R' Akiva Blum wrote:<br><<br><pre>I don't understand this cheshbon.<br>Accoring to RT, visable shkiah is irrelavant. It should follow that calculating the length of a 'hour' would include the 72 minutes after shkiah and 72 before haneitz. Total minutes in a day is 864 (instead of 720 acc. To Gra).
<br>One hour is 864/12=72 minutes. One and a quater hours is 72*1.25=90 minutes. So plag is 90 minutes before 72 after shkiah which is... 18 minutes (zmanios, in the winter) before shkiah.<br><br><Where did I go wrong?
<br><br>The Rishonim (Ramban, Rashba) say explicitly that plag hamincha is 1/6 of a mil before shkia which is approximately 3 minutes before shkia. Based on this and other calculations R' Willig (he printed this in his sefer on berachos and Shabbos) claims that according to these Rishonim a mil is
22.5 minutes and therefore tzeis is 90 minutes after shkia.<br></pre>