Bag, A K.. "Early System of Nakṣatras, Calendar, Antiquity o Vedic and Harappan Traditions." Indian Journal of History of Science 50, no. 1 (2015).

The fixation of time for Fire-worships and rites was of prime importance in the Vedic traditions. The apparent movement of the Sun, Moon, and a Zodiacal system along the path of the Sun/Moon withnakatras (asterisms or a group of stars) were used to develop a reasonable dependable calendar maintaining a uniformity in observation of nakatras, from which the antiquity of these early traditions could be fixed up. The gvedic tradition recognized the northern and the southern (uttarāyana and dakiāyana) motions of the Sun, referred originally to six nakatras (raised to 28 or 27) including Aśvinī nakatra citing it about 52 times. It recommended the beginning of the Year and a calendric system with the heliacal rising of Aśvinī at the Winter solstice. When Aśvinī was no longer found at Winter solstice because of the anti- clockwise motion of the zodiacal nakatras due to precision (not known at the time), the Full-moon at Citrā nakatra in opposition to the Sun at Winter solstice was taken into account as a marker for the Year- beginning, resulting in the counting of the lunar months from Caitra at the Winter solstice during Yajurvedic Sahitā time. The same system continued during the Brāhmaic tradition with the exception that it changed the Year-beginning to the New-moon of the month of Māgha (when Sun and Moon were together after 15 days of Full-moon at Maghā nakatra), resulting in the corroboration of the statement, ‘Kttikānakatra rises in the east’. The Vedāga-jyautia continued the same counting system from the New- moon, assigning the beginning of Śravihā segment of the nakatras as the beginning of 5-year Yuga at Winter solstice. The antiquity of these gvedic, Yajurvedic, Brāhmaic and Vedāga-jyautia traditions may be found by comparing the old and new longitudes of nakatras and fixed at 6500 BC, 5000 BC, 2500 BC and 1000 BC respectively after corrections due to visibility error. This system of astronomical dating, based on long uniform pattern of observations, are possible in a culture obsessed with satisfactory domestic cultivation and regular worships. The Harappan tradition around c.2000 BC followed the Yajurvedic tradition of counting of month from the Full-moon in a star in opposition, still prevalent in some parts of North India, unlike New-moon Brāhmaic system in South India. The calendric elements were found to be luni-solar, and in the process, the types of years, months, days, day-lengths, intercalation, seasons, nakatras & nakatra space (aśa, bhāśa), tithis, full-moon & new-moon in a Yuga, eighteen/ nineteen years’ cycle for adjustment of synodic tropical year with lunar year have been explained and discussed.