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.