Enormously large (12)
(Other definitions for astronomical that I've seen before include "Of the study of stars" , "Really, really big" , "Tremendous" , "Inconceivably large" , "Absolutely huge, in a heavenly way" .)
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">enormously large</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for astronomic that I've seen before include "Sky-high (colloq.)" , "whacking" , "Immense" , "Incredibly large - so romantic (anag)" , "Relating to science" .)