Gathas (underline mine) - meaning 'epics'. It is the same word in Sanskrit and other Indian languages even today. Another word which has remained the same over millenia is 'vera or beda' (a compound) that Ahur Mazda asked Yima, the king of men, to prepare, so that two of each species could be saved from a deluge by ice which was imminent. I think astronomical references and description of stellar positions should be taken as empirical proof. Vedas clearly mention that at one time the Aryan year began when sun rose in the asterism of Orion on the day of vernal equinox. (4,000 BC). At other times in the asterism of Pleiades (2,000 BC). Presently it is Arietis and waits for a change to Pisces. The earliest mention is of Castor and Pollux (earlier than 4,000 BC). * You can shorten it to Aup. as many people do.