Q: When a deacon assists a celebrant who is incensing the altar, does he bow as the incensation begins, again crossing in front of the altar, and again as the priest completes the incensation? Likewise, some celebrants include incensation of the crucifix at the beginning of Mass. Does the deacon mimic the priest in bowing at these times?
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A: The rules for incensation are in GIRM 277. It includes this statement: “Before and after an incensation, a profound bow is made to the person or object that is incensed, except for the altar and the offerings for the Sacrifice of the Mass.”
So the priest is not supposed to bow to the altar at all at the beginning of Mass—not even when circling it. He is to incense the cross, as the same GIRM 277 says later on. And he bows to the cross before and after incensing it.
As you see, the instruction for bowing is in the passive voice and doesn’t say who bows. But I believe the implication is that the one who incenses is the one who bows. I’ve seen many deacons bow with the priest when he bows, but to me it is no more appropriate for the deacon to bow than it would be, say, for the cantor to bow. It’s more the responsibility of the one incensing.