bain
This method returns the beginning of bain hashmashos (twilight) according to the Yereim (Rabbi Eliezer of Metz) calculated as the sun's position 3.05 above the horizon around the equinox / equilux, its position 18 minutes or 3/4 of a 24-minute mil before sunset. According to the Yereim, bain hashmashos starts 3/4 of a Mil before sunset, and tzais /nightfall starts at sunset. Note that lechumra (of about 14 seconds) a refraction value of 0.5166 as opposed to the traditional 0.566 is used. This is more inline with the actual refraction in Eretz Yisrael and is brought down by Rabbi Yedidya Manet in his Zmanei Halacha Lema’aseh (p. 11). That is the first source that I am aware of that calculates degree-based Yereim zmanim. The 0.5166 refraction is also used by the Luach Itim Lebinah. Calculating the Yereim's bain hashmashos using 18-minute based degrees is also suggested in the upcoming 8th edition of the zmanim Kehilchasam. For more details, see the article The Yereim’s Bein Hashmashos.
Return
the Date of the sun's position 3.05 minutes before sunset. If the calculation can't be computed such as in the Arctic Circle where there is at least one day a year where the sun does not rise, and one where it does not set, a null will be returned. See detailed explanation on top of the AstronomicalCalendar documentation.