If the ancestral formation is *h₂ewsreh₂, Lithuanian aušrà, dial. auštrà (“dawn”), is identical in formation, as is Ancient Greek αὔρᾱ (aúrā, “(esp. cool) breeze, fresh air of the morning”), if from Proto-Greek *aúhrā (< *h₂éwsreh₂). Sanskrit उस्रा (usrā́-, “dawn, morning”) (from *h₂usreh₂) only differs by the zero-grade in the root.
Also compare Proto-Slavic *utro (“morning, dawn”) (with variants including *ustro, from Proto-Balto-Slavic *auš(t)ra- (“dawn, morning”)) and Albanian err (“darkness”) (if from *ausra (“twilight”)). For the epenthetic -t- in Proto-Germanic (also widespread in Balto-Slavic), compare *þimistraz (“dark, dusky”).
All of these may go back to a Proto-Indo-European adjective *h₂us-ró- (> Sanskrit उस्र (usrá-, “reddish, ruddy, bright, matutinal”)) ~ *h₂éws-ro- (“of the dawn or morning, matutinal; eastern”), derived from a r-stem noun *h₂us-r- (forms belonging to such a formation are attested in the oldest Vedic), explaining the differences in ablaut and gender. Proto-Celtic *wāsrī from *h₂wōsrih₂ (> Old Irish fáir) is close as well.
- *austraz ("east, dawn")
- Old English: ēastre, ēostre, Ēostre
- Old Frisian: āsteron
- Old Saxon: *ōstara
- Old High German: ōstra, ōstrūn, ōstarūn
- → Slavic: