Before the Motu Propio "Sacram Communionem. On Laws of Fasting and the Evening Mass" of Pope Pius XII in 1957, the rule of c. 821 § 1 CIC/1917 applied in the Catholic Church as a ruling on the times for celebrating mass. The codex is from 1917, but I suspect the rule is older (maybe Council of Trent?).
c. 821 § 1 CIC/1917: Missae celebrandae initium ne fiat citius quam una hora ante auroram vel serius quam una hora post meridiem.
The beginning of the celebrated mass shall not be before one hour before aurora and later than one hour after midday. [my translation]
What does aurora means in this context? Is it sunrise, the beginning of the red sky at morning or something else? Was there a definition or practical formula in the canonical literature? As this question was practically relevant for every priest there should be at least some rule of thumb.
This question is different from Can Mass be celebrated at any hour of the day?. The scope of that question is broader and in Geremia's Answer the word aurora is just translated with sunrise without explanation. I want the details here.