Implied Question #1: Rashi asks why the Gemara cannot ask (that, seeing as it is discussing a case where, according to both R. Yehudah and R. Yossi, Tekufas Tishri falls only one day late), why not simply declare Ellul a full month?

Answer: And he answers - because if they did, Rosh Hashanah would fall on Sunday, Wednesday or Friday (something which they would try and avoid at all costs, as the Gemara explains in the first Perek of the Rosh Hashanah).

Implied Question #2: Then why does the Gemara ask later 'Let us declare Adar a full month'? Why are we not afraid there too, that this will subsequently cause Tishri to fall on Sunday, Wednesday or Friday?

Answer #1: It is specifically there according to Acherim, who is discussing Tekufas Nisan, answers Rashi, that we can consider declaring Adar a full month, seeing as it is possible to declare any of the subsequent summer months a short month, thereby resolving the problem - but not Tekufas Tishri ...

Answer #2 (Part 1): ... as opposed to Rashi, Rabeinu Tam maintains that the Gemara's Kashya 've'le'Abreih la'Adar' goes according to all the Tana'im. This is because there are many Tana'im who do not hold of the principle 'Lo Adu Rosh' - like we find in the Gemara at the end of 'Eilu Kesharim' (Shabbbos, 113a), where R. Yishmael and R. Akiva dispute bringing the fat pieces from the Shabbos Korbanos on Yom-Kipur.

Answer #2 (Part 2): ... and when the Gemara answers shortly that 'he is reckoning from up to down', it goes according to all the Tana'im - the Tana Kama, R. Yehudah, R. Yossi and R. Shimon.

Question #1: This is not correct however, since the answer will not go according to R. Yossi, who states that sixteen days before Pesach (if Tekufas Nisan falls on the seventeenth) one declares a leap-year, but sixteen days before Sukos, one does not'?

Question #2: Furthermore, at the end of 'ha'Chalil' (Sukah, 54b), we do not find a Tana who can author the Mishnah of 'Chelvei Shabbos Kereivim be'Yom-ha'Kipurim' other than Acherim?

Question #3: Moreover, it seems that Ravina, who concludes that really Acherim is speaking in Tishri (and not Nisan), he is coming to preclude the need to answer that he (Acherim) is 'reckoning from up to down'.

Retraction (Part 1): Even Rashi will have to admit however, that Ravina does not answer the Kashya 've'le'Abreih la'Adar' at all, seeing as he is referring to Acherim (in whose personal opinion, the calendar is fixed and) who is (therefore) not concerned with 'Lo Adu Rosh'.

Retraction (Part 2): And we will therefore still require the previous answer that 'he is reckoning from up to down'.

Question: And for the same reason, Rashi 'pushed' in vain when he explained that Me'abrin does indeed refer to Adar, and it is always possible to rectify Tishri by declaring one of the summer months Chaser. This is not necessary, seeing as it goes according to Acherim (as we just explained)

Implied Question (on Rashi's original answer): One cannot ask that now too, if we declare a leap-year, Rosh ha'Shanah will still occur on Sunday, Wednesday or Friday, according to those who hold that the extra month is always Chaser?

Answer: Since it is always possible to rectify it by declaring an additional month in the summer Malei (a full month); And likewise, according to those who hold that the extra month is always Malei, it is possible to declare an additional month Chaser.

Clarification: This means that sixteen days in Nisan pass, and Tekufas Nisan falls on the seventeenth. And the reason for this is because then the Tekufah of Tishri will fall on the twenty-second of Tishri, leaving not even one day of Sukos to fall in Tekufas Tishri.

Implied Question: In fact, even if Tekufas Nisan would fall on the sixteenth - either at the beginning of the day or after six hours, Tekufas Tishri would still fall on the twenty-second. This is because the difference between one Tekufah and the next is ninety-one days and eight a half hours. So why does R. Yossi mention specifically the seventeenth of Nisan, and not the sixteenth?

Answer: The reason for this is because when Tekufas Nisan falls on the seventeenth, then Tekufas Tishri will always fall on the twenty-second. Whereas when it falls on the sixteenth, should it fall at the beginning of the night, or even in the middle of the night, then Tekufas Tishri will fall on the tewnty-first, and it will not be necessary to declare a leap-year.

Implied Question: Why do we not apply the principle here the principle 'The last Tana always comes to add'?

Answer: Since other Tana'im are cited in between the two, on whose opinion the latter Tana comes to add.

Precedent: And so we find in Perek Bameh Madlikin (Shabbos, 24a), where the Gemara states that the difference between the two Tana'im is the ruling of Rav B'runa, only we don't know which opinion is which.

Explanation #1: Rashi explains that the spring of Tekufas Nisan (the Nisan of the sun year) should take place, at least a little, within the period of the new moon; and what is considered the period of the new moon? Fourteen days! Beyond that is considered 'the old moon'.

