Is it ensure, insure, or assure?
1550s, from astronomy + -ical. Popular meaning "immense, concerning very large figures" (as sizes and distances in astronomy) is attested from 1899. Astronomical unit (abbreviation A.U.) "mean distance from Earth to Sun," used as a unit of measure of distance in space, is from 1909. Related: Astronomically.
Inconceivably large: an astronomical price for that car