Why is facing in the same direction called ad orientem?
The term ad orientemliterally translates "to the east." Historically, Churches were built facing east. They built them in this direction in order to connect the actions of the Mass with the natural world. Christ who is the light of the world is symbolized by the rising sun which the people were facing as they attended Mass. As Western thought became more abstract, Christians began building churches that were not facing east. But, they still recognized that they were all symbolically facing east during the Mass and therefore kept the term.