tzais
This method returns tzais (nightfall) based on the opinion of the Geonim calculated as 45 minutes after sunset during the summer solstice in New York, when the neshef (twilight) is the longest.
The sun's position at this time computes to 7.75˚ below the western horizon.
See
Igros Moshe Even Haezer 4, Ch. 4 (regarding tzais for krias Shema).
It is also mentioned in Rabbi Heber's Shaarei Zmanim on in chapter 10 (page 87) and chapter 12 (page 108).
Also see the time of 45 minutes in Rabbi Simcha Bunim Cohen's The radiance of Shabbos as the earliest zman for New York.
This zman is also listed in the Divrei Shalom Vol. III, chapter 75,
This zman is also listed in the Divrei Shalom etc. chapter 177 (FIXME - could not be located).
Since this zman depends on the level of light, Rabbi Yaakov Shakow presented this degree-based calculation to Rabbi Rabbi Shmuel Kamenetsky who agreed to it.
Return
the Date representing the time when the sun is 7.67 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.