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