Astronomy dictionary – P

Browse through our astronomy dictionary to find definitions for some of the most common terms used in practical astronomy and space science.

Click on one of the letters below to search for a term.


Pancam camera

This is the high-resolution stereo panoramic camera (pancam) that the Mars Exploration Rovers Spirit and Opportunity are equipped with. It is used to give a geological overview of a region and to navigate the martian surface, as well as to choose what places are potentially interesting for the science instruments to study.


The apparent shift of a nearby star relative to distant background stars caused by the Earth’s rotation around the Sun. The parallax angle is equal to half of the angular distance that the nearby star appears to move by.


The distance to a star if it has a parallax angle of 1 arcsecond (1/3,600 of a degree). One parsec is equal to 3.26 lightyears.

Peak to valley wavefront (PV)

This is a reading of how smooth a parabolic mirror is by measuring the height difference from the centre (valley) of the mirror to the edge (peak) of it. It is typically a fraction of the wavelength of optical light.


These are a type of salt that have been found on the surface of Mars by the Phoenix lander. On Earth certain microbes are known to use pechlorates as an energy source.


The part of the shadow cast by an eclipsing body that results from the Sun being only partly eclipsed.


The lighter shadow cast by an eclipsing body during a partial eclipse; also the outer region of a sunspot group.

Penumbra (lunar eclipse)

This is the out lighter part of the Earth’s shadow cast out into space. It is the first part of the Earth’s shadow that the Moon enters at the beginning of a lunar eclipse.


The point of closest approach for an object in orbit around the Earth.


The point in the orbit of an object that takes it closest to the Sun.


A measurement of the intensity of electromagnetic radiation emitted by a celestial body.


Light can be considered to take the form of waves or particles; these particles are called photons.


The optical surface of the Sun. All of the Sun’s visible light that reaches Earth comes from here.


Plage are a similar phenomenon to faculae, but in the Sun’s chromosphere. They are seen as a bright regions usually near an active region.

Planetary nebula

A glowing shell of gas formed by certain stars at the end of their lives. William Herschel coined the name because of their likeness to giant gas planets such as Jupiter.


This is the name given to the small rocky bodies that form in young planetary systems. They are the precursors to full blown planets and usually range from 100 metres to 100 km in size.


Another term for a minor planet – the official name for asteroids, as used by the International Astronomical Union (IAU).


A star chart with an overlay that shows you what stars are visible at a specific time, date and latitude.


A gas of highly charged particles, which is electrically neutral because it contains equal numbers of positively and negatively charged ions. Plasma is often referred to as the fourth state of matter (the other three being solid, liquid and gas).


If an object in our Solar System has an orbit that takes it beyond Neptune, is near spherical and hasn’t cleared its orbit of detritus like small asteroids and meteoroids, it’s now classed as a plutoid. Currently, there are only two known plutoids: Eris and Pluto itself. The new category was invented by the International Astronomical Union (IAU). It follows the downgrading of Pluto, from planet to dwarf planet in 2006. Who knows when the next plutoid will be discovered or, for that matter, when the IAU might come up with a new planet category?


A small scope that goes into equatorial mounts to help you achieve an accurate polar alignment for your telescope. Accurate polar alignment is essential to take good quality, long-exposure images, for which the star field needs to be tracked.

Prime Meridian

An imaginary line on the surface of a planet from which longitude is measured. On Earth, the line passes through the Royal Observatory at Greenwich, England.


A subatomic particle that resides in the nucleus of an atom and carries a positive electrical charge.

Protoplanetary disc

The disc of gas and dust surrounding a newly formed star. New planets may form in the protoplanetary disc.

Protoplanetary nebula

The initial stage in the formation of a ‘planetary nebula’ – a glowing cloud of gas and dust surrounding a star that has ceased nuclear fusion and blown off its outer layers of gas. The term is also sometimes used in astronomy to describe the disc of matter around a star in which planets may form.


A dense accumulation of dust and gas that is on the verge of collapsing to form a full blown nuclear burning star.


A rapidly rotating neutron star. Jets of electromagnetic radiation that emanate from pulsars, in the form radio waves, are observed in pulses because of this rotation.


The type of rock formed from fragments or ash resulting from volcanic activity, especially the hot, fast-moving dense clouds that occur during an eruption.