tzais
A method that returns tzais (nightfall) when the sun is 8.5˚ below the geometric horizon (90) after sunset, a time that Rabbi Meir Posen in his the Ohr Meir calculated that 3 small stars are visible, which is later than the required 3 medium stars. See the ZENITH_8_POINT_5 constant.
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.
See also
ComplexZmanimCalendar.getTzaisGeonim8Point5Degrees