tzais90
Method to return tzais (dusk) calculated as 90 minutes after sea level sunset. This method returns tzais (nightfall) based on the opinion of the Magen Avraham that the time to walk the distance of a Mil according to the Rambam's opinion is 18 minutes for a total of 90 minutes based on the opinion of Ula who calculated tzais as 5 Mil after sea level shkiah (sunset). A similar calculation tzais19Point8Degrees uses solar position calculations based on this time.
Return
the Date representing the time. If the calculation can't be computed such as in the Arctic Circle where there is at least one day a year where the sun does not rise, and one where it does not set, a null will be returned. See detailed explanation on top of the AstronomicalCalendar documentation.