Diurnal adj. 1.Relating to the daytime; belonging to the period of daylight, distinguished from the night; opposed to nocturnal; as, diurnal heat; diurnal hours.
2.Daily; recurring every day; performed in a day; going through its changes in a day; constituting the measure of a day; as, a diurnal fever; a diurnal task; diurnal aberration, or diurnal parallax; the diurnal revolution of the earth. "Ere twice the horses of the sun shall bring Their fiery torcher his diurnal ring."
3.(Bot.) Opening during the day, and closing at night; said of flowers or leaves.
4.(Zool.) Active by day; applied especially to the eagles and hawks among raptorial birds, and to butterflies (Diurna) among insects.
Diurnal aberration (Anat.), the aberration of light arising from the effect of the earth's rotation upon the apparent direction of motion of light.
Diurnal arc, the arc described by the sun during the daytime or while above the horizon; hence, the arc described by the moon or a star from rising to setting.
Diurnal circle, the apparent circle described by a celestial body in consequence of the earth's rotation.
Diurnal motion of the earth, the motion of the earth upon its axis which is described in twenty-four hours.
Diurnal motion of a heavenly body, that apparent motion of the heavenly body which is due to the earth's diurnal motion.
Diurnal parallax. See under Parallax. Diurnal revolution of a planet, the motion of the planet upon its own axis which constitutes one complete revolution.
Diurnal n. 1.A daybook; a journal. (Obs.)
2.(R. C. Ch.) A small volume containing the daily service for the "little hours," viz., prime, tierce, sext, nones, vespers, and compline.
3.(Zool.) A diurnal bird or insect.
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