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The time lapse between sunset and nightfall (i.e. the length of twilight) depends mainly on the observer's latitude and the time of year (and also, to a lesser degree, on atmospheric conditions.) Somewhere, Rabbi David Tzvi Hoffman writes that the degree of sky darkness at nightfall (tzeit hakochavim) occurs when the Sun's depression angle below the horizon is 7°5' (the time of which can be calculated for any place and date). Does anyone know where that ruling of his is to be found (presumably in his Melamed L'ho'il)?

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It can be found in Melamed LeHo'il I:30.

The measure of darkness of 7°5' for tzeit hakochavim was proposed by a R. Baruch HaKohen of Rawicz to R. Hoffmann. After some back and forth, R. Hoffmann writes:

לענין לוחותיך על זמן צאת הככבים אין לפקפק, רק טוב שתעיר על זה שאין השיעור בצמצום כולי האי ופורתא לא דק לחומרא.‏

Regarding the time for tzeit hakochavim in your calendars [7°5'] there is nothing to dispute, although it would be good if you were to note that it is not totally precise, and one should be stringent and leave a small margin.

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  • Thank you, Joel, for your reply giving the reference and for the translation. About the latter, I think what you translated as "and leave a small margin," are the words "ופורתא לא דק לחומרא," which, but for the presence of the word לא, I would have translated as "and allow a small fraction of a minute (דק) as a margin for extra stringency." But why is the word לא there?
    – Moongazer
    Apr 8 at 23:57
  • @Moongazer To be honest, I'm not totally confident that I've translated that phrase in the way R. Hoffmann intended. In any event, he's using Talmudic language - see for example Sukkah 8a where we have the phrases לא דק פורתא (he was a little imprecise) and לחומרה לא דק (he was imprecise in the direction of stringency). I don't think R. Hoffmann is using the word דק here to mean a minute. Rather לא דק has the connotation of being imprecise.
    – Joel K
    Apr 9 at 7:24

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