tzais72Zmanis
Method to return tzais (dusk) calculated as 72 minutes zmaniyos, or 1/10th of the day after sea level sunset. This is the way that the Minchas Cohen in Ma'amar 2:4 calculates Rebbeinu Tam's time of tzeis. It should be noted that this calculation results in the shortest time from sunset to tzais being during the winter solstice, the longest at the summer solstice and 72 clock minutes at the equinox. This does not match reality, since there is no direct relationship between the length of the day and twilight. The shortest twilight is during the equinox, the longest is during the the summer solstice, and in the winter with the shortest daylight, the twilight period is longer than during the equinoxes.
Return
the Date representing the time. 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.