The Latin word here is sidquidem (= si quidem), and if you look it up, you will find that it can mean both things:
The first, condicional meaning is certainly the original one, since si pretty unequivocally means “if.” The second, causal meaning must have developed out of it. A similar thing can sometimes be observed in English when people say things like: “Well, if that's how he thanks me for my efforts, I won't feel very motivated to do him a favour in the future” – where the speaker clearly means “since that's how etc.”
Anyway, seeing that the expression is ambiuguous, how can we determine what meaning the author intended?
Notice that the two translations differ not just in the translation of siquidem, but in fact in what they think is the subject of the sentence introduced by it, and in how they interpret in totum. In fact, the two translators came to totally different conclusions:
- … if indeed they all of them shine by light borrowed from him [the sun] …
- … as in fact she [the moon] shines with a light entirely borrowed from him [the sun] …
This is ambiguous in Latin because the verb is an infinitive. Also notice how strange it is that si should be followed by an infinitive? What is going on here?
In my opinion the first translation is mistaken. The subject of the long string of infinitives is clearly each time the moon. It divides the year into twelve months, it is governed by the sunshine, later on it only shows as much light as it receives itself from the sun, etc. It is obvious that Pliny knows that the moon is illuminated entirely by the sun, and it seems far-fetched to imagine that the subordinate clause introduced by siquidem refers to the other stars, just because they happened to be mentioned before in a terse ut reliqua siderum.
And if we imagine siquidem functioning as an adverb like nam, just modifying a clause without subordinating it, then we may have an explanation for the somewhat puzzling continuation of the AcI.
And finally, Pliny only ever used siquidem in the sense of “since.” I found an interesting treatment on Google Books by one Antonius Ludewig: Quomodo Plinus maior, Seneca philosophus, Curtius Rufus, Quintilianus, Cornelius Tacitus, Plinius minor particula quidem usi sint. (= Prager Philologische Studien No. 3, Prague 1891) It has this to say on si quidem:
Ac primum quidem alii scriptores frequenter, alii raro si quidem usurpant. Apud Plinium maiorem semel et sexagiens legitur [⋯]
In omnibus his locis compositioni si quidem Plinius eam subicit significationem, ut sit idem fere ac quoniam aut nam (enim) da ja, indem, nemlich, denn, numquam vero si = wenn. Quid eo mirabilius videtur, quia qui ante Ciceronem scripserunt numquam ei causalem vim tribuerunt [⋯]. Atque ut probem eodem modo quo nam etiam siquidem a Plinio poni, ex multis haec seligo exempla: n. h. [sequuntur plerique loci]
Quae cum ita sint, non admodum mirabimur infinitivum poni II 45 (in constructione accusativi cum infinitivo): [hic refertur locus noster]
Hoc loco excepto si quidem cum indicativo coniungitur, ut hac in re idem de usu Pliniano et Ciceroniano statui possit.
First, some writers use si quidem frequently, others rarely. In Pliny, it is found 61 times. [⋯]
In all these cases, Pliny places more or less the same meaning in this phrase as quoniam or nam (since), da ja, indem, nemlich, denn [these are all German phrases meaning “since, as, for,” etc.], but never si = wenn [“if”]. Which is curious, because the writers before Cicero never assigned any causal force to it. Now in order to show siquidem is used by Pliny in the same way as nam, I present the following of many examples: Naturalis Historia [here follow many citations from Naturalis Historia]
This being so, we will not be very surprised that the infinitive is put II 45 (in an AcI construction): [here follows our example]
With the exception of this case, si quidem is used with the indicative, so that in this regard we can state that the usage of Pliny and Cicero is the same.
Thus, in my opinion the correct translation is:
… since the moon, like the other stars, is influenced by the light of the sun, because it shines entirely with light borrowed from it, …