(a) RASHI and TOSFOS explain that the Chachmei Yisrael agree with the
opinion of Rebbi Yehoshua, who says that the world was created in Nisan.
This is evidenced by the fact that they count the Tekufah like Rebbi
Yehoshua, and the calculation of the Tekufah affects the times of the Molad
and Birkas ha'Chamah (the blessing recited upon seeing the sun when it
reaches the same position that it was in when it was created). The only
reason why they rule that we count the number of years of the calendar from
Tishrei, like Rebbi Eliezer, is because Tishrei is a more important time
since it is considered Rosh Hashanah for various matters (Shemitah, Yovel,
Orlah, Ma'aser, etc.), as the Gemara (8a) derives from verses.
The RITVA learns like Rashi and Tosfos, that the Chachmei Yisrael agree with
the opinion of Rebbi Yehoshua. However, l'Halachah, the Ritva rules that we
follow the opinion of Rebbi Eliezer even for calculation of the Tekufah,
because Rav (27a) states that the world was created in Tishrei, and Rav has
the authority to dispute a Beraisa ("Rav Tana Hu u'Palig"). That is why we
say, in the Shemoneh Esreh of Rosh Hashanah, that "this day is the beginning
of Creation," following the opinion of Rav (who rules like Rebbi Eliezer)
that the world was created in Tishrei.
(b) RABEINU TAM (27a, cited by Tosfos DH k'Man) says that even Rebbi
Yehoshua, who says that the world was created in Nisan, agrees that Hashem
*wanted* to create the world in Tishrei. This explains why, even according
to Rebbi Yehoshua, we say in the Shemoneh Esreh of Rosh Hashanah that "this
day is the beginning of Creation" -- Hashem *planned* the Creation for
Tishrei. That, too, might be why we count the years from Tishrei even though
we rule like Rebbi Yehoshua, for that is the time at which Hashem originally
*planned* to create the world.
(Alternatively, Rebbi Eliezer, might agree with Rebbi Yehoshua that the
world was *planned* to be created in Tishrei but was actually created in
Nisan. The only argument is whether we consider the Creation, with regard to
counting the years, to be at the time that Hashem planned to create the
world, or at the time that he actually created it. Accordingly, both Rebbi
Eliezer and Rebbi Yehoshua agree that we calculate the Tekufah from Nisan,
and the only argument is how to count the years. If so, we effectively rule
like Rebbi Eliezer, since we count the years from Tishrei.)
(c) RABEINU CHANANEL is not bothered by the contradiction in the ruling of
the Chachmei Yisrael. He says that regarding matters related to the moon, we
count from Tishrei, like Rebbi Eliezer, and regarding matters related to the
sun, we count from Nisan, like Rebbi Yehoshua. What does Rabeinu Chananel
mean? How can we count the Molad of the moon from Tishrei, saying that the
moon was created in Tishrei, while at the same time we count the Tekufah
from Nisan, saying that the sun was created in Nisan?
Rav David Metzger, in his annotations to the commentary of Rabeinu Chananel,
writes that Rabeinu Chananel seems to be of the opinion that the Birkas
ha'Chamah is *not* a blessing recited when the sun returns to the position
it was at when created. There is no blessing of that sort.
How does Rabeinu Chananel understand the Gemara in Berachos (59b) which
mentions says that the Birkas ha'Chamah is to be recited when the "sun is in
its Tekufah" ("Chamah b'Tekufasah")? Rabeinu Chananel there records two
opinions of what Birkas ha'Chamah means. The first explanation is like Rashi
explains there, that Birkas ha'Chamah is the blessing recited when the sun
returns to the position at which it was when it was created, which is
actually our Girsa in the Gemara in Berachos. The second opinion is based on
the Yerushalmi (Berachos 9:2). The Yerushalmi says that the Birkas ha'Chamah
is recited when the sun reappears after not being visible for three
consecutive days (due to cloud cover, for example). That is what "Chamah
b'Tekufasah" means, and not that the sun is in its original position.
According to this second explanation, the calculation of the Tekufah is not
related at all to the blessing of Birkas ha'Chamah. According to Rabeinu
Chananel, the Beraisa here in Rosh Hashanah which does not consider it
contradictory to count Nisan as the first month for Tekufah and Tishrei for
the Molad follows the opinion of the Yerushalmi, that there is not Berachah
recited when the sun returns to its original spot. (There are those who took
Rabeinu Chananel's position about Birkas ha'Chamah into account
Halachically. According to the grandson of the Maharal, the Maharal did not
recite Birkas ha'Chamah with Hashem's name (Rebbi Akiva Eiger OC 229).
Similarly, the Sheyarei Keneses ha'Gedolah (ibid.) writes that they had a
Minhag in his community not to recite the Birkas ha'Chamah at all.)
As far as the Molad or Tekufah is concerned, it does not make a difference
what is considered to be the beginning of the year. That is, the world was
created in Tishrei like Rebbi Eliezer. When the Beraisa says that the
Chachmei Yisrael counted the Tekufah of the sun from Nisan, it means that we
consider the Tekufah of Nisan to be the first of the four Tekufos of the
year. We calculate the Tekufah starting from when Tekufas Nisan *would have
been* in the Nisan before Creation (see Rambam Kidush ha'Chodesh 9:3). As
for the Molados, these are calculated from the Tishrei (actually, from
Tishrei of the year *before* the Tishrei of Creation, see TOSFOS 8a DH
This approach also answers a question of Tosfos (8b). Tosfos asks how could
Rebbi Yehoshua and Rebbi Eliezer argue when the sun and moon were created.
It is obvious that they were created in Nisan, because we can examine the
astronomical factors and see that if they were created in Tishrei, the sun
and moon would be in different places than they are now. How could Rebbi
Yehoshua argue about that? The answer is that their argument has absolutely
no bearing on the actual time of the Molad or Tekufah.
Why, though, should we calculate the Tekufah from the Nisan before Creation,
extrapolating back from Tishrei? Why do we not simply calculate the Tekufah
and Molad from Tishrei, when the world was created?
The RAN asks this question (3a of the pages of the Rif). He explains (as
does the Mefaresh on the Rambam, Hilchos Kidush ha'Chodesh 9:3) that Nisan
is a more preferable time from which to calculate the Tekufah, because it is
brighter at that time of year, and all of the produce is beginning to grow.