tzais
This method returns the tzais (nightfall) based on the opinion of the Geonim calculated as 60 minutes after sunset around the equinox / equilux, the day that a solar hour is 60 minutes in New York. The sun's position at this time computes to 9.75˚ below the western horizon. This is the opinion of Rabbi Eliyahu Henkin. This also follows the opinion of Rabbi Shmuel Kamenetsky. Rabbi Yaakov Shakow presented these degree-based times to Rabbi Shmuel Kamenetsky who agreed to them.
Return
the Date representing the time when the sun is 9.75 below sea level. 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.