from
The Collaborative International Dictionary of English v.0.48
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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from
The Collaborative International Dictionary of English v.0.48
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.
[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.
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