2. (Astron.) The apparent difference in position of a body
(as the sun, or a star) as seen from some point on the
earth's surface, and as seen from some other conventional
point, as the earth's center or the sun.
{Annual parallax}, the greatest value of the heliocentric
parallax, or the greatest annual apparent change of place
of a body as seen from the earth and sun; as, the annual
parallax of a fixed star.
{Binocular parallax}, the apparent difference in position of
an object as seen separately by one eye, and then by the
other, the head remaining unmoved.
{Diurnal}, or {Geocentric}, {parallax}, the parallax of a
body with reference to the earth's center. This is the
kind of parallax that is generally understood when the
term is used without qualification.
{Heliocentric parallax}, the parallax of a body with
reference to the sun, or the angle subtended at the body
by lines drawn from it to the earth and sun; as, the
heliocentric parallax of a planet.
{Horizontal parallax}, the geocentric parallx of a heavenly
body when in the horizon, or the angle subtended at the
body by the earth's radius.
{Optical parallax}, the apparent displacement in position
undergone by an object when viewed by either eye singly.
--Brande & C.
{Parallax of the cross wires} (of an optical instrument),
their apparent displacement when the eye changes its
position, caused by their not being exactly in the focus
of the object glass.
{Stellar parallax}, the annual parallax of a fixed star.
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. --Shak.
3. (Bot.) Opening during the day, and closing at night; --
said of flowers or leaves.
4. (Zo["o]l.) 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.
2. (R. C. Ch.) A small volume containing the daily service
for the ``little hours,'' viz., prime, tierce, sext,
nones, vespers, and compline.
3. (Zo["o]l.) A diurnal bird or insect.