tzais
This method returns the tzais (nightfall) based on the opinion of the Geonim calculated as 3/4 of a 24-minute Mil, based on a Mil being 24 minutes, and is calculated as 18 + 2 + 4 for a total of 24 minutes. It is the sun's position at 5.88˚ below the western horizon. This is a very early zman and should not be relied on without Rabbinical guidance.
Return
the Date representing the time when the sun is 5.88 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.