Question #1: In that case, what is the Gemara's Kashya 've'le'Abreih la'Adar?' Why can we not answer that he holds 'Yom Tekufah Gomer' (the day of the Tekufah concludes the Tekufah), in which case, even if they do declare Adar Malei and postpone it by one day, the Tekufah will fall on the fourteenth; the Tekufah of Nisan of the sun will fall on the fifteenth and will not be within the period of the new moon?

Question #2: Besides, the Gemara in 'Hayu Bodkin' (later, Daf 41b) implies that the period of the new moon lasts until the fifteenth?

Explanation #2 (Part 1): We must therefore explain that Acherim hold 'Yom Tekufah Gomer', and that the period of the new moon lasts until the fifteenth. Consequently, when the Tekufah of Nisan falls on the fifteenth, they declare a leap-year, since otherwise, the Tekufah of Nisan of the sun will not fall within the period of the new moon at all.

Explanation #2 (Part 2): Then the Gemara asks 've'le'Abreih la'Adar?', in order that the Tekufah should fall on the fourteenth, and that the Nisan of the sun should begin on the fifteenth, leaving one day of the new Tekufah within the period of the new moon.

Explanation of Gemara Rosh ha'Shanah (1): The Gemara in the first Perek of Rosh ha'Shanah (Daf 21a) states 'When you see that the Tekufah of Teives lasts until the sixteenth of Nisan, declare a leap-year. And don't worry about it!, as the Torah writes "Guard the month of spring!" - 'Guard the spring of the Tekufah!'

Explanation of Gemara, Rosh ha'Shanah (2): according to the above explanation, that seems to be speaking where the Tekufah falls on the sixteenth iteself (see Maharsha), and the Sugya holds 'Yom Tekufah Gomer'. Consequently, even if one declares Adar Malei, and the Tekufah falls on the fifteenth, the spring of the Tekufah will not fall within the period of the new moon.

Explanation of Gemara, Rosh ha'Shanah (3): Whereas if it would have lasted until the fifteenth of Nisan, then it would have sufficed to declare Adar Malei, thereby eliminating the need to declare it a leap-year.

Explanation of Gemara, Rosh ha'Shanah (4): And when the Gemara says there says 'And don't worry about it!', it means that the author is Acherim, who is an indivudual, and the Halachah is therefore not like him, as Rashi explains there.

Question: The question arises however, why does the Gemara there need to come on to the reason of 'Aviv shel Tekufah'? Why does it not give the reason because of Tekufas Tishri, because when Tekufas Nisan falls on the sixteenth, then Tekufas Tishri falls either on the twenty-first or on the twenty-second, and even if it falls on the twenty-first, not even part of the Yom-Tov falls in the new Tekufah, seeing as this Tana holds 'Yom Tekufah Gomer'?

Answer #1: According to Rabeinu Tam (earlier) it fits nicely, since he holds that when Tekufas Teives lasts until the beginning of the sixteenth of Nisan or at least until the first six hours of the night, then it is not necessary to declare a leap-year on account of Tekufas Tishri, since adding one day to Adar or to Ellul will result in Teufas Tishri falling on the twentieth.

Answer #2: Even according to Rashi however, one can also answer that the Gemara is coming to teach us that specifically if the Tekufah lasts up until the sixteenth, is it necessary to declare a leap-year, but not if it lasts only until the fifteenth - even if it falls at the beginning of the day or after six hours, because then it will suffice to declare Adar Malei. And even though this will result in Tekufas Tishri falling on the twenty-first, this is nothing to worry about, seeing as we rule like Acherim, who is not concerned about Tekufas Tishri, since he does not Darshen "Chag ha'Asif".

Clarification: Even according to Rabeinu Tam, who explained (on Amud Alef) that the Gemara is asking according to all the Tana'im, it is evident from Rav Acha bar Ya'akov's answer, that it is asking according to Acherim as well.

Question: What is now the question (on Acherim), because in the first Perek of Rosh ha'Shanah (Daf 20a), Rabah bar Shmuel cites a Beraisa which, based on the Pasuk "he'Chodesh ha'Zeh", learns that when the need arises, one may declare a leap-year, but not a full month ...

Question (cont.): ... and Rava establishes Rabah bar Shmuel's Beraisa like Acherim, who holds 'Ein Bein Rosh ha'Shanah le'Rosh ha'Shanah Ela Arba'ah Yamim Bil'vad' (the months run 'Malei, Chaser' alternatively and cannot be changed, so that their are always four days between Rosh ha'Shanah of one year and Rosh ha'Shanah in the next)?

Clarification: This is not coming to answer the Kashya 've'le'Abreih la'Adar', as Tosfos already explained earlier (on Amud Alef).