Equinoctial full moon of the Brahmāṇḍa Purāṇa and the nakṣatra solar zodiac starting from summer solstice

The first theoretical system of tracking sun in the tropical annual cycle is cryptically mentioned in the Maitrāyaṇīya Āraṇyaka Upaniṣat (MAU) of the Kṛṣṇa Yajurveda, as the southern sojourn of sun starting at the summer solstice. This is called maghādyaṁ, the first point of the maghā nakṣatra, identified most likely with the early morning visibility of ε-Leo, near the azimuth of the sunrise point on the horizon as observed at Kurukshetra. Twenty seven equal nakṣatra sectors named in the traditional sequential order cover one tropical circuit of sun of 366 days with the winter solstice falling exactly at the middle of the śraviṣṭhā sector. Even though MAU mentions each nakṣatra to be made up of four quarters, no practical application of this ¼-nakṣatra sky part amounting to 3º20´ in longitude is seen in Vedic texts till we come to the Brahmāṇḍa Purāṇa, a text closer to the Vedas. This Purāṇa states, observed equinoctial full moon positions corresponding to spring equinox at ¼-kṛttikā and autumn equinox at ¾-viśākha exactly 180º apart as they should be. This statement is analysed in this paper by computer simulation of full moon time series for the years − 2400 to − 800 to show that the Purāṇa data would be realistically valid for the period 1980 BCE to 1610 BCE. It is further demonstrated that the Purāṇa has followed the maghādi system of solar nakṣatra system stated in the MAU. The central epoch circa 1800 BCE of this maghādi equal nakṣatra solar zodiac got modified, due to precession effects, to the śraviṣṭhādi scheme of Parāśara, Vṛddha Garga and Lagadha dateable to circa 1300 BCE.