Par·al·lax n.
 1. The apparent displacement, or difference of position, of an object, as seen from two different stations, or points of view.
 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; 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.
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 Hor·i·zon·tal a.
 1. Pertaining to, or near, the horizon. “Horizontal misty air.”
 2. Parallel to the horizon; on a level; as, a horizontalline or surface.
 3. Measured or contained in a plane of the horizon; as, horizontal distance.
 Horizontal drill, a drilling machine having a horizontal drill spindle.
 Horizontal engine, one the piston of which works horizontally.
 Horizontal fire Mil., the fire of ordnance and small arms at point-blank range or at low angles of elevation.
 Horizontal force Physics, the horizontal component of the earth's magnetic force.
 Horizontal line Descriptive Geometry & Drawing, a constructive line, either drawn or imagined, which passes through the point of sight, and is the chief line in the projection upon which all verticals are fixed, and upon which all vanishing points are found.
 Horizontal parallax. See under Parallax.
 Horizontal plane Descriptive Geometry, a plane parallel to the horizon, upon which it is assumed that objects are projected. See Projection. It is upon the horizontal plane that the ground plan of the buildings is supposed to be drawn.
 Horizontal projection, a projection made on a plane parallel to the horizon.
 Horizontal range Gunnery, the distance in a horizontal plane to which a gun will throw a projectile.
 Horizontal water wheel, a water wheel in which the axis is vertical, the buckets or floats revolving in a horizontal plane, as in most turbines.
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 horizontal parallax
      n : the maximum parallax observed when the celestial body is at
          the horizon