Parallax

parallax
    n 1: the apparent displacement of an object as seen from two
         different points that are not on a line with the object
    

Parallax \Par"al*lax\, n. [Gr. ? alternation, the mutual
   inclination of two lines forming an angle, fr. ? to change a
   little, go aside, deviate; para` beside, beyond + ? to
   change: cf. F. parallaxe. Cf. {Parallel}.]
   1. The apparent displacement, or difference of position, of
      an object, as seen from two different stations, or points
      of view.
      [1913 Webster]

   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.
      [1913 Webster]

   3. (Astron.) The annual parallax. See {annual parallax},
      below.
      [PJC]

   {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; it is equivalent
      to the parallax of an astronomical object which would be
      observed by taking observations of the object at two
      different points one astronomical unit (the distance of
      the Earth from the sun) apart, if the line joining the two
      observing points is perpendicular to the direction to the
      observed object; as, the annual parallax of a fixed star.
      The distance of an astronomical object from the Earth is
      inversely proportional to the annual parallax. A star
      which has an annual parallax of one second of an arc is
      considered to be one parsec (3.26 light years) distant
      from the earth; a star with an annual parallax of
      one-hundredth second of an arc is 326 light years distant.
      See {parsec} in the vocabulary, and {stellar parallax},
      below.

   {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 parallax} 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.
      [1913 Webster]