Who is here signified by the Sun, if not the Saviour or by the Moon, if not the Church?
Until the Lord was withdrawn from her sight (that is, by His Ascension), His Holy Church was pale before the hostile glare of the world.
But after He was ascended, she waxed stronger, and distinctly shed forth the beams of that faith which had hitherto dwelt hiddenly in her.
“The sun was lifted up, and the moon stood still in her habitation” when the Lord was gone away into heaven, His holy Church waxed stronger in her enlightening power.
Hence it is that Solomon has put into the mouth of the (same) Church the words: Behold, He cometh! leaping upon the mountains, skipping upon the hills.
These hills are his lofty and noble achievements. “Behold, He cometh leaping upon the mountains.”
When He came to redeem us, He came, if I may so say, in leaps. My dearly beloved brethren, would you know what His leaps were?
From heaven he leapt into the womb’ of the Virgin, from the womb into the manger, from the manger on to the Cross, from the Cross into the grave, and from the grave up to heaven.
Lo, how the Truth made manifest in the Flesh did leap for our sakes, that He might draw us to run after Him for this end did He rejoice, as a strong man to run a race.
Therefore, dearly beloved brethren, it behoves us in heart and mind thither to ascend, where we believe Him to have already ascended bodily.
Let us fly from earthly lusts. For us, who have a Father in heaven, let nothing be sweet below. And very much must we keep in our minds this thought, that He Which ascended up in peace, will return in dreadful Majesty and will require from us with justice an account of our keeping of those commandments which He gave us in mercy.
Let no man therefore reckon lightly this season which is given unto us that we may repent ourselves, nor be reckless touching the state of his soul.
Gregory the Great (c.540-604): Homilies on the Gospels, 29, from Mattins of Thursday in the Octave of the Ascension in the Old Breviary @ http://lzkiss.net/cgi-bin/horas/brevi.pl.