What's the new rite in Tantum Ergo? St. Thomas Aquinas' hymn Tantum Ergo, which is usually sung at Benediction, always seems better in Latin:


  Et antiquum documentum
  Novo cedat ritui:


Mainly because in both the transliterated and the literal translation it doesn't make a ton of sense:


  And let the old practice
  Give way to the new rite;


Given this is written over 1000 years after the rite was established, what is this line in the hymn about?
"New" in this sense, is as in, "New covenant (or new testament, if you're going by a literal translation of the word covenant)." It is "new" as in it only 1200 (Aquinas wrote it), as opposed to the 4000(?) year old covenant of Abraham and the 2500 year old covenant of Moses.

It means that the believer is able to relate directly to the Godhead through the sacrifice of Christ, as opposed to the defective worship of the Old Testament (where the priest was mortal and sinful and had to offer sacrifice for his own sins in addition to the sins of the people and all of that other quasi-Pauline goodness from the book of Hebrews).