/ Dictionary / Index N nocturnal arc: Meaning and Definition of Find definitions for: noctur'nal arc' Pronunciation: [key] — Astron. Astron. the portion of the diurnal circle that is below the horizon of a given point. Cf. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. nocturnal nocturnal emission Related Content Daily Word Quiz: procrastinate Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language