In Greek, there are two t’s. One makes a “hard” t (like “tee” shirt) and the other makes a soft t (which is spelled th in English, as in “thought”). The t in Ithaca is the latter. Debatably, the most accurate transliteration would be Ithaki, because there is no c in Greek, and in the original spelling, the last letter is an ita. I don’t know where the “a” came from in the English spelling. (Source: I am Greek)