The sign of the cross

During the Regina Coeli prayer, when praying for the deceased, the Holy Father, uttering the words Requiem aeternam…, made the sign of the cross over the book. In the same way the priest does, celebrating a Requiem Mass (according to the old rite, obviously): during the introit the celebrant makes the sign of the cross over the Missal. Why did the pope do this gesture? We do not know.