Objection 1. It would seem that the brightness of the heavenly bodies will not be increased at this renewal. For this renewal as regards the lower bodies will be caused by the cleansing fire. But the cleansing fire will not reach the heavenly bodies. Therefore the heavenly bodies will not be renewed by receiving an increase of brightness.
Objection 2. Further, just as the heavenly bodies are the cause of generation in this lower world by their movement, so are they by their light. But, when generation ceases, movement will cease as stated above (Article 2). Therefore in like manner the light of the heavenly bodies will cease rather than increase.
Objection 3. Further, if the heavenly bodies will be renewed when man is renewed, it follows that when man deteriorated they deteriorated likewise. But this does not seem probable, since these bodies are unalterable as to their substance. Therefore neither will they be renewed when man is renewed.
Objection 4. Further, if they deteriorated then it follows that their deterioration was on a par with the amelioration which, it is said, will accrue to them at man's renewal. Now it is written (Isaiah 30:26) that "the light of the moon shall be as the light of the sun." Therefore in the original state before sin the moon shone as much as the sun does now. Therefore whenever the moon was over the earth, it made it to be day as the sun does now: which is proved manifestly to be false from the statement of Genesis 1:16 that the moon was made "to rule the night." Therefore when man sinned the heavenly bodies were not deprived of their light; and so their light will not be increased, so it seems, when man is glorified.
Objection 5. Further, the brightness of the heavenly bodies, like other creatures, is directed to the use of man. Now, after the resurrection, the brightness of the sun will be of no use to man: for it is written (Isaiah 60:19): "Thou shalt no more have the sun for thy light by day, neither shall the brightness of the moon enlighten thee," and (Apocalypse 21:23): "The city hath no need of the sun, nor of the moon to shine in it." Therefore their brightness will not be increased.
Objection 6. Further, it were not a wise craftsman who would make very great instruments for the making of a small work. Now man is a very small thing in comparison with the heavenly bodies, which by their huge bulk surpass the size of man almost beyond comparison: in fact the size of the whole earth in comparison with the heaven is as a point compared with a sphere, as astronomers say. Since then God is most wise it would seem that man is not the end of the creation of the heavens, and so it is unseemly that the heaven should deteriorate when he sinned, or that it should be bettered when he is glorified.
On the contrary, It is written (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."
Further, the whole world will be renewed for the better. But the heaven is the more noble part of the corporeal world. Therefore it will be altered for the better. But this cannot be unless it shine out with greater brightness. Therefore its brightness will be bettered and will increase.
Further, "every creature that groaneth and travaileth in pain, awaiteth the revelation of the glory of the children of God" ['The creature also itself shall be delivered from the servitude of corruption, into the liberty of the children of God. For we know that every creature groaneth and travaileth in pain,' etc.] (Romans 8:21-22). Now such are the heavenly bodies, as a gloss says on the same passage. Therefore they await the glory of the saints. But they would not await it unless they were to gain something by it. Therefore their brightness will increase thereby, since it is their chief beauty.
I answer that, The renewal of the world is directed to the end that, after this renewal has taken place, God may become visible to man by signs so manifest as to be perceived as it were by his senses. Now creatures lead to the knowledge of God chiefly by their comeliness and beauty, which show forth the wisdom of their Maker and Governor; wherefore it is written (Wisdom 13:5): "By the greatness of the beauty and of the creature, the Creator of them may be seen, so as to be known thereby." And the beauty of the heavenly bodies consists chiefly in light; wherefore it is written (Sirach 43:10): "The glory of the stars is the beauty of heaven, the Lord enlighteneth the world on high." Hence the heavenly bodies will be bettered, especially as regards their brightness. But to what degree and in what way this betterment will take place is known to Him alone Who will bring it about.
Reply to Objection 1. The cleansing fire will not cause the form of the renewal, but will only dispose thereto, by cleansing from the vileness of sin and the impurity resulting from the mingling of bodies, and this is not to be found in the heavenly bodies. Hence although the heavenly bodies are not to be cleansed by fire, they are nevertheless to be Divinely renewed.
Reply to Objection 2. Movement does not denote perfection in the thing moved, considered in itself, since movement is the act of that which is imperfect: although it may pertain to the perfection of a body in so far as the latter is the cause of something. But light belongs to the perfection of a lightsome body, even considered in its substance: and consequently after the heavenly body has ceased to be the cause of generation, its brightness will remain, while its movement will cease.
Reply to Objection 3. A gloss on Isaiah 30:26, "The light of the moon shall be as the light of the sun," says: "All things made for man's sake deteriorated at his fall, and sun and moon diminished in light." This diminishment is understood by some to mean a real lessening of light. Nor does it matter that the heavenly bodies are by nature unalterable, because this alteration was brought about by the Divine power. Others, however, with greater probability, take this diminishment to mean, not a real lessening of light, but a lessening in reference to man's use; because after sin man did not receive as much benefit from the light of the heavenly bodies as before. In the same sense we read (Genesis 3:17-18): "Cursed is the earth in thy work... Thorns and thistles shall it bring forth to thee"; although it would have brought forth thorns and thistles before sin, but not as a punishment to man. Nor does it follow that, supposing the light of the heavenly bodies not to have been lessened essentially through man sinning, it will not really be increased at man's glorification, because man's sin wrought no change upon the state of the universe, since both before and after sin man had an animal life, which needs the movement and generation of a corporeal creature; whereas man's glorification will bring a change upon the state of all corporeal creatures, as stated above (Question 76, Article 7). Hence there is no comparison.
Reply to Objection 4. This diminution, according to the more probable opinion, refers not to the substance but to the effect. Hence it does not follow that the moon while over the earth would have made it to be day, but that man would have derived as much benefit from the light of the moon then as now from the light of the sun. After the resurrection, however, when the light of the moon will be increased in very truth, there will be night nowhere on earth but only in the center of the earth, where hell will be, because then, as stated, the moon will shine as brightly as the sun does now; the sun seven times as much as now, and the bodies of the blessed seven times more than the sun, although there be no authority or reason to prove this.
Reply to Objection 5. A thing may be useful to man in two ways. First, by reason of necessity, and thus no creature will be useful to man because he will have complete sufficiency from God. This is signified (Apocalypse 21:23) by the words quoted, according to which that "city hath no need of the sun," nor "of the moon." Secondly, on account of a greater perfection, and thus man will make use of other creatures, yet not as needful to him in order to obtain his end, in which way he makes use of them now.
Reply to Objection 6. This is the argument of Rabbi Moses who endeavors to prove (Dux errantium iii) that the world was by no means made for man's use. Wherefore he maintains that what we read in the Old Testament about the renewal of the world, as instanced by the quotations from Isaias, is said metaphorically: and that even as the sun is said to be darkened in reference to a person when he encounters a great sorrow so as not to know what to do (which way of speaking is customary to Scripture), so on the other hand the sun is said to shine brighter for a person, and the whole world to be renewed, when he is brought from a state of sorrow to one of very great joy. But this is not in harmony with the authority and commentaries of holy men. Consequently we must answer this argument by saying that although the heavenly bodies far surpass the human body, yet the rational soul surpasses the heavenly bodies far more than these surpass the human body. Hence it is not unreasonable to say that the heavenly bodies were made for man's sake; not, however as though this were the principal end, since the principal end of all things is God.
Source: Summa Theologica (New Advent)