Dear McComas Taylor,
According to uṇādi-sūtra ‘divo dve dīrghaś cābhyāsakhyaḥ’ (Siddhāntakaumudī no. 495/Ujjvaladatta 4.55), the word dīdivi is derived from the root div (class 4) with the affix KviN. In his Bālamanoramā under this sūtra, Vāsudeva Dīkṣita quotes ṛgveda 1.1.8 “gopām ṛtasya dīdivim” and says that the word is used with meaning of ‘dyotamāna’.
Yours
McComas Taylor,
Professor of Sanskrit
CHL Deputy Director, Languages
College of Asia and Pacific, Australian National University
Secretary-General, International Assoc. of Sanskrit Studies