tzais16Point1Degrees
This calculates the time of tzais at the point when the sun is 16.1 below the horizon. This is the sun's dip below the horizon 72 minutes after sunset according Rabbeinu Tam's calculation of tzais around the equinox / equilux in Jerusalem. The question of equinox VS equilux is complex, with Rabbi Meir Posen in the Ohr Meir of the opinion that the equilux should be used. See Yisrael Vehazmanim vol I, 34:1:4. Rabbi Yedidya Manet in his Zmanei Halacha Lema'aseh (4th edition part 2, pages and 22 and 24) and Rabbi Yonah Metzbuch (in a letter published by Rabbi Manet) are of the opinion that the astronomical equinox should be used. The difference adds up to about 9 seconds, too trivial to make much of a difference. For information on how this is calculated see the comments on alos16Point1Degrees.
Return
the Date representing the time. If the calculation can't be computed such as northern and southern locations even south of the Arctic Circle and north of the Antarctic Circle where the sun may not reach low enough below the horizon for this calculation, a null will be returned. See detailed explanation on top of the AstronomicalCalendar documentation.