Astronomy Dictionary

Browse through Sky at Night Magazine’s comprehensive astronomical dictionary, with over 300 entries.


Calcium-K is a specific wavelength (roughly 393.3 nanometres) of ultraviolet light, emitted by calcium atoms that are missing one electron. Specific solar telescopes filter all of the Sun’s light apart from this particular wavelength.

Caldwell catalogue

A listing of 109 bright star clusters, nebulae and galaxies that weren’t included in the Messier catalogue. The catalogue was compiled by Sir Patrick Caldwell-Moore.


A brand name for the commercial abrasive used by amateur telescope makers to grind mirrors.


This is an optical system that uses both reflective and refractive elements. The Schmidt and Maksutov-Cassegrain telescopes are both catadioptric designs, having a lens at the front and a mirror at the back of the scope.

Catadioptric telescope

A telescope that contains both lenses and mirrors to focus light. Catadioptric telescopes include both Maksutov- and Schmidt-Cassegrain models.

Celestial sphere

This is the name given to the projection of the night sky on to an imaginary sphere around the Earth. The astronomical co-ordinates of right ascension and declination are also mapped on to this sphere.

Cepheid variable

A type of variable star whose periodicity in brightness is proportional to its absolute magnitude. From the star’s absolute magnitude, its distance from Earth can be calculated, and so Cepheids are referred to as ‘standard candles’.

Chroma (or chrominance noise)

This is the blotchy variation in colour sometimes seen in DSLR images.

Chromatic aberration

The introduction of false colour into an image, caused by a lens bending different wavelengths of light unevenly, which disperses the light.


The chromosphere is the layer of the Sun’s atmosphere above the visible ‘surface’ of the photosphere and beneath the outer tenuous corona. Meaning ‘sphere of colour’, it is the place where dynamic prominences and other similar events occur.


A star or constellation that doesn’t set (i.e. doesn’t disappear below the horizon) over the course of the night, due to its proximity to one of the poles. The constellation of Cassiopeia is circumpolar.

Cold dark matter

This is the favoured model to describe dark matter (a mysterious matter that doesn’t emit any light). Its constituent particles move slowly (which is why it’s called ‘cold’), and therefore it was easy for dark matter to clump together in the early Universe.


In order for a telescope to perform to its full ability the optics (including primary mirror, secondary mirror or lenses) need to be accurately aligned. This can be done by collimating a telescope via several methods including studying the shape of un-focussed stars or by using special laser collimating devices.

Colour fringing

Another name for chromatic aberration.


Coma is a optical aberration, where stars at the edge of a field of view appear to broaden out into triangle or fan shapes, caused by an imperfection in the lens or mirror.

Comatic aberration (coma)

An optical defect in a lens, which means light rays that enter the edge of the lens at an angle converge so as not to be brought to a sharp focus. The result is a smearing of detail towards the edge of the field of view.

Comet nucleus

The solid, central part of a comet, composed of rock, dust and frozen gases.

Contact binary stars

Two gravitationally bound stars (a binary star system) that have both filled their ‘Roche lobes’. These are teardrop-shaped regions around each star in the binary system, within which matter is gravitationally bound to each star. Any matter beyond a star’s Roche lobe will fall onto the other star in the binary system.

Contact eclipsing binary

Eclipsing binary stars transit each other during their orbit around each other and their combined brightness varies as they do so. In some cases the two stars’ outer atmospheres fill the Roche lobe making contact with each other to make a ‘contact eclipsing binary’.

Convection cells

These are huge ‘bubbles’ (some the size of Jupiter) under the surface of the Sun, caused by the rising and sinking of super-hot gas in the Sun’s interior. Smaller convection cells cause ‘granules’ on the surface. These can be observed with a specially filtered H-alpha telescope.


This is the Sun’s outer atmosphere seen from Earth during a total solar eclipse and also with a coronagraph on an orbiting space observatory. The temperature of the corona reaches around 2 million degrees K.

Coronal mass ejection

A vast cloud of hot plasma, mainly composed of electrons and protons, that is ejected from the surface of the Sun.

Coronal mass ejection (CME)

A vast cloud of hot plasma, mainly composed of electrons and protons, that leaves the surface of the Sun.

Corrector plate

This is a lens plate that fits on the front of Schmidt-Cassegrain and Maksutov-Cassegrain telescopes to correct optical aberrations.

Cosmic rays

These are energetic particles that originate outside Earth’s atmosphere. The most energetic cosmic rays are of an unknown, extragalactic origin and travel at nearly the speed of light.


A small crater ranging from a few millimetres across to a few metres.


An adjective used to describe things related to the planet Venus. The correct form is really ‘Venereal’, but this is rarely used. Astronomers use ‘Cytherean’ because the island of Cytherea crops up in the mythology of Aphrodite (also called Venus).

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