The Collaborative International Dictionary of English v.0.48:

Astronomic \As`tro*nom"ic\, a.
   Astronomical.
   [1913 Webster]

WordNet (r) 3.0 (2006):

astronomic
    adj 1: relating or belonging to the science of astronomy;
           "astronomic telescope" [syn: astronomic,
           astronomical]
    2: inconceivably large [syn: astronomic, astronomical,
       galactic]

Moby Thesaurus II by Grady Ward, 1.0:

98 Moby Thesaurus words for "astronomic":
   Atlantean, Brobdingnagian, Cyclopean, Cynthian, Gargantuan,
   Herculean, Homeric, abysmal, amplitudinous, anagalactic,
   asteroidal, astral, astrologic, astrologistic, astrologous,
   astronomical, astrophysical, awesome, boundless, bulky, celestial,
   circumplanetary, cislunar, colossal, cosmic, elephantine, empyreal,
   empyrean, enormous, epic, equinoctial, extensive, extragalactic,
   galactic, giant, giantlike, gigantic, heavenly, heliacal, heroic,
   huge, immeasurable, immense, infinite, intercosmic, interplanetary,
   intersidereal, interstellar, jumbo, king-size, large, lunar,
   lunary, lunate, lunular, lunulate, mammoth, massive, massy,
   meteoric, meteoritic, mighty, monster, monstrous, monumental,
   mountainous, nebular, nebulose, nebulous, outsize, overgrown,
   planetal, planetarian, planetary, planetesimal, prodigious,
   profound, semilunar, sidereal, sizable, solar, spacious, sphery,
   star-spangled, star-studded, starry, stellar, stellary, stupendous,
   terrestrial, titanic, towering, tremendous, uranic, vast,
   voluminous, weighty, zodiacal