Yes, I know there are already proposed philosophies out there, in which the entire point is to establish a regular ritual acknowledging Hashem's creation of the universe, such that the inaccuracy of the proposed time when compared with the vernal equinox is irrelevant. I wonder if, rather than assuming that the sources are saying things other than what they explicitly say, we can intuit a philosophy behind this birchat hachama which comes out of the sources themselves. Maybe others have said this already, or maybe this is entirely off base. Regardless, these are my musings.
The now-famous gemara, in Berachot 59b:
אמר אביי כל כ"ח שנין והדר מחזור ונפלה תקופת ניסן בשבתאי באורתא דתלת נגהי ארבע:
There also seems to something to the perfection of the regularity of the movements of the constellations, of the sun, of the moon, that is being appreciated here, and acknowledging Hashem's wondrous Creation. And thus these are instituted at the start of their cycle.
ו,ח הרואה את הקשת בענן אומר נאמן בבריתו ברוך זוכר הברית.
ו,ט היה הולך בבית הקברות אומר ברוך יודע מספר כולכם הוא יודע הוא עתיד לדון הוא עתיד להקימכם ברוך מחיה מתים במאמרו.
ו,י את החמה ואת הלבנה ואת הכוכבים ואת המזלות כסדרן אומר ברוך עושה בראשית ר' יהודה אומר המברך על החמה זו דרך אחרת וכן היה רבי יהודה אומר הרואה את הים תדיר ונשתנה בו דבר צריך לברך.
In our gemara, we have a redefinition of the tekufah of the Sun, by Abaye. Some claim, based on various sources, that this was a late interpolation. I am not persuaded by this. Yes, I agree that certain things are late interpolations, but this has Abaye, an Amora, attached to it. A maginal comment can be appended to the gemara, but to make this a maamar by an Amora, it would need to be be a forgery; either that, or we would have to assert that by some mechanism (usually shibush based on surrounding text) the Amora's name was attached to the interpolation.
So I do believe that Abaye said it. What is Abaye doing? One could say he is reducing the tekufah in the brayta to a specific one of twenty-eight tekufot. Or one could say that he is redefining tekufah. The Chama in its tekufah is the sun in its turning point. Abaye is clarifying that the brayta meant a different turning point than we might expect. Not the vernal equinox, but rather, the start of the Machzor Gadol.
Why would Abaye say this? One possibility is simply that he had a tradition that it was true, and he was clarifying what otherwise we would not know. Another possibility is that this is a "clarification" -- a way of reinterpreting a brayta almost out of existence. A once-a-year blessing of the vernal equinox is too close to pagan practice, or sun-worship, so instead, relegate it to a blessing once in 28 years, if you happen to catch it. That is, Abaye holds like Rabbi Yehuda in the Tosefta, and he "interprets" the brayta from its plain meaning, the vernal equinox, to mean the beginning of the Machzor Gadol, in order to sequester it.
Meanwhile, the parallel Yerushalmi, in Yerushalmi Berachot 65a, has no such redefinition, so it still refers to the vernal equinox. (The interpretations of that Yerushalmi to include a redefinition of tekufah to the sun after three cloudy days, in the rainy season, is without merit, as that is referring to the rakia rather than the sun.)
If this is correct, then Abaye was not trying to be mataken, establish, a super-duper-special once-in-28-years festival; nor was he trying to tie this is to maaseh bereishit even more; or insert-your-own-attributed-reason-here. Rather, he was trying to redefine the brayta to reduce the chances of performing this ritual, while not arguing against it directly.
Could he have used R' Ada's calculation, which is more accurate? No, because there is not really a cycle there. Or if a cycle, it is not a cycle that would repeat from maaseh bereishit for several thousand years, such that it would not make sense as a reinterpretation of the brayta.
But doesn't this calendar of Shmuel eventually take us away from the true vernal equinox? Well, we cannot really pretend to know the motivations or understanding here. (Even though I am, above :) Perhaps Abaye did not perform the complicated calculations himself, but relied on the calendar which had a cycle, and so while he knew it was an approximation, did not realize that the shift would occur so soon and so significantly, moving away from the true tekufah so drastically in such short order. Perhaps he did know, but his point was to establish the cycle, and he figured that later generations would of course adjust the calendar to a new time in a generation or two to keep the approximation close enough. One thing I would not leap to is that Abaye doesn't care that the cycle is fictional and the time is not the tekufah, but rather has deep mystical or philosophical reasons he is not sharing which we get to attribute to him, and which are against the plain meaning of his words.
So we get this approximation, and the approximation was good enough in Abaye's time. And the intent brayta is still to say these words when the circumstances arise. One does not chase after thunderclouds. And perhaps one does not chase after the Sun.
The idea I am trying to get across here is that this is not being mesaken a new festival, once in 28 years, such that we rely on an instituted time. Rather, it is still dependant upon seeing the actual thing. And this would be seeing the Sun in the beginning of its new cycle. We should then still expect it to be in its vernal equinox, more or less, based on adjusting every century or so, so that it is indeed the beginning of its new cycle, from its starting position. (Even if that means changing the day of the week, or month.)
This is not a "festival" which should exist whether or not the phenomenon exists, and such that we should necessarily pay heed to the "established" time rather than the existence of the phenomenon. The brayta again, was הרואה חמה בתקופתה לבנה בגבורתה וכוכבים במסילותם ומזלות כסדרן אומר ברוך עושה בראשית. If it is a cloudy day, and we don't actually see the sun, we obviously would not say the bracha. Here, we are not seeing the Chama in its tekufah -- certainly not the vernal equinox, but also not the beginning of what used to be considered the Machzor Gadol, the uber-vernal equinox at the beginning of a rough cycle. (One could argue this last point.)
As I noted in the comments on another post, we could state this as a parsing issue. The brayta states הרואה חמה בתקופתה. Is בתקופתה an adverb or an adjective. Is it the adjective: "one who sees the sun in its tekufah?" Or is it the adverb: "one who sees, during the tekufah, the sun?"
That is my suggestion. Of course, others can interpret it otherwise. One point I would make is that encoding the halacha perhaps based purely on kelalei horaah and what appears in the gemara, rather than second-guessing the opinions, is not necessarily making a positive point of the underlying nature of the mitzvah -- that it is not X, but rather Y. It can make no sense to us, based on astronomy we know, but we can still encode it. Or it can have some other meaning.
The Rambam encodes it lehalacha, and he surely knows astronomy. Thus,
I am unfortunately harried right now, so where I would have liked to elaborate in several places, I cannot. There is too much noise and I cannot think. So I apologize if this ended up disjointed.