Or HaChaim on Genesis 1:14/אור החיים על בראשית א׳:י״ד

1And God said, "let there be luminaries:" The reason for it saying, "let there be," in the singular form, about two [luminaries] is to hint with this that [only] one of them would shine during the day, and for this reason it did not use the plural form. And through God speaking in this way, it allowed for the thing that happened to result in the moon getting reduced - as they, of blessed memory, stated (Chullin 60b) - since if God had said at the beginning of His words, "let [them] be," the matter would have stood like that [with their size being equal] perforce, because the word of Our God stands forever.

2It also appears, in its making a point of stating, "to separate between the day and between the night," [to be saying that] if the luminaries were equal and one of them serves during the day and one serves during the night, in what way could night be distinguishable from day, since night 'shines like the day?'

3Indeed, the verse [also] wants [to teach] that - given that He preceded on the first day to create the light and you will find that it states [the word] light five times in the first section; and the Rabbis, of blessed memory, hinted (Bereishit Rabbah 3:6) that five lights were created and God separated them for His inheritance in the future, for the righteous ones, and then - now on the fourth day, He emanated from the lights a portion sufficient for the world and suspended it in the firmament of the skies; and that is [why] it states, let there be luminaries (meohrot, which can also be understood as 'from the lights'), the explanation of which is that it be one portion from the lights that were created on the first day. And He embedded that they should not be fixed but should rotate, such that through their rotation, night and day be distinguishable, and this He did as a sign for the recognition of the holy days, as it states it, "for signs and for appointed times etc. And when the luminary was emanated, two [different] parts were emanated and both of them were created to serve the purposes of day. And this is why you will find (Chullin 60b) that the moon claimed that it it impossible for two kings to use one crown, the explanation [of the 'one crown'] is at one time, and this is how it is brought in the words of our Rabbis; because if the boundary of the moon had been the night and that of the sun, the day, there is not [here] one crown for both of them, but [rather] each one has a boundary of its own. And so you will find that it states, "and they will be for luminaries in the firmament of the skies to shine on the earth," without [mention] of the setting of a time.