How are dates assigned to Vedas and Scriptures according by historians? Do they really indicates the date of origin.
Stephan Hillyer Levitt's paper The Dating of the Indian Tradition provides a good summary of how different historians and indologists have attempted to date the Vedas and the Vedic period.
F. M. Müller suggested that Buddhism is simply a reaction against Brahmanism, and it presupposes the existence of the whole Veda — the Vedic hymns. the Brāhmaṇas, the Āraṇyakas, and the Upaniṣads. The whole of this literature, therefore, is pre-Buddhist, the Buddha's parinirvāṇa having been in the early part of the 5th c. B. C. The Vedāṇga and Sūtra literature might be of approximately the same date as the origin and first spread of Buddhism. This literature, which necessarily presupposes the Brāhmaṇa literature, was dated to a period of 600 to 200 B. C. Now the Brāhmaṇas, Müller argued, cannot possibly have been composed in less than 200 years. Therefore, these were dated from 800 to 600 B. C. The Brāhmaṇas presuppose the Vedic Saṃhitas, the collections of songs and prayers, and so 200 years, roughly 1000 to 800 B.C., were allowed for these collections to be arranged. Before the compilation of these collections, which were already regarded as sacred sacrificial poetry and authorized prayer books, there must have been a period at which hymns themselves arose as popular or religious poems. This, he concluded, must have been before 1000 B.C. And as 200 years had already been assumed for the Brāhmaṇas and for the period he called the mantra period, 200 years was also now assumed for the arising of the poetry, and this period of 1200 to 1000 B.C. was arrived at as the period of the composition of the Vedic hymns.
The paper itself, however, noting some similarities between ancient Mesopotamian and Vedic religions proposes late 4000 B. C. as the period when the first of the Rig-vedic hymns were composed. The author says:
My argument in this paper, though, is that we can date the early Indic tradition on the basis of comparable points in ancient Mesopotamia. By this, the Ṛgveda would date back to the beginning of the 3rd millennium B. C., with some of the earliest hymns perhaps even dating to the end of the 4th millennium B. C., the composition of the Ṛgveda would end at about 1500 B. C. with the end of Indus Valley civilization and with the first period of doubt and severe crisis of faith in Mesopotamian civilization. We then have Müller's mantra period, the composition of the Atharvaveda coinciding in the main with this and with the growth of an interest in magic in ancient Mesopotamia in the latter half of the 2nd millennium B. C., and the composition of the Brāhmaṇas and Āraṇyakas which texts also indicate this interest in magic as well as the development of monotheism from the late Ṛgveda. The development of monotheistic deities in India can be seen as reflecting the emphasis on personal deities in ancient Mesopotamia in the 2nd millennium B. C. Tentatively, I would date the Upaniṣads to the beginning of the 1st millennium B. C. coinciding with the second crisis of faith in ancient Mesopotamia.
THE CULTURE OF THE ṚG VEDA
No real synchronisms are contained in the Ṛg Veda itself, to give us any certain information on the date of its composition. Some authorities in the past claimed an exceedingly early date for it, on the basis of tradition and ambiguous astronomical references in the hymns themselves—it was even believed by one very respected Indian scholar that it was as old as 6000 B.C. The discovery of the Indus cities, which have nothing in common with the culture described in the Veda and are evidently pre—Vedic, proves that the hymns cannot have been composed before the end of Harappā. The great development in culture, religion and language which is evident in the later Vedic literature shows that a long period must have elapsed between the time of the composition of the last hymns of the Ṛg Veda and the days of the Buddha—perhaps as much as 500 years. It is therefore probable that most of the Ṛg Veda was composed between 1500 and 1000 B.C., though the composition of some of the most recent hymns and the collation of the whole collection may have taken place a century or two later.
The 'Indian scholar' Basham was referring to was none other than the Indian independence activist Bal Gangadhar Tilak who wrote The Orion or the Antiquity of the Vedas in 1893. In his article Lokamanya Tilak and the Astronomical Dating of the Vedas, Indian astrophysicist Jayant V. Narlikar writes:
Tilak therefore concluded that the most important period in the Aryan civilization was the so-called Orion period which occurred around 4000 B.C. to 2500 B.C.
During this period the spring equinox shifted from the constellation of Ardra to Krittikas. This was the period during which Vedic Suktas were written and sung. During the latter part of this period, according to Tilak, the Aryans divided and went three ways, to Greece, Persia and India. This period was followed by the Krittika period which extended from 2500 B. C. to 1400 B. C. The latter part of this period is recorded in the Vedanga Jyotisha.
According to Tilak the Vedic period extended even further back to around 6000 B. C. There are references in the Vedas for commencing sacrifices at Aditi the presiding deity of Punarvasu. Tilak argued that this epoch corresponds to this spring equinox being at or near Punarvasu.
In another article, Narlikar says Tilak was led to this approach while contemplating on a particular verse (10.35) from the Bhagavad-gītā where Kṛṣṇa identifies himself with the Mārgaśirṣa month from the Hindu calendar.
This was the clue that Tilak worked on. He was led to it by a shloka from the Bhagavad Gita in which Lord Krishna, identifying himself with the best and most important in any class of objects or people, says: "I am Margashirsha amongst the months and spring amongst the seasons".
In modern times Margashirsha does not fall in the spring season; rather it falls closer to the autumn. So why this discrepancy? The discrepancy is resolved if we argue that the statement was made when Margashirsha fell in the spring season. By turning the earth clock backwards, we move the equinoxes backwards until the spring equinox was in the zodiacal group identified with Margashirsha. This gave him an estimate of the antiquity of the statement.
Tilak used this approach to look at astronomical allusions in Vedic literature and from them sought to build up the stellar framework that must have existed when the statement was made.
His monograph, The Orion: Research into the Antiquity of the Vedas, is a scholarly discussion of this approach. He arrived at an age for the Vedas much older than the age estimated by Western scholars. This opened the door to controversy as to which method is correct. While Tilak's reliance on astronomical data gave him a reliable clock, the weakness of his method probably lay in the authenticity of the allusions he had used. Whatever the eventual outcome of this ongoing exercise of dating our ancient literature, we have to give credit to Tilak for his ingenious approach.
Do they really indicate the date of origin?
As of now, the dates are just a guesstimate. Regarding his own calculations, Müller said:
We cannot hope to fix a terminus a quo. Whether the Vedic hymns were composed 1000, 1500, or 2000, or 3000 years B. C., no power on earth will ever determine.