When Is P’lagh HaMinha

Question
In a shiur—http://machonshilo.org/en/eng/list-ask-the-rav/55-shabbath-yom-tov/388-making-shabath-early—the Rav spoke of making Shabbath early, i.e. davening Minha before Plag HaMinha and Arvit (the Shemona Esre) after Plag HaMinha. My Question is by which opinion does the Rav hold: a) Gra & Baal Tanya, or b) Magen Avraham using “Fixed 72 minutes…”, or both, i.e. Minha before the earlier time and Arvit after the later?

Answer
The Halakhic day is divided into 12 equal parts or ‘Halakhic hours’ also known as Sha’oth Z’maniyoth. According to the shitta usually associated with the Gra and the Ba’al HaTanya, the time from sunrise to sunset is divided into 12 parts. This is the opinion one should follow.
According to the other shitta usually associated with the Maghen Avraham, the time from dawn (first light) till Sseth HaKokhavim is divided into 12 equal parts. Most people do not realize that this view can only work according to the shitta of Rabenu Tam and other Rishonim regarding the onset of nightfall or Sseth HaKokhavim (SH), because the time from to dawn to sunrise is equal to the time from sunset to Rabenu Tam’s SH (which refers to the total absence of light in the sky including the western horizon, which is the inverse of ‘Aloth HaShahar (AH) or first light on the eastern horizon in the morning). On an average day in EY this corresponds to approximately 65-70 minutes. According to this view, Hassoth HaYom (HH) or midday, i.e. the mid-point of the sun’s transit across the sky from east to west, corresponds to the end of the 6th hour. If, however, one divides the elapsed time from AH until the appearance of three stars in the evening (SH according to the Gra and Ba’al HaTanya) which occurs much earlier than Rabenu Tam’s SH, HH will not correspond to the end of the 6th hour.
The opinion of Rabenu Tam and many other Pos’qim regarding nightfall is based on a misunderstanding of the TM P’sahim 93b-94a, a fact which was pointed out by the MaHaRal, the Gra, the Ba’al HaTanya, the Hazon Ish and many other Hakhamim. It also contradicts what we observe with our own eyes, a point stressed by all the above-mentioned Pos’qim. There are numerous proofs that this shitta is mistaken, but I refrain from elaborating here.
It follows, therefore, that the Maghen Avraham’s opinion regarding Sha’oth Z’maniyoth must be rejected.
Rabbi David Bar-Hayim