Ad orientem

In the Christian liturgy the expression ad orientem (Latin for "to the east") describes a particular orientation of a priest celebrating Mass.

The literal meaning of the expression indicates that the priest faces eastward, an orientation that has been described as linked with the "cosmic sign of the rising sun which symbolizes the universality of God." Outside of Rome, it was an ancient custom for churches to be built with the entrance at the west end and for priest and people to face eastward to the place of the rising sun.

However, ad orientem is more often used to mean facing the apse or wall behind the altar, with priest and people looking in the same direction (as opposed to the versus populum orientation, in which the priest faces the congregation), even if they are not facing to the east or even have their backs to the east.

This distinction between ad orientem and versus populum differs from the usage in the Tridentine Roman Missal, which says: "Si altare sit ad orientem, versus populum, celebrans versa facie ad populum, non vertit humeros ad altare, cum dicturus est Dóminus vobiscum, Oráte, fratres, Ite, missa est, vel daturus benedictionem ..." (If the altar is ad orientem, towards the people, the celebrant, facing the people, does not turn his back to the altar when about to say Dominus vobiscum ["The Lord be with you"], Orate, fratres [the introduction to the prayer over the offerings of bread and wine], and Ite, missa est [the dismissal at the conclusion of the Mass], or about to give the blessing ...)

The earliest churches in Rome had a façade to the east and an apse with the altar to the west; the priest celebrating Mass stood behind the altar, facing east and so towards the people.

According to Louis Bouyer, not only the priest but also the congregation faced east at prayer, a view strongly criticized on the grounds of the unlikelihood that, in churches where the altar was to the west, they would turn their backs on the altar (and the priest) at the celebration of the Eucharist. The view prevails therefore that the priest, facing east, would celebrate ad populum in some churches, in others not, in accordance with the churches' architecture.

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Wikipedia