page_title: Glossary of astronomy
section_title: P
hierarchical_section_title: Glossary of astronomy / P
context_page_description: This glossary of astronomy is a list of definitions of terms and concepts relevant to astronomy and cosmology, their sub-disciplines, and related fields. Astronomy is concerned with the study of celestial objects and phenomena that originate outside the atmosphere of Earth. The field of astronomy features an extensive vocabulary and a significant amount of jargon.
context_section_description: parsec (pc)
  A unit of length defined as the distance at which a star would show a parallax shift of exactly one arcsecond as observed from Earth's orbit. It is equal to 3.2616 light-years or 206,265 astronomical units. The word "parsec" is a portmanteau of the words parallax and second.
  partial solar eclipse
  periapsis
  The point at which an orbiting body is closest to its primary. Contrast apoapsis.
  perigee
  The point at which a body orbiting the Earth (such as the Moon or an artificial satellite) is closest to the Earth. Contrast apogee.
  perihelion
  The point at which a body orbiting the Earth's Sun is closest to the Sun. Contrast aphelion.
  perturbation
  The complex motion of an astronomical body that is subject to forces other than the gravitational attraction of its primary alone, or any force which complicates the orbital characteristics of the body such that the idealized Kepler orbit of the two-body problem is not an accurate representation of the body's actual orbit. Perturbing forces may include the gravitational forces exerted by any number of additional bodies, the off-center gravitational forces which are consequences of bodies not being perfectly spherical, and/or atmospheric resistance.
  phase angle
  The elongation or angle between an orbiting body and the Sun as viewed from a particular perspective such as the Earth. It determines the amount of a planet or moon's visible surface that lies in shadow. Inferior planets such as Venus generally have low phase angles as seen from Earth, so they are often viewed as a crescent; superior planets such as Mars and Jupiter usually have high phase angles, so that little of the shadowed side is visible.
  photometric system
  photosphere
  plane of reference
  An arbitrarily chosen, imaginary plane from which to measure and define orbital elements such as inclination and longitude of the ascending node. The ecliptic plane, invariable plane, and equatorial plane are all commonly used as reference planes in various contexts.
  planet
  A type of astronomical body orbiting a star or stellar remnant which is massive enough to be rounded by its own gravity (but not massive enough to achieve thermonuclear fusion) and has cleared its neighbouring region of all planetesimals.
  planetary
  Of or relating to a planet or planets.
  planetary body
  Any secondary body that is geologically differentiated or in hydrostatic equilibrium and therefore has a planet-like geology, such as a planet, dwarf planet, or other planetary-mass object, but excluding smaller objects such as planetesimals.
  planetary differentiation
  The process of separating out different constituents of a planetary body, causing it to develop compositionally distinct layers (such as a metallic core).
  planetary nebula
  A type of emission nebula formed from a glowing shell of expanding plasma that has been ejected from a red giant star late in its life. The name derives from their resemblance to a planet. An example is the Ring Nebula.
  planetary science
  planetary system
  Any set of gravitationally bound non-stellar objects in or out of orbit around a star or star system. In general, planetary systems include one or more planets, though such systems may also consist of dwarf planets, moons, asteroids, meteoroids, planetesimals, and debris discs, among other objects.
  planetary-mass object (PMO)
  planetesimal
  Any solid object (generally larger than 1 kilometre (0.62 mi) in diameter) that arises during the formation of a planet whose internal strength is dominated by self-gravity and whose orbital dynamics are not significantly affected by gas drag. The term is most commonly applied to small bodies thought to exist in protoplanetary disks and debris disks during the process of planet formation, but is also sometimes used to refer to various types of small Solar System bodies which are left over from the formation process. There is no precise distinction between a planetesimal and a protoplanet.
  planetoid
  Another name for a minor planet or dwarf planet.
  polar orbit
  An orbit in which the orbiting object passes directly over or nearly over both poles of 
caption: The parallax shift of a star at a distance of one parsec as seen from the Earth (not to scale)