tzais
A method that returns tzais (nightfall) when the sun is 6 below the western geometric horizon (90) after sunset. For information on the source of this calculation see ZENITH_6_DEGREES.
Return
The Date of nightfall. 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.