Exact meaning of aurora in canon law?

Question

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.

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I asked a similar question on christianity.SE, but got no good answer.

Answer 1

I found a source suggesting the choice is the astronomical dawn, according to the definition you cite:

• This book, dated 1958, (FWIW, it was granted a nihil obstat, but I can't find any further assessment of its authority), gives the following definition of dawn, specifically in the context of the celebration of the Eucarist:

  In canone 821.1 legitur 'Missae celebrandae initium ne fiat citius quam una hora ante auroram' (...) Aurora incipit si stellae sexti gradus seu magnitudinis non amplius videntur seu disparent (...) Hoc generaliter fit, si sol decem et octo gradus (secundum alios tamen 16 vel 24) sub horizonte est.

  It says it's 18° below the horizon, hence the astronomical dawn. But it adds that according to some other authors there are different definitions. This makes me think there is no sharper definition.

• The author goes to cite a different source that also supports the 18° limit:

  Per auroram, seu crepusculum matutinum intellegitur (...). Qui (...) iuxta sententiam communiter receptam tunc ad nos perveniunt quando sol attingit gradum 18mm. infra horizontem.

The fact that the book was written by a 1950's moralist makes me think there is neither a sharper nor a more official definition. (But of course, I might be wrong.)

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FWIW, this other book, from 1760, hesitates about the one-hour margin. It does not give a sharp definition of dawn, but cites some authors estimating varying amounts of time before sunrise meant by dawn at different times of the year.