The nakṣatra system is the oldest stratum of Vedic light-science. Whereas the 12-fold rāśi division comes from the ecliptic geometry of solar motion, the 27-fold nakṣatra division comes from the sidereal sky as the Moon traverses it across roughly 27.32 days. Each mansion subtends 13°20' of the ecliptic and is further sub-divided into four pādas of 3°20' each.
The Taittirīya Brāhmaṇa preserves the canonical devatā-assignment in its Nakṣatra-Sūkta (3.1.1 to 3.1.5). The first mansion Aśvinī is presided by the Aśvin twins; the second Bharaṇī by Yama; the third Kṛttikā by Agni; and so on through the 27. These are not ornamental labels — each devatā encodes the qualities the tradition associates with the moon-position.
In daśā computation, Pārāśara assigns each nakṣatra to a graha-lord, generating the 120-year Vimśottarī cycle. The pāda of the natal Moon determines the starting fraction of the first daśā. This module gives you the foundation; later modules in the BPHS and Practitioner paths use it operationally.