- astronomical 1550s, adj. from ASTRONOMY (Cf. astronomy) + -ICAL (Cf. -ical). Popular meaning "immense, concerning very large figures" (as sizes and distances in astronomy) is attested from 1899. Astronomical unit (abbrev. A.U.) "mean distance from Earth to Sun," used as a unit of measure of distance in space, is from 1909. Related: Astronomically.
Etymology dictionary. 2014.