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10 Topic Pages Found in Astronomy and starting with the letter P
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Open parallax

( pâr'əlăks ), any alteration in the relative apparent positions of objects produced by a shift in the position of the observer. In astronomy the term

Open Particle Physics
Particle Physics

Study of the particles that make up all atoms, and of their interactions. More than 300 subatomic particles have now been identified by physicists,

Open perturbation

( pŭr´´tərbā'shən ), in astronomy and physics, small force or other influence that modifies the otherwise simple motion of some object. The term is

Open Phobos

The innermost and larger of the two satellites of Mars, both of which were discovered in 1877 by Asaph Hall. Observations from Mars-orbiting


optical device used to project a representation of the heavens onto a domed ceiling; the term also designates the building that houses such a device.

Open Pluto (Dwarf planet)
Pluto (Dwarf planet)

Dwarf planet. Until its reclassification in 2006 it was considered to be the smallest and, usually, outermost planet of the Solar System. The

precession of the equinoxes

westward motion of the equinoxes along the ecliptic . This motion was first noted by Hipparchus c.120 B.C. The precession is due to the gravitational

Open Ptolemaic system
Ptolemaic system

( tŏl´´əmā'ĭk ), historically the most influential of the geocentric cosmological theories, i.e., theories that placed the earth motionless at the

Open Ptolemy

(Claudius Ptolemaeus), fl. 2d cent. A.D., celebrated Greco-Egyptian mathematician, astronomer, and geographer. He made his observations in Alexandria

Open pulsar

Celestial source that appears to flash at radio and other wavelengths at regular intervals, ranging from a few seconds to a few thousandths of a