Shabbos 34b: How long is Bein Hashmashos? Rabbah said in the name of Rav Yehuda who said in the name of Shmuel: ¾ of the amount of time it takes to walk a mil.
Shulchan Aruch Orach Chaim 261:2: Some say that one must start keeping Shabbos early, and the period when one can add is from the beginning of sunset, when the sun is no longer visible, until bein hashmashos, which is a period of 3.25 mil. One may start keeping Shabbos early for some or all of this period, as long as he adds some part of the definite weekday onto Shabbos. And the length of bein hashmashos is ¾ of a mil, or 1500 amos walking time, before nightfall.
שבת לד ע”ב: שיעור בין השמשות בכמה? אמר רבה אמר רב יהודה אמר שמואל: שלשה חלקי מיל.
שולחן ערוך או”ח רס”א ס”ב: י״א שצריך להוסיף מחול על הקודש וזמן תוספת זה הוא מתחלת השקיעה שאין השמש נראית על הארץ עד זמן בין השמשות והזמן הזה שהוא ג׳ מילין ורביע רצה לעשותו כולו תוספת עושה רצה לעשות ממנו מקצת עושה ובלבד שיוסיף איזה זמן שיהיה ודאי יום מחול על הקודש. ושיעור זמן בין השמשות הוא ג׳ רבעי מיל שהם מהלך אלף ות״ק אמות קודם הלילה.
On the last page of one of the volumes of the Chasam Sofer’s commentary on Shas (the volume on Pesachim, Hilchos Pesach, Megillah and Lulav), we find the following words: “One must know that we hold that from sunset to the emergence of the stars is 24 minutes or 35 minutes, and we follow the stricter of the two … and the lighting of Shabbos and Chanukah candles on Erev Shabbos must be done before sunset, which means 35 minutes before the emergence of the stars…”
When the Chasam Sofer’s chiddushim were reprinted in 1954 by the Debreciner Rav, this paragraph was omitted. Some people protested against this censorship with a poster hung in Williamsburg, showing the original text side-by-side with the censored version, and saying, “Recently, people have arrived in this country keeping the chumra of their country of origin, to keep Shabbos until 72 minutes after sunset. Of course, they have the right to do so, and may they be blessed for it. The problem, however, is that they enter Shabbos late and do work after sunset, whereas the custom here is to start Shabbos 18 minutes before sunset. This creates confusion and disunity among religious Jews: one person is going to daven while the other is carrying his wallet and going to the mikvah. How do they square themselves with the Mishnah, “One must accept the stringencies of the place where he arrives, and one should not deviate so as not to cause fighting.”
And the main problem is that when [those following the earlier zman] publicized the Chasam Sofer’s ruling, it bothered these people so much that they had the audacity to tamper with the Torah of Moshe and tear out this page, replacing it with this falsified version. This is a tremendous, terrible chutzpah!!! We are upset at the Reform movement for removing prayers from the Siddur. But how are we any different, if we also practice censorship, and someday, if someone doesn’t like a halacha in Shulchan Aruch, he can simply omit it or change it? This leads to the destruction of all our holy sefarim.”
The Debreciner Rav wrote a letter responding that the sefer was censored by someone else without his knowledge. After the Debreciner Rav passed away in 1997, Rabbi Meir Amsel, editor of Hamaor, wrote that the Debrecener Rav had told him that the Satmar Rebbe had commanded him to delete this section.
Rabbi Yisroel Dovid Harfenes explained that the Satmar Rebbe was not practicing censorship here. On the contrary, it was the previous printers of the Chasam Sofer who had committed a falsification by inserting this section, which was actually the opinion of the Chasam Sofer’s teacher, Rabbi Nosson Adler. It was first printed by the Likutei Chaver Ben Chaim, quoting notes written by the Maharam Schick of what the Chasam Sofer told him in 1786, in his teacher’s name.
The Chasam Sofer himself disagreed with this ruling, as can be seen in his teshuva in Orach Chaim 80. There he was asked about when to make the bris for a baby born at 8:30 PM on Shabbos, the 28th of Sivan 5563 (1803), which he calls “bein hashmashos, on a cloudy day such that it was impossible to see whether the stars were out.” His response was that we hold like Rabbeinu Tam that bein hashmashos (the uncertain period between Shabbos and weekday) is the last ¾ of a mil, where a mil is 22 minutes, before the stars are visible. This period starts from the “end of sunset” (the term used by Rabbeinu Tam for the point in time that is 3 and ¼ mil after the sun sets below the horizon). Thus the uncertain period is between 71.5 and 88 minutes after sunset. He adds that the practice in these countries is that we do work on Friday until “an hour or 45 minutes” before the stars emerge. Thus we accept Rabbeinu Tam as the definite halacha, even to be lenient. We don’t take into account Rabbeinu Tam’s opponent, the Re’em, who holds that beis hashmashos is the last 16.5 minutes before visible sunset.
Now, on that particular day, the Chasam Sofer writes that nightfall was at 8:55 PM. The uncertain period thus began 16.5 minutes earlier, at 8:38 PM. If the baby was born at 8:30, then he was definitely born on Shabbos and the bris should be on Shabbos.
This teshuva, saying that we do work and light candles after sunset on Friday, and that Shabbos ends 88 minutes after sunset, definitely disagrees with Rabbi Nosson Adler’s statement that candles are lit before sunset and Shabbos ends at most 35 minutes after sunset. The teshuva was written in 1803, as opposed to the quote from his teacher which was written down in 1786, so it is more authoritative. Therefore, the Satmar Rebbe asked for the early zman quote to be deleted so as to more faithfully represent the Chasam Sofer’s opinion.
Source: Changing the Immutable, p. 104; Yisroel Vehazmanim, p. 873
[Why does the Chasam Sofer say that we do work on Friday until 45 minutes before the stars become visible, when actually one may work until 16.5 minutes before the stars? The answer is probably that the Jews of his time and place used to accept Shabbos early, following the Shulchan Aruch’s statement that one should add on to Shabbos. In any case, the fact that they worked after sunset shows that they followed Rabbeinu Tam; therefore the baby born 25 minutes before the stars emerged would be considered definitely born on Shabbos.
It also seems that the Chasam Sofer does not count 88 minutes forward from sunset to determine when tzeis hakochavim is; rather he goes by the actual appearance of the stars, and then counts 16.5 minutes backward to determine when bein hashmashos occurs. This is evident from his words at the outset of the teshuva that the baby was born “on a cloudy day such that it was impossible to see whether the stars were out.” Why not just determine tzeis hakochavim by counting from sunset? It seems that sunset was not visible from his location, or else it wasn’t clear to him which exact moment Chazal call sunset. Similarly, at the end of the teshuva he states that the stars came out at 8:55 PM that day, and does not state when sunset occurred.
It’s also notable that the Chasam Sofer doesn’t even mention the opinion, advocated by the Gra and later popularized by the Mishnah Berurah, that bein hashmashos begins at sunset. To the Chasam Sofer, the only two opinions on this subject were Rabbeinu Tam and the Re’em.]