 Because of this practice, Tahanun is also known as nefilat apayim ("falling on the face").Because Joshua fell on his face before the Ark of the Covenant, Ashkenazi custom is that one puts one's head down only when praying in front of an Ark containing a Torah scroll.Otherwise, it is proper to sit with the head up.One source says that where the ark, containing a valid (non-Pasul) Sefer Torah can be seen from where one is sitting, then head down, if not, not.