Parshas Lech Lecha (and Vayeira) are always good weeks to talk about the inyanim of bris mila. This week I spoke about a few interesting shailos regarding Bris Mila and Bein HaShemashos.
Just as a brief hakdama, there are two main opinions regarding when nightfall starts. The shitta of the Geonim (and the GR"A) is that we hold that tzeis is 3/4 of a mil after shkiah and Bein Hashemashos lasts for 3/4 of a mil and is the time between shkiah and tzeis. . Rabbeinu Tam however holds that tzeis is 4 mil after shkiah and Bein Hashemashos is the last 3/4 of a mil before his tzeis. The time before Bein Hashemashos (3 1/4 after shkiah) is considered day.
To put this in terms we can understand, let us say a mil is 18 minutes. Also, lets assume sunset (what we call shkiah) is 6:30. According to the Geonim, Bein Hashemashos lasts for (3/4) * 18 or 13.5 minutes, which means tzeis is 6:43 and 30 seconds. According to Rabbeinu Tam, tzeis is 4 mil or 72 minutes after shkiah which would be 7:42. Bein Hashemashos doesn't begin until 3 1/4 mil or 58.5 minutes after shkiah. So Bein Hashemashos for Rabbeinu Tam is from 7:29 30 seconds until 7:42. The time between 6:30 and 7:29.30 is daytime according to Rabeeinu Tam.
Now for the questions. Assume shkiah is 6:30.
We pasken that if a baby is born Bein Hashemashos, the bris is the next day.
1) Let's say a child is born Tuesday night 5 minutes after shkiah at 6:35. According to the Geonim, the baby is born Bein Hashemashos and the bris should be Wdnesday. According to Rabbeinu Tam, the baby was born on Tuesday and the bris should be Tuesday. Rav Moshe writes that this is a case of sefek sfeika. Safeik who we pasken like and even if we pasken like the Geonim, it is a safeik what day it is since it would beBein Hashemashos acc. to Geonim. Rav Moshe wants to say that a safeik in din and safeik in metzius creates a sefek sefeika. Therefore, Rav Moshe says the bris should be Tuesday. However, if the baby was born Motzei Shabbos at 6:35 then Rav Moshe says because of chumra of Shabbos we don't make the bris on Shabbos but do it Sunday. But if one wants to do the bris on Shabbos we don't stop them.
This case happened to me. My son was born a few minutes after shkiah on Thursday, but the Rav I asked said the bris should be on Friday.
Also, Rav Moshe's sefek sfeika would not apply when the baby is born at 7:00. In this case according to the Geonim it is night and Rabeeinu Tam it is day so there is no sefek sefeika.
2) Let's say the baby was born Friday night at 7 pm. According to the Geonim the bris is Shabbos and acc. to Rabbeinu Tam the bris is Friday. The father follows the Geonim and wants to make the bris on Shabbos. However, the mohel follows Rabbeinu Tam. Can the mohel do the bris since it would involve chilul Shabbos. Rav Yechezkel Roth at first says the mohel can't do the bris. But then he says there is a shitta of the Re'ah that if a mohel does a bris sh'lo bizmano on Shabbos it is only mikalkeil. Only the father has a mitzvah on a mila shelo bizmano and since there is no mitzvah for the mohel it is mikalkeil. Therefore, the mohel could do the bris in our case. I don't understand it since mikalkeil is still chayav m'd'rabanan.
3) If a baby is born Friday before shkiah but after you were m'kabeil Shabbos the bris is Friday. Kabbalas Shabbos is meaningless in this case. We follow the day. A similar question is if a Friday bris was delayed and you were mikabeil Shabbos before shkiah and then you were able to do the bris. Do you do the bris? The Taz paskens in Siman 600 you do the bris. However, the Minchas Kohein in Ma'mar 2 Perek 1 disagrees and says you are machmir for chilul Shabbos.