The Chasam Sofer Shut O’CH 80 questions why don’t we light late like the Rabbenu Tam. Could be that by a bris as in SA YD 266 Shach s’k 11, if the child is born within the 72 minutes we don’t make the bris on shabbos but push it away to the next day. In order to satisfy everyone, the Rabonon got together to light early erev shabbos and wait 72 minutes motzei shabbos or some 50 minures like Rav Moshe. The Geonim pasken that 13 1/2 minutes after shkia (3/4 of a mil, 18) starts bein hashmoshas after which is night. The Rabbenu Tam’s view is that there is another shkia and 72 minutes after is night so 58 1/2 minutes after the first shkia is ben hashmoshas. The Yereim, Rebbi Eliezer Mimetz’s view is that 13 1/2 minutes before shkiah is ben hashmoshas which we follow erev shabbos taking a mil 24 min and 3/4 is 18.
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