Per Vasmer, the first element more likely derives from *radъ than folk etymology’s *rajь. The second element from *dǫga would be cognate with Lithuanian dangùs (“sky, heaven”).
*radǫga f
singular | dual | plural | |
---|---|---|---|
nominative | *radǫga | *radǫdzě | *radǫgy |
genitive | *radǫgy | *radǫgu | *radǫgъ |
dative | *radǫdzě | *radǫgama | *radǫgamъ |
accusative | *radǫgǫ | *radǫdzě | *radǫgy |
instrumental | *radǫgojǫ, *radǫgǫ** | *radǫgama | *radǫgami |
locative | *radǫdzě | *radǫgu | *radǫgasъ, *radǫgaxъ* |
vocative | *radǫgo | *radǫdzě | *radǫgy |
* -asъ is the expected Balto-Slavic form but is found only in some Old Czech documents; -axъ is found everywhere else and is formed by analogy with other locative plurals in -xъ.
** The second form occurs in languages that contract early across /j/ (e.g. Czech), while the first form occurs in languages that do not (e.g. Russian).