tzais
This method returns the tzais (nightfall) based on the opinion of the Geonim as calculated by Rabbi Yechiel Michel Tucazinsky. It is based on of the position of the sun no later than 31 minutes after sunset in Jerusalem the height of the summer solstice and is 28 minutes after shkiah around the equinox / equilux. This computes to 6.45 below the western horizon.
Return
the Date representing the time when the sun is 6.45 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.