I’m reading Fearful Majesty, a biography of Ivan IV “the Terrible” by Benson Bobrick, and I just ran across this bit of information:
Upon his return to Moscow on December 12 , Ivan [announced] that he intended “to study the coronation formula of his ancestors,” specifically that of Grand Prince Vladimir Monomakh and, in emulation of that prince, to be crowned Grand Prince and “Tsar,” meaning Emperor. Etymologically, the word “tsar” derived from caesar,but had entered Church Slavic through the Greek as a translation of basileus, meaning “emperor.” However, from the days of the Mongol conquest, Russians had applied it not only to the Byzantine emperor but to the Tartar khans. At the Moscow court only Tartar descendents of Genghis Khan who had also been rulers in their own right were honored by the name.
I imagine John Emerson knew the Tatar khans were called “tsar,” but I sure didn’t.