Archaeoastronomy and Vedic chronology
The history of Indian astronomy begins with the Lagadha, a treatise of the Mauryan era composed well after the end of the Vedic period. Astronomical knowledge in India reached an early peak in the 5th century CE, with the Āryabhaṭīya. Its author, Aryabhata, uses astronomical calculations to determine the date of the Battle of Kurukshetra as 18 February 3102 BCE. This date has become traditional and is still widely cited in Hindu literature.
Modern authors attempted to date the Vedic period based on archaeoastronomical calculations. Thus, William Jones who tried to show, based on information gathered from Varaha Mihira, that Parashara muni lived at 1181 BCE. Jacobi (1909) has argued that in the Rigveda and Atharvaveda the sun was in Phalguni,[clarification needed] and in the Sankhayana and Gobhila Grhyasutra the Full moon was in Bhadrapada during the summer solstice,[clarification needed] which would have occurred at 4500-2500 BCE.
Jacobi and Tilak have both noted that the terms of the naksatras Mula (root), Vicrtau (dividers) and Jyestha (oldest) suggest that these names originated from a time when Mula marked the beginning of the year, i.e. about 4500-2500 BCE. Tilak has also noted that the two week long pitrs period after the full moon in Bhadrapada occurred at the beginning of the pitryana, which would have been true at about 4500-2500 BCE.
Subhash Kak in his Astronomical Code of the Rgveda dates the Rigveda to "4000-2000 BCE"; Kak's results have been criticized by Plofker as having "no statistical significance whatsoever", even if overlooking their being based on the structure of the Iron Age shakha (recension) of Shakala rather than the content of the actual Rigvedic texts.
"The One" referred to in the Nasadiya Sukta and other hymns has been suggested to have originally referred to the axis mundi, and "The One who dwells beyond the seven sages" as the polar star, at the time referring to Thuban (α Draconis).
The samvatsara "full year" in the Yajurveda has 360 days, and 12 (TS) or 13 (VS) months.
The visuvant (summer solstice) period is 21 days in Aitreya Br. and 7 days in Pancavimsa Br., the summer solstice being in the middle of the period.
The gavam ayana ritual in SB 4.6.2. is based on the motion of the sun.
In the Maitrayana Brahmana Upanishad (6.14), the year is said to be into two portions, with the part from Magha to half of Śraviṣṭha associated with Agni, and the part from Sārpa to half of Śraviṣṭha associated with Varuna and Saumya (the moon). Aiyar has argued that Agni suggests the warm half and similarly Varuna the cool half of the year, suggesting the summer solstice at the beginning of Maghā and thus implying the vernal equinox in Kṛttikā. This, according to Kak, would correspond to 1660 BCE.
The Shatapatha Brahmana mentions that the Krttikas (the Pleiades) "do not swerve from the east". This would have been the case with precision at 2950 BCE and was true also about 2000 BCE, but was still true to within 8-13 degrees (viz., East by north) around the 8th to 6th centuries BC, the assumed date of the text's composition.
The positions of the solstices and equinoxes in the Vedanga Jyotisha, with the sun very close to the Krittika at the Vernal Equinox., would correspond to about 1370 BCE, although the text in its present form is from a later date.
The Vedanga Jyotisha, in common with Mesopotamian texts, asserts a 3:2 ratio between the durations of daylight on the longest and shortest days of the year. This corresponds to a latitude of about 35 degrees. A latitude of 34 degrees would correspond to Northern India.
- Bryant 2001:251
- Bryant 2001:254; Jacobi 1909
- Bryant 2001:255
- S. Kak, The Astronomical Code of the Rgveda. New Delhi: Aditya Prakashan 1994.
- Plofker, K. Review of Kak (1994), Centaurus 38 (1996), 362-364; see also Michael Witzel, "Autochthonous Aryans? The Evidence from Old Indian and Iranian Texts," Electronic Journal of Vedic Studies, Vol. 7 (2001) issue 3 (May), §28
- Bjorn Merker, Rig Veda Riddles In Nomad Perspective, Mongolian Studies, Journal of the Mongolian Society XI, 1988.
- Bryant 2001:253
- Kak, Subhash: Archaeoastronomy and Literature, Current Science, vol. 73, no.7, 1997
- paraphrased and cited by Bryant 2001:257-8
- Subhash Kak. Birth and Early Development of Indian Astronomy. In Astronomy across cultures: The History of Non-Western Astronomy, Helaine Selin (ed), Kluwer, 2000
- ŚBM 22.214.171.124: kṛttikāsv agnī ādadhīta ... etā ha vai prācyai diśo na cyavante "One should found one's fires under the Krttikas ... These do not swerve from the eastern direction...All other Nakshatras deviate from the Eastern direction."
- The SB reference was first noted by Sankar B. Dikshit (Bryant 2001:256). Dikshit, Sankar B. 1985. "The Age of the Satapatha Brahmana" Indian Antiquary 24:245-6
- (or during the third millennium BCE) e.g., Kak, Subhash: Archaeoastronomy and Literature, Current Science, vol. 73, no.7, 1997
- Texts of the Brahmana period do only distinguish intermediate directions, viz. eighths of the compass, and the rising of the Pleiades fell into the Eastern eighth until well after the Vedic period. See also Michael Witzel, The Pleiades and the Bears viewed from inside the Vedic texts, EVJS Vol. 5 (1999), issue 2 (December)  (Witzel speculates that the reference may be based on centuries old priestly traditions of times when the Pleiades were even closer to due East) and Witzel EJVS Vol. 7 (2001) issue 3 (May) 
- Sastry 1985
- Bryant 2001:259. Keith 1912
- Pingree (1978)
- Bryant 2001:261. Yukio Ohashi 1997
- Bryant, Edwin (2001), The Quest for the Origins of Vedic Culture, Oxford University Press
- David Frawley. 1991. Gods, Sages, and Kings, Lotus Press, Twin Lakes, Wisconsin ISBN 0-910261-37-7
- Kak, Subhash, The Astronomical Code of the Rigveda (1994, 2000).
- Tilak, Bal Gangadhar. The Orion or Researches into the antiquities of the Vedas, The Arctic home in the vedas, Vedic Chronology and Vedanga Jyotisha. Poona: Messrs Tilak Bros.