Objection 1. It would seem that towards the time of the judgment the sun and moon will be darkened in very truth. For, as Rabanus says, commenting on Matthew 24:29 "nothing hinders us from gathering that the sun moon, and stars will then be deprived of their light, as we know happened to the sun at the time of our Lord's passion."
Objection 2. Further, the light of the heavenly bodies is directed to the generation of inferior bodies, because by its means and not only by their movement they act upon this lower world as Averroes says (De Subst. Orbis.). But generation will cease then. Therefore neither will light remain in the heavenly bodies.
Objection 3. Further, according to some the inferior bodies will be cleansed of the qualities by which they act. Now heavenly bodies act not only by movement, but also by light, as stated above (Objection 2). Therefore as the movement of heaven will cease, so will the light of the heavenly bodies.
On the contrary, According to astronomers the sun and moon cannot be eclipsed at the same time. But this darkening of the sun and moon is stated to be simultaneous, when the Lord shall come to judgment. Therefore the darkening will not be in very truth due to a natural eclipse.
Further, it is not seemly for the same to be the cause of a thing's failing and increasing. Now when our Lord shall come the light of the luminaries will increase according to Isaiah 30:26, "The light of the moon shall be as the light of the sun, and the light of the sun shall be sevenfold." Therefore it is unfitting for the light of these bodies to cease when our Lord comes.
I answer that, If we speak of the sun and moon in respect of the very moment of Christ's coming, it is not credible that they will be darkened through being bereft of their light, since when Christ comes and the saints rise again the whole world will be renewed, as we shall state further on (74). If, however, we speak of them in respect of the time immediately preceding the judgment, it is possible that by the Divine power the sun, moon, and other luminaries of the heavens will be darkened, either at various times or all together, in order to inspire men with fear.
Reply to Objection 1. Rabanus is speaking of the time preceding the judgment: wherefore he adds that when the judgment day is over the words of Isaias shall be fulfilled.
Reply to Objection 2. Light is in the heavenly bodies not only for the purpose of causing generation in these lower bodies, but also for their own perfection and beauty. Hence it does not follow that where generation ceases, the light of the heavenly bodies will cease, but rather that it will increase.
Reply to Objection 3. It does not seem probable that the elemental qualities will be removed from the elements, although some have asserted this. If, however, they be removed, there would still be no parallel between them and light, since the elemental qualities are in opposition to one another, so that their action is corruptive: whereas light is a principle of action not by way of opposition, but by way of a principle regulating things in opposition to one another and bringing them back to harmony. Nor is there a parallel with the movement of heavenly bodies, for movement is the act of that which is imperfect, wherefore it must needs cease when the imperfection ceases: whereas this cannot be said of light.
Source: Summa Theologica (New Advent)