Free Astronomy Quiz

  1. > Resources >
  2. Terminology - P

As with any field of scientific study, astronomy has a long list of important terminology. So we've put together this glossary where you can check you fully understand the meaning of the common terms, as well as some of the more unusual words.



A -- B -- C -- D -- E -- F -- G -- H -- I -- J -- K -- L -- M -- N -- O -- P -- Q -- R -- S -- T -- U -- V -- W -- X -- Y -- Z



Words beginning with 'P' :



Parallax : the shift of position of an object when the observer's perspective changes. The distance of nearby stars can be measure by measuring their tiny positional changes (parallax) at separate points of Earth's orbit around the Sun.

Parsec : a unit for large distances in space. It was defined as the distance of 1 AU subtended at an angle of 1 arcsecond. It is equivalent to 3.26 light years.

Penumbra : a region where some of the light illuminating an object is obstructed. It is the lighter part of a shadow.

Periastron : the point of minimum separation in an elliptical orbit between an object and its star, applies more specifically to binary systems (two stars orbiting each other).

Perigee : refers to objects orbiting the Earth. When an object in an elliptical orbit around the Earth is at its closest distance to the Earth.

Perihelion : in an elliptical orbit, the point at which an object is closest to the Sun.

Phases : the varying visible fractions of an illuminated astronomical object. The Moon, for example, has different phases such as first quarter, full Moon etc.

Photon : a distinct particle of pure energy and no mass.

Photosphere : the outer layer of the Sun. This is from where its light is emitted.

Planet : an astronomical object which orbits a star and fulfills the following requirements: has enough mass, therefore enough gravity to be round doesn't have enough mass to ignite (cause thermonuclear fusion), and has cleared its path of other objects in its orbit around its star

Planisphere : a star chart/map using two adjustable disk rotating on common pivot to display the night sky at any one time and date. It is often used to help learn constellations and positions of stars.

Plutinos : objects found in the Kuiper Belt. They have orbital resonances with Neptune, this means that for every 3 orbits of Neptune a plutino has 2.

Precession : the change of orientation of the rotation axis. Earth's polar axes point in different direction over a period of 25,800 years (one precession).

Protostar : the early stage of star formation. It is a young star still gathering mass from the gas cloud it originates, but before it starts its hydrogen fusion phase.

Pulsar : a rotating neutron star or white star emitting a beam of electromagnetic radiation. They can only be observed when the beam is pointing at Earth. They have very regular, short and rapid pulses of radiation.




Resources :


To support the quizzes in each section, we've put together some extra resources to provide further information: