The Gemora says that when Koresh was good, he was given status as jewish king and thus counts from Nissan; while when he was bad, he was given status as a non-/jew, and thus counts from Tishrey.
Furthermore, the Gemora says that Tzedoka shelo leshaym shamayim by a non-Jew is not a Tzadik - and this was a proof that Koresh went bad.
Furthermore, Rashi says that koresh was with a dog, something not allowed even to a ben noach.
On all these these things, can't we ask: Wasn't Koresh a Jew - he was ben Esther.
Daniel Braude, Har Nof, Israel
Shalom Rav,
The Gemara definitely implies that Koresh the Second was a non-Jew. Rashi, in fact, learns this way as well when -- in connection with Koresh -- he cites the Halachah that a Ben Noach is subject to the prohibition of bestiality (4a, DH Kalbesa).
The quoted source that Koresh was the son of Esther is Tosfos 3b, DH Shenas Esrim, but in fact it is a Midrash Rabah. I would therefore have thought that this is a Machlokes between the Gemara and the Midrash.
If, as it would seem, Tosfos maintains that the Gemara agrees with the Midrash, then your question is valid and I have no answer.
B'Virchas Kol Tuv,
Ekliezer Chrysler