"ac Nostra"

Happy feast day! Let us praise and adore almighty God for the favour he has bestowed on us by calling the Virgin Mary to be the Mother of the Divine Word, by preserving her from all stain of sin from the moment of her conception, and by her consequent assumption, body and soul, to the glory of heaven where she is Queen of all the angels and saints. Let us thank God also for allowing her to be our Mediatrix, Advocatrix, Auxliliatrix and co-Redemptrix.

I liked the British Pathé video because of the clipped English accent of former days and the unaffected respect that the announcer has for the event even though he is presumably not a Catholic. (Though it doesn't have the formula of the proclamation.) You can see some other videos at Rorate Caeli. Unfortunately none of them has the all-important beginning of the magnificent formula:

auctoritate Domini Nostri Iesu Christi, Beatorum Apostolorum Petri et Pauli ac Nostra pronuntiamus, declaramus et definimus divinitus revelatum dogma esse : Immaculatam Deiparam semper Virginem Mariam, expleto terrestris vitae cursu, fuisse corpore et anima ad caelestem gloriam assumptam

by the authority of our Lord Jesus Christ, of the Blessed Apostles Peter and Paul, and Our own, we pronounce, declare, and define it to be a divinely revealed dogma: that the Immaculate Mother of God, the ever Virgin Mary, having completed the course of her earthly life, was assumed body and soul into heavenly glory.

The "ac Nostra" - "and our own [authority]" affirms that the Bishop of Rome, the successor of St Peter, has the same apostolic authority to teach, as shepherd of all the faithful, ex cathedra, what pertains to the deposit of faith committed by Christ to the apostles, enjoying, in the exercise of his extraordinary magisterium, that infallibility which Christ willed to give to His Church.