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Glossary: P

Parallax The apparent change in position of two objects viewed from different locations.
Parsec 30.856 802 5 Pm (Peta = 10^15).  3.26 light-years.  An object 1 parsec from the Sun has a parallax angle of 1 arcsec.    = 1 AU/tan(1 arcsec)   =149597870700/TAN(2*PI()/360/3600) metres
Patera A shallow crater with a complex, scalloped edge.
Penumbra The area of partial illumination surrounding the darkest part of a shadow caused by an eclipse.
Periapsis Point in orbit closest to body it is orbiting.
Perigee The point in the orbit of the Moon or other satellite at which it is closest to the Earth.
Perigee  Point in orbit closest to Earth.  (Greek: peri (near) geo (earth))
Perihelion  Point in orbit closest to Sun.  (Greek: peri (near) Helios (Sun))
Perturb To cause a planet or satellite to deviate from a theoretically regular orbital motion.
Phase The apparent change in shape of the Moon and inferior planets as seen from Earth as they move in their orbits.
Photon A particle of light composed of a minute quantity of electromagnetic energy.
Photosphere The bright visible surface of the Sun.
Planemo A large planet or planetary body that does not orbit a star. Planemos instead wander cold and alone through the cosmos. It is believed that most planemos once orbited their mother star but were ejected from the star system by gravitational interaction with another massive object.
Planet A celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighboring region of planetesimals.
Planetary Nebula A shell of gas surrounding a small, white star. The gas is usually illuminated by the star, producing a variety of colors and shapes.
Planetesimal A solid object that is believed to exist in protoplanetary disks and in debris disks. Planetesimals are formed from small dust grains that collide and stick together and are the building blocks that eventually form planets in new planetary systems.
Planitia A low plain.
Planum A high plain or plateau.
Plasma A form of ionized gas in which the temperature is too high for atoms to exist in their natural state. Plasma is composed of free electrons and free atomic nuclei.
Pluto Second-most-massive known dwarf planet.  Formerly known as 9th planet.
Precession The apparent shift of the celestial poles caused by a gradual wobble of the Earth's axis.
Prograde Orbit In reference to a satellite, a prograde orbit means that the satellite orbits the planet in the same direction as the planet's rotation. A planet is said to have a prograde orbit if the direction of its orbit is the same as that of the majority of other planets in the system.
Prominence An explosion of hot gas that erupts from the Sun's surface. Solar prominences are usually associated with sunspot activity and can cause interference with communications on Earth due to their electromagnetic effects on the atmosphere.
Proper Motion The apparent angular motion across the sky of an object relative to the Solar System.
Protoplanetary Disk A rotating circumstellar disk of dense gas surrounding a young newly formed star. It is thought that planets are eventually formed from the gas and dust within the protoplanetary disk.
Protostar Dense regions of molecular clouds where stars are forming.
Proxima Centauri Closest star to the Earth, other than the Sun.
Pulsar A spinning neutron star that emits energy along its gravitational axis. This energy is received as pulses as the star rotates.

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