The Rabbenu Taams’s view is that there are two shkios, one the normal sun set and 72 min after that is another shkiah when it is nigth. 13 1/2 min before the second shkiah or 58 1/2 min after the first shkiah is ben hashmoshos. There is the Geonim’s view that 13 1/2 min from the shkiah is night. The yereim’s view is earliest mostly followed Erev Shabbos that 18 min before is ben hashmoshas. Ben Hasahmoshos is 3/4 of a mil which is an argument in its measurement. Some hold 18 min, 22.5 or 24 min. The Geonim and Rabbenu Tam go by 18 min and the Yereim by 24 min. The GRA holds 22.5 min.
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