'astronomic' originates from Greek, specifically the word 'astronomia', where 'astron' meant 'star' and 'nomos' meant 'law' or 'arrangement'.

'astronomia' passed into Latin as 'astronomia', then into Old French and Middle English (as 'astromie'/'astronomie'), and eventually produced the modern English adjective 'astronomic' (and related forms such as 'astronomy' and 'astronomical').

Initially, it meant 'the law or arrangement of the stars' (i.e., the study of celestial bodies); over time it evolved to mean both 'relating to the science of astronomy' and, by figurative extension, 'extremely large' or 'vast.'