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ve; one of the conic sections formed by the intersection of the surface of a cone with a plane parallel to one of its sides. It is a curve, any point of which is equally distant from a fixed point, called the focus, and a fixed straight line, called the directrix. See Focus.

rotation of a parabola about its axis; any surface of the second order whose sections by planes parallel to a given line are parabolas.

nce from a center. Paracentric curve (Math.), a curve having the property that, when its plane is placed vertically, a body descending along it, by the force of gravity, will approach to, or recede from, a fixed point or center, by equal distances in equal times; -- called also a paracentric. -- Paracentric motton or…

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 o…

ed in the same direction, and in all parts equally distant; as, parallel lines; parallel planes. Revolutions . . . parallel to the equinoctial. Hakluyt.

The ratio of the three crystallographic axes which determines the position of any plane; also, the fundamental axial ratio for a given species.

With the lower focal points all in the same plane; -- said of sets of eyepieces so mounted that they may be interchanged without varying the focus of the instrument (as a microscope or telescope) with which they are used.

To scrape or beat with the forefoot. His hot courser pawed the Hungarian plane. Tickell.

et fall from a fixed point upon the straight lines tangent to a given curve, or upon the planes tangent to a given surface. -- Pedal note (Mus.), the note which is held or sustained through an organ point. See Organ point, under Organ. -- Pedal organ (Mus.), an organ which has pedals or a range of keys moved by the f…

A plane figure having five angles, and, consequently, five sides; any figure having five angles. Regular pentagon, a pentagon in which the angles are all equal, and the sides all equal.

A genus of plants including water purslane.

t is sometimes, but rarely, used of the nearest points of other orbits, as of a comet, a planet, etc. Called also epigee, epigeum.

That point of the orbit of a planet or comet which is nearest to the sun; -- opposed to Ant: aphelion.

olution; proceeding in a series of successive circuits; as, the periodical motion of the planets round the sun.

The region surrounding a moving body, such as the wing of a bird or a gliding aëroplane, within which cyclic or vortical motion of the air occur.

Exactly upright or vertical; pointing to the zenith; at right angles to the plane of the horizon; extending in a right line from any point toward the center of the earth.

Pertaining to the art, or in accordance with the laws, of perspective. Perspective plane, the plane or surface on which the objects are delineated, or the picture drawn; the plane of projection; -- distinguished from the ground plane, which is that on which the objects are represented as standing. When this plane is ob…

e force additional to that which causes its regular motion; as, the perturbations of the planets are caused by their attraction on each other. Newcomb.

Of or pertaining to perturbation, esp. to the perturbations of the planets. "The perturbational theory." Sir J. Herschel.

t to quantity of illumination or form of enlightened disk; as, the phases of the moon or planets. See Illust. under Moon.