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Until now, it was generally assumed that Tolkien’s decision to change the length of the Valian year to make it equal to 144 solar years (instead of 10 or 9.582 solar years as in earlier writings) was a consequence of the shift that occurred in the cosmology of his imaginary world in the late 1950s. This view was in fact expressed by Christopher Tolkien himself in his comments on text XI of Myths Transformed (Morgoth’s Ring, p. 430, n. 2) and became one of the main reasons underlying the common dislike of the new cosmology due to the incompatibility of the new conception of the Valian year with the chronology established in Tolkien’s earlier writings (such as the Annals of Aman and the Tale of Years).
One might wonder how the length on the Valian year was related to the shape of the Earth or the nature of the Sun and Moon and why changes regarding the latter would trigger a change in the former. If any such relation exists, it is not obvious, and evidence against such relation can be found in text I of the chapter The Valian Year in part one of The Nature of Middle-earth, which shows that when he wrote it, Tolkien had decided that the world must be round and coëval with the Sun and Moon, but the Valian year in that text was still equal in length to 10 solar years. Another point of interest in that text is the following passage:
The yên, which is merely a mode of reckoning, has nothing to do with the life of the Elves. In Aman this depended on the years of the Trees, or really on the days of the Trees; in Middle-earth on the cycles of growth, Spring to Spring, or löar. In Middle-earth, one löa aged an Elf as much as a year of the Trees, but these were in fact 10 times as long (The Nature of Middle-earth, p. 7).
The statement here that the yên, which had been equated to 144 solar years in Appendix D to The Lord of the Rings, “has nothing to do with the life of the Elves”, is in striking contradiction with the idea of the Elvish life-year of the same length, which is well-established in Tolkien’s later writings (particularly those published in part one of The Nature of Middle-earth). All the more curious is the fact that text II of the same chapter, while dealing with the same matters as text I, differs from it in that it introduces the equation of both the Valian year and the year of Elvish life to 144 solar years. This correspondence between the Valian year and the Elvish life-year reoccurs in all later texts which concern Elvish ageing, and this begs the question whether it is intentional.
The answer can be found in text XI of Myths Transformed published in Morgoth’s Ring (see esp. pp. 425–6), which tells that the Valian year was the minimal unit of time in which the “Ageing of Arda” could be perceived by the Valar, and all corporeal living things (such as plants and animals) that the Valar brought into being in Aman for their delight and use aged no quicker than Arda itself, so that the year of their life was the Valian year (see also The Nature of Middle-earth, p. 89). It is told here that the rate of ageing natural to the Elves accorded with the unit of Valian time, and this was the reason that made it possible for the Valar to bring the Elves to dwell in Aman, and a source of their bliss:
For the Eldar this was a source of joy. For in Aman the world appeared to them as it does to Men on Earth, but without the shadow of death soon to come. Whereas on Earth to them all things in comparison with themselves were fleeting, swift to change and die or pass away, in Aman they endured and did not so soon cheat love with their mortality (Morgoth’s Ring, p. 426).
Another important evidence can be found in text B of the chapter The Awaking of the Quendi in part one of The Nature of Middle-earth, where Tolkien considers the problems of the chronology given in the Tale of Years:
No scale of Quendian “growth” or “ageing” is devised, but in Valinor events seem to show that they lived at about the rate of 1 VY = 1 year of Elvish life. This fits events in Valinor, for which it was arranged, but makes all the Eldar far too old in later narrative, unless we suppose that they remained unchanged, after maturity, for an indefinite time (The Nature of Middle-earth, p. 34).
It seems very likely that here in these words lies the reason why Tolkien introduced the idea that the Elves aged in units of time equal in length to 144 solar years, the purpose of which was to prevent them from being too old in the course of the Second and Third Ages, and after the concept of the Elvish life-year emerged, the Valian year was equated to it in length because it was meant to correspond to the rate of Elvish ageing. If that is true, then it must have been Tolkien’s post-LotR conception of Elvish ageing and not his reshaping of the cosmology that resulted in the new conception of the Valian year and the abandonment of the chronology of the Annals of Aman, which was never replaced in full.