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The difference in apparent direction or position of an object when viewed from different points. For bodies of the solar system, parallax is the difference in the direction of the body due to the dis- placement of the observer from the center of the earth, and is called geocentric parallax, varying with the body’s altitude and distance from the earth. The geocentric parallel when a body is in the horizon is called horizontal parallax, as contrasted with the parallax at any altitude, called parallax in altitude. Parallax of the moon is called lunar parallax. In marine navigation it is customary to apply a parallax correction to sextant altitudes of the sun, moon, Venus, and Mars. For stars, parallax is the angle at the star subtended by the semimajor axis of the earth’s orbit and is called heliocentric or stellar parallax, which is too small to be significant as a sextant error.

Related Terms

POINT-BLANK RANGE

Distance to a target that is so short that the trajectory of a bullet or projectile is practically a straight, rather than a curved, line.

PISTON DISPLACEMENT

The volume which a piston in a cylinder displaces in a single stroke, equal to the distance the piston travels times the internal cross section of the cylinder.

PHOTOFLASH BOMB

A missile dropped from aircraft; it contains a photoflash mixture and a means for ignition at a distance above the ground, to produce a brilliant light of short photoelectric effect See photoelectricity.

PHONOTELEMETER

A device consisting essentially of a stopwatch, for estimating the distance of guns in action by measuring the interval between the flash and the arrival of the sound.

PENETRATION NUMBER

The consistency of greases, waxes, petrolatum, and asphalt or other bituminous materials expressed as the distance that a standard needle penetrates the sample under specified American Society for Testing and Materials test.

PEDOMETER

1. An instrument for measuring and weighing a newborn child. 2. An instrument that registers the number of footsteps and distance covered in walking.

PASSIVE RADAR

A technique for detecting objects at a distance by picking up the microwave electromagnetic energy that is both radiated and reflected by all bodies.

PARSHALL FLUME

A calibrated device for measuring the flow of liquids in open conduits by measuring the upper and lower beads at a specified distance from an obstructing sill.

OROGRAPH

A machine that records both distance and elevations as it is pushed across land surfaces; used in making topographic maps.

ODOMETER

1. An instrument for measuring distance traversed, as of a vehicle. 2. The indicating gage of such an instrument. 3. A wheel pulled by surveyors to measure distance traveled.

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