The Vulgate reads:
Dixitque Deus: fiat lux. Et facta est lux.
But I would have expected:
Dixitque Deus: sit lux. Et fuit lux.
This is based on scientific texts, where "let x" be is rendered as sit x.
Would my version still be correct Latin? How does it differ from the first?
Perhaps the following comparison makes this understandable:
sit lux ≈ may light exist
fiat lux ≈ may light come into existence
The English translation "let there be light" is gives a slightly different impression than the Latin fiat lux. In Latin the wish or order is for light to come or to be made, not for it to be.
The choice of fiat is paralleled by facta in the next sentence. One possible English translation would have been: "May light be made. And light was made." That would arguably have been less poetic. Perhaps someone can comment on the English translation tradition and why it ended up speaking of the light being instead of coming or being made. For what it's worth, in the Finnish translations light comes, so the English wording is not used globally.
The perfect tense of fuit sounds odd and the meaning of the sentence unclear. This tense often signifies the completion of an act, making it better suited for facta est than fuit. It depends on context, but fuit lux can well be closer to "light has been" than "light was".
It's just barely possible it's simpler than that. First, note that the Hebrew does use a verb corresponding to sit rather than fiat:
וַיֹּאמֶר אֱלֹהִים יְהִי אוֹר וַיְהִי־אוֹר׃
wayyomer ĕlohîm: "yəhî ʾôr" wayəhî-ʾôr.
יְהִי yəhî is a 3rd m. sg. jussive of היה 'to be'.
The Septuagint turns this into the following:
καὶ εἶπεν ὁ Θεός· γενηθήτω φῶς· καὶ ἐγένετο φῶς.
γενηθήτω is the 3rd sg. aorist imperative of γίγνομαι 'to become'. Latin's 3rd person imperative is rarely used for most verbs, and gigno, the cognate of γίγνομαι, is pretty cumbersome and certainly more marked than γίγνομαι is in Greek, so fiat is a more than fair rendering of that; the Vulgate clearly closely follows the Septuagint here.
(This does mean that fiat isn't acting as the passive of facio here but just has its own meaning of 'to become' (factus est, too, is the active perfect of semi-deponent fio, not the passive of facio), but it's not clear that this is a line Jerome or any contemporary speaker of Latin would have drawn.)
So why didn't the Septuagint use a form of εἰμί 'to be'? It did! εἰμί is suppletive, with γίγνομαι supplying its aorist stem!
Was Jerome simply unaware of that? Probably not, admittedly, and he certainly did also know Hebrew and have access to the Hebrew version of Genesis, so his choice of fiat is almost certainly a conscious decision made for the reasons Joonas discussed (and the fact that he didn't use gignatur is actually a point in favour of that). But I thought it was an interesting observation.
