M - morning/eastern to Sun, first visibility with outers
E - evening/western to Sun, last visibility with outers
For a modern astronomer's way of computing arcus visionis, see https://academic.oup.com/mnras/article/84/9/731/951742
Our fellow astrologer Kolev, however, disagrees with Schoch - http://www.babylonianastrology.com/downloads/Ancient_Priest_vrs_Modern_Scholar.pdf
It's a very tentative area and mostly overlooked by astronomers, since it takes into account many factors as well as the local atmosphere. I would, however, disagree with Kolev's method of taking atmosphere astrologically (but not astronomically), since we strive to observe not appearances and clouds, but the ambient. So, I would prefer to see a precise theoretical model built only on superior/inferior conj and latitude for an average quality atmosphere, but unfortunately there is not enough interest it seems and I don't have the necessary skill to do so myself.
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