Tosfos in R"H (27a) says regarding the machlokes between Rebbi Yehushua and Rebbi Eliezer if b'tishrei nivra or b'nissan nivra hoalom eilu v'eilyu: in tishrei it was ulu b'machshava and in nissan it was actually created.
According the the gemarah here in a.z. daf ches that asks b'shleima according the the opinion that b'tsihrei etc. it makes sense that Adam was thinking the days were getting shorter, i.e. the world was ending. but according to the opinion b'nissan etc. then he already knew there were longer and shorter days. From the gemarah it seems clear that it is an actual machlokes and not eilu v'eilu in the way Tosfos understands?
Y. Biberfeld, Philadelphia, USA
This is an outstanding question.
1. I found that your question can be answered on the basis of the words of the Sefer Yom Teru'ah (Rav Moshe ben Chaviv (Maharam Ibn Chaviv), circa 1700 C.E., author of Kapos Temarim on Sukah and Tosfos Yom ha'Kipurim on Yoma). In his commentary to the Tosfos in Rosh Hashanah that you cite, he writes that there is no doubt that there is in fact a dispute between Rebbi Yehoshua and Rebbi Eliezer, as you also wrote. However, the opinion of Rabbi Eliezer ha'Kalir, whom Tosfos in Rosh Hashanah cites, is a third opinion and does not agree entirely with either Rebbi Yehoshua or Rebbi Eliezer.
2. This answer of the Yom Teru'ah can be understood with the help of Tosfos in Chagigah (13a, DH v'Raglei) who writes that Rebbi Eliezer ha'Kalir, who wrote so many Piyutim which we say at different seasons of the year in the prayer service, was in fact a Tana. Tosfos writes that he was none other than Rebbi Eliezer the son of Rebbi Shimon bar Yochai. Since ha'Kalir was a Tana, we now can understand how he was able to disagree with both Rebbi Yehoshua and Rebbi Eliezer, and maintain a middle opinion that Hash-m thought of creating the world in Tishrei but only actually created it in Nisan. Rabeinu Tam's answer in Rosh Hashanah is valid only according to ha'Kalir, not according to the other Tana'im.
3. This opinion of ha'Kalir is not consistent with the opinion of the Gemara in Avodah Zarah 8a, as you accurately point out, because ha'Kalir could not have said that according to the opinion that the world was created in Tishri, Adam had never seen long days by the time the days in the winter started getting shorter, because according to ha'Kalir everyone agrees that Adam ha'Rishon was not actually created until Nisan. It must be that ha'Kalir disagrees with the Gemara, which he is capable of doing since he is a Tana.
May we all have a good Yom ha'Din on Rosh Hashanah, the day on which we were created.
Kesivah v'Chasimah Tovah,
Here is a slight addition to the above reply. I wrote above in (2) that Hakalir maintained a middle opinion between the views of R. Eliezer and R. Yehoshua. However we can now express this in a somewhat different way:- Hakalir maintained that whilst Rabbi Eliezer's view was that the world was actually physically created in Tishrei and that is the way he is cited in Avoda Zara 8a, nevertheless Hakalir believed that he had in fact offered an improvement on R. Eliezer's stance, namely that the world was not physically created in Tishrei but rather the seeds of the creation were sewn, so to speak, by Hashem in Tishrei when he decided to create the world. So Hakalir taught that there is some accuracy in R. Eliezer's opinion that the world was created in Tishrei, but in fact there is something deeper behind this, and the truth of the matter is that the start of the Creation in Hashem's plan was in Tishrei but was only put into action in Nisan. According to this, Hakalir did not believe that he was making some kind of compromise between the 2 opinions, as I seemed to be suggesting above, but rather he was building on R. Eliezer's opinion and improving it by learning that the Creation of the World took place in 2 stages. Both Tishrei and Nisan are crucial times in the history of the world, but Tishrei was the time of Hashem's intention and thought to create the World, and this good intention only bore fruit - in a way that finite, physical creatures like ourselves can appreciate - when Nisan came along.
Possibly we can suggest that this adds more meaning to what we do on Rosh Hashanah, because we have two days when we spend most of our time in Shul and have an opportunity to think about and appreciate how Hashem is the King of the entire universe. In the same way that at the time of the Creation of the World, the chief purpose of Tishrei was to be the season when Hashem decided to bring the World into existence, so too we possess every year an opportunity to decide how we are going to conduct our lives in a better way in the coming year. Rosh Hashana is the time for decisions and resolutions and if we have the correct intentions next Thursday and Friday, this will make all our actions throughout the coming year so much more meanigful.