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civil day[′siv·əl ′dā]
A mean solar day beginning at midnight instead of at noon; may be based on either apparent solar time or mean solar time.
A day of constant 24-h duration. It is the time interval between two successive lower transits of the mean sun for the meridian of that place. The civil day begins at midnight and the civil time is given by the hour angle of the mean sun plus 12 h. A civil day is divided into 24 equal parts—mean solar hours—which in turn are divided into minutes and seconds. Also called a solar day or a mean day.