Glossar - P

  • Parsec
    Parsec is a portmanteau of the words parallax and second. This is 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.
  • Periapsis
    Periapsis is the point of closest approach between two orbiting objects.
  • Phase angle
    Phase angle is 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 a low phase angle as seen from Earth, so they are often viewed as a crescent. Superior planets such as Mars and Jupiter usually have a high phase angle, so little of the shadowed side is visible.
  • Planetary differentiation
    Planetary differentiation is the process of separating out different constituents of a planetary body, causing it to develop compositionally distinct layers (such as a metallic core).
  • Precession
    Precession can refer to a slow change in the orientation of an object's axis of rotation. For the Earth, this is referred to as the precession of the equinoxes. Apsidal precession means a steady change in the orientation of an orbit, such as the precession in the orbit of Mercury that was explained by the theory of general relativity.
  • Projected separation
    Projected separation is the minimum physical separation of two astronomical objects, as determined from their angular separation and estimated distance.[12] For planets and double stars, this distance is usually given in Astronomical Units. The actual separation of the two objects depends on the angle of the line between the two objects to the line-of-sight of the observer.
  • Proper motion
    Proper motion is the rate of angular motion of an object over an interval of time, usually years. For stars, this is typically given in milliarcseconds per year.
  • Protostar
    Protostar is a concentration of mass formed out of the contraction of a collapsing interstellar cloud. Once sufficient mass has fallen onto this central core, it becomes a pre-main-sequence star.