tzais72
This method returns the tzais (nightfall) based on the opinion of Rabbeinu Tam that tzais hakochavim is calculated as 72 minutes, the time it takes to walk 4 Mil at 18 minutes a Mil. According to the Machtzis Hashekel in Orach Chaim 235:3, the Pri Megadim in Orach Chaim 261:2 (see the Biur Halacha) and others (see Hazmanim Bahalacha 17:3 and 17:5) the 72 minutes are standard clock minutes any time of the year in any location. Depending on the isUseElevation setting) a 72 minute offset from either sunset or sea level sunset is used.
Return
the Date representing 72 minutes after 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.