For anyone interested in philosophy of science, here's a paper on how to classify Galileo's science of local motion according to the principles of St. Thomas laid out in his commentary on Boethius' De Trinitate.

Discovering why St. Luke spends so much time describing the Nativity, when according to St. Thomas his main purpose is extolling the priesthood of Christ, involves looking at the details of the Evangelist's account:

1. St. Luke begins by noting the occasion of Mary and Joseph's journey to Bethlehem: a world-wide census. Why record this detail? The Tradition tells us that the enrollment is a figure of Christ enrolling the whole company of the elect in the Book of Life at the Last Judgment.

   GREGORY: But the registering of the whole world when our Lord was about to be born was mystical; for He appeared in the flesh Who should write down the names of His own elect in eternity.

   AMBROSE: There is described a secular registration, implied a spiritual, to be laid before the King not of earth but of Heaven; a registering of faith: a census of souls. For the old census of the Synagogue was abolished, a new census of the Church was preparing. And to decide that the census was not of Augustus, but of Christ, the whole world is ordered to be registered. For who could demand the registration of the whole world but He who had dominion over it, for the earth is not of Augustus, but the earth is the Lord's?

   ORIGEN: To those who attentively consider it, there seems to be expressed a kind of sacrament, in its being necessary that Christ should be put down in the registration of the whole world; in order that His name being written with all, He might sanctify all, and being placed in the census with the whole world, He might impart to the world the communion of Himself.

   AMBROSE: This was then the first public enrollment of souls to the Lord, to Whom all enroll themselves not at the voice of the crier, but of the Prophet, who says, O clap your hands, all you people.

   ...to cite a few authorities. It is fitting that St. Luke should include this detail , as it is pre-eminently the role of Christ the High Priest to offer redemption to the world and salvation to the elect.


2. Our Lord is conceived in Nazareth, but St. Luke notes that he is born in Bethlehem. This name, which means "House of Bread," has a mystical significance:

   THEOPHYLUS: He condescended to become incarnate at that time, that after His birth He might be enrolled in Caesar's taxing, and in order to bring liberty to us might Himself become subject to slavery. It was well also that our Lord was born at Bethlehem, not only as a mark of the royal crown, but on account of the sacrament of the name.

   GREGORY: Bethlehem is by interpretation the house of bread. For it is the Lord Himself who says, I am the bread of life which came down from heaven. The place therefore where the Lord was born was before called the house of bread, because it was there that He was to appear in His fleshly nature who should refresh the souls of the elect with spiritual fullness.

   Christ's birth in the House of Bread can be taken as a foreshadowing of the Blessed Sacrament, or as a figure of the redemptive nourishment Christ provides to his elect. Either way, this detail is intimately connected with his priesthood.


3. St. Luke goes on to describe Christ's birth, noting his rest in a manger. The detail also has a priestly significance:

   THEOPHYLUS: He is confined in the narrow space of a rude manger, whose seat is the heavens, that He may give us ample room in the joys of His heavenly kingdom. He Who is the bread of Angels is laid down in a manger, that He might feast us, as it were the sacred animals, with the bread of His flesh.

   CYRIL: He finds man in his corrupt affections become like the beasts that perish, and therefore He is laid in the manger, in the place of food, that we changing the life of beasts, might be brought to the knowledge that befits man, partaking not of hay, but of the heavenly bread, the life-giving body.

   Once again, Luke's description of our Lord's birth is ordered toward demonstrating his role as High Priest and Redeemer.


4. Next, we have the angel's message to the shepherds: "Fear not; for, behold, I bring you good tidings of great joy that shall be to all the people: For, this day is born to you a Saviour, who is Christ the Lord, in the city of David." Compare this announcement of Christ's birth, in which he is referred to as "Saviour" with that of the Magi, who declare him "King."


5. Finally, the heavenly army's Gloria testifies to the saving power of Christ's Priesthood. For they not only praise God, singing, "Glory to God in the highest," but also add, "and on earth peace to men of good will." Why should the angels include the peace of men in their song of joy at Christ's birth? Once again, the Fathers interpret:

   GREGORY: At the same time they also give praises because their voices of gladness accord well with our redemption, and while they behold our acceptance, they rejoice also that their number is completed.

   THEOPHYLUS: They wish also peace to men, as they add, On earth peace to men, because those whom they had before despised as weak and abject, now that our Lord has come in the flesh they esteem as friends.

   CYRIL: This peace has been made through Christ, for He has reconciled us by Himself to God and our Father, having taken away our guilt, which was the ground of offense also. He has united two nations in one man, and has joined the heavenly and the earthly in one flock.

   THEOPHYLUS: For whom they ask peace is explained in the words, Of good will. For them, namely, who receive the new born Christ. For there is no peace to the ungodly, but much peace to them that love the name of God.

   ORIGEN: But the attentive reader will ask, How then does the Savior say, I came not to send peace on the earth, whereas now the Angels' song of His birth is, On earth peace to men? It is answered, that peace is said to be to men of goodwill. For the peace which the Lord does not give on the earth is not the peace of good will.

   AUGUSTINE: For righteousness belongs to good will.

   CHRYSOSTOM: Behold the wonderful fill working of God. He first brings Angels down to men, and then brings men up to heaven. The heaven became earth, when it was about to receive earthly things.

   ORIGEN: But in a mystery, the Angels saw that they could not accomplish the work committed to them without Him Who was truly able to save, and that their healing fell short of what the care of men required. And so it was as if there should come one who had great knowledge in medicine, and those who before were unable to heal, acknowledging now the hand of a master, grudge not to see the corruptions of wounds ceasing, but break forth into the praises of the Physician, and of that God who sent to them and to the sick a man of such knowledge; the multitudes of the Angels praised God for the coming of Christ.