testimony to bais din, the killer will not be given the death penalty since this is a testimony that can't lead to hazama.
It seems from this gemarah (as well as elsewhere in this perek) that bais din that sees an event can act upon what they see without the requirement for witnesses.
However, the mishnah in Rosh Hashana states that if bais din sees the new moon, two of the members of bais din need to act as witnesses and others are appointed to the bais din to complete the required number.
How can we resolve the apparent difference - in Sanhedrin testimony of witnesses is not required in front of a seeing bais din, but in Rosh Hashana testimony of witnesses is required for a seeing bais din?
Naftoli Willner, Baltimore
Sholom Rav,
The Mishnah in Rosh Hashanah is speaking where the Beis-Din saw the new moon at night (See Bartenura), when Kidush ha'Chodesh cannot be performed, which is why testimony is required the following day.
Had they seen it during the day, they would have been permitted to declare Rosh Chodesh on the basis of what they saw (because 'hearing is not better than seeing').
The answer to this question can be found in Tosfos here DH Bifnei. Tosfos writes that the Beis Din saw the treifah kill in the daytime. In contrast, the Gemara Rosh Hashanah 25b states that the witnesses saw the New Moon at night. The difference is that if they saw it happen during the day the Beis Din can judge immediately on the basis of what they saw and do not require witnesses to testify; but if they saw it at night the Beis Din cannot act immediately because one cannot conduct a Din Torah at night. They must wait until the daytime, when they will require witnesses. Since they require witnesses this is impossible against a treifa that killed, because it is a testimony that cannot lead to hazama. [see also Rashi Bava Kama 90b DH She-Ra'ou]
KOL TUV
Dovid Bloom