horizontal parallax
    n 1: the maximum parallax observed when the celestial body is at
         the horizon

Horizontal \Hor`i*zon"tal\, a. [Cf. F. horizontal.]
   1. Pertaining to, or near, the horizon. "Horizontal misty
      air." --Milton.
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   2. Parallel to the horizon; on a level; as, a horizontalline
      or surface.
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   3. Measured or contained in a plane of the horizon; as,
      horizontal distance.
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   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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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.
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   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.
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   3. (Astron.) The annual parallax. See annual parallax,
      below.
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   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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