1) I recall reading that the new moon is not visible at nighttime, only around sunset. Is that accurate?
2) How long is it after the old moon disappears before it is visible again?
3) Where was the courtyard that the people gathered in to give eidus? Was it on the premises of the Beis HaMikdash?
4) Where did the dayanim sit (the Mishnah calls it the aliya)?
Thanks again,
Dan
1) The new moon sets immediately after sunset. (That is what I meant by comment #2 that I sent to you earlier in a private communication.) After it is under the horizon it cannot be seen. When it rises (about 12 hrs. later) it is already daybreak, and since the moon is small, it is usually not visible until the following nightfall.
(On other days of the month, the moon is visible for about 12 hrs. out of every day, some of those hours by night and some by day.)
2) You mean to ask how long after the scientific Molad , I presume. (That is, the moon disappears the same number of hours before the Molad as it takes for it to reappear after the Molad).
Rashi in Rosh Hashanah 20b says that it can theoretically be seen at night even 6 hours after the Molad. The Rambam gives it 18 hours.
Practically speaking, recent naked-eye moon-sighting records are in the 14-hour range. But bear in mind that the Halachic Molad is not identical to the scientific one. It is a mean Molad, and can vary be about 3 hours from the true Molad. (In addition, the calculations we make are not as accurate as the scientific calculations, and our Molad has drifted a bit over the years from the true Molad.)
3) The place designated for reviewing the testimonies was called "Makom ha'Vaad" (Rosh Hashanah 31a). The Seudos were made within the Mechitzos of that Makom ha'Vaad, so that Edim who came on Shabbos from out of the Techum could go there. If the Sanhedrin accepted the witnesses in Lishkas ha'Gazis, then it is possible that the feasts were given on Har Habayis. It is possible, though, that they saw the witnesses elsewhere - but it is clear that it was in Yerushalayim (see Baal ha'Me'or, Beitzah 6a).
Of course, later the Sanhedrin left Yerushalayim and the Makom ha'Vaad moved with it. In principle, the witnesses could have been received, and the feasts celebrated, anywhere in Eretz Yisrael (see Rambam in Milchamos, ibid.)
4) In general, Batei Midrash and Batei Din were convened in an upper story room, or "Aliyah". (See, for example, Rashi Rosh Hashanah 11a DH Hashkimu Li, and 10a DH Amudei.)
Best wishes,
Mordecai Kornfeld