alos60
Method to return alos (dawn) calculated as 60 minutes before sunrise.
This is the time to walk the distance of 4 Mil at 15 minutes per Mil (4 * 15 = 60).
Opinions on how long it takes to walk a mil:
This (15 minutes per mil) seems to be the opinion of the Chavas Yair in the Mekor Chaim, Orach Chaim Ch. 90, though the Mekor Chaim in Ch. 58 and in the Chut Hashani Ch. 97 states that a person walks 3 and a 1/3 mil in an hour, or an 18-minute mil.
Also see the Divrei Malkiel Vol. 4, Ch. 20, page 34) who mentions the 15 minute mil lechumra by baking matzos.
Also see the Maharik Ch. 173 where the questioner quoting the Ra'avan is of the opinion that the time to walk a mil is 15 minutes (5 mil in a little over an hour).
There are many who believe that there is a ta'us sofer (scribe's error) in the Ra'avan, and it should 4 mil in a little over an hour, or an 18-minute mil.
Time based offset calculations are based on the opinion of the Rishonim who stated that the time of the neshef (time between dawn and sunrise) does not vary by the time of year or location but purely depends on the time it takes to walk the distance of 4 mil.
tzaisGeonim9Point75Degrees is a related zman that is a degree-based calculation based on 60 minutes.
Return
the Instant 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.