sof
This method returns the latest zman krias shema (time to recite Shema in the morning) calculated as 3 hours (regular clock hours and not sha'os zmaniyos) before ZmanimCalendar.chatzos.
Generally known as part of the "Komarno" zmanim after Rav Yitzchak Eizik of Komarno, a proponent of this calculation, it actually predates him a lot.
It is the opinion of
the Shach in the Nekudas Hakesef (Yoreh Deah 184),
Rav Moshe Lifshitz in his commentary Lechem Mishneh on Brachos 1:2.
It is next brought down about 100 years later by the Yaavetz
(in his siddur, Mor Uktziah Orach Chaim 1,
Rav Yitzchak Eizik of Komarno in the Ma'aseh Oreg on Mishnayos Brachos 11:2,
Shevus Yaakov,
Chasan Sofer
and others.
See Yisrael Vehazmanim vol. 1 7:3, page 55 - 62.
A variant of this calculation (sofZmanShmaGRASunriseToFixedLocalChatzos) uses fixed for calculating this type of zman.
Return
the Date of the latest zman krias shema. 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.