Aurora means dawn, that's well known. But there is more than one type of dawn. The English Wikipedia knows three types: astronomical (18°), nautical (12°) and civil dawn (sun 6° below the horizon). Latin Wikipedia knows - in the stub article Aurora - only the definition 6° below the horizon.
I am searching for an exakt definition (or a rule of thumb) for the time of aurora in canon law. As it is used here:
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]
So the priest has to know when aurora is. There could be an specific canon-law definition or just the normal, traditional latin/roman definition. So I am interested in every definition avaivible, best canonistic/ecclesiastical, but ancient or medieval is okay too. It could use brightness, degree below the horizon or some other criterion.
I asked a similar question on christianity.SE, but got no good answer.