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A Short Glossary of Astronomical Terms

by Martin J. Powell

C/1995 O1 - an official designation by the International Astronomical Union for newly discovered comets. The prefix C/ is used for a comet which has an orbit of longer than 200 years, or which has a parabolic or hyberbolic orbit (i.e. it will never return to the solar system). The year of the comet's discovery is then given (in this case 1995). This is followed by an upper case letter identifying the half-month in which the discovery took place (the letters I and Z are not used). Hence the letter 'A' refers to comets discovered between January 1st and 15th, the letter 'B' refers to comets discovered between January 16th to 31st, and so on. The final number represents the order of the discovery announcement within the half-month. Hence Hale-Bopp has an orbit of greater than 200 years, and was the first comet to be discovered between July 16th and July 31st 1995. Similarly, Comet Hyakutake received the designation C/1996 B2. Comets with well computed orbits of less than 200 years (known as "periodic comets") have the prefix P/.

Astronomical Unit  (abbrev AU) - a standard unit for measuring distances within the Solar System. By definition, 1 AU is equal to the average distance of the Earth from the Sun during the course of a year, i.e. 92,955,806 miles (149,597,870 km). For example, Mercury orbits at an average distance of 0.387 AU and Pluto at an average distance of 39.44 AU from the Sun.

Averted Vision - the act of looking slightly to one side, rather than directly at, a faint object. When looked at directly, a faint object often cannot be seen, however it becomes visible when one looks slightly to one side of it. This is because the most sensitive part of the retina is a circular area around its centre, where the highest concentration of rods occurs.

Circumpolar - an expression used for celestial bodies which are above the horizon throughout the day, i.e. they do not rise or set. As the Earth rotates they appear to prescribe a circle around the celestial pole. An object will be circumpolar when its declination (q.v.) is greater than the value of the observer's latitude subtracted from 90, i.e. the co-latitude. For example, stars observed at the latitude of London (51░ .5 North) will be circumpolar if their declination is greater than (90 - 51.5) = +38░.5.

Co-latitude - a value equal to (90 - observer's latitude). See also circumpolar.

Coma - the principal part of most comets, consisting of a diffuse cloud of gas and dust which surrounds the nucleus (q.v.)

Declination - the angle North (+) or South (-) of the celestial equator, measured relative to the Earth's centre. Together with the Right Ascension (RA), which is measured in hours and minutes, it is used to pinpoint an object's position on the celestial sphere.

Degree of Condensation (abbrev DC) - a measure of the light distribution within the coma (q.v.), measured on a scale from 0 (diffuse) to 9 (stellar or disk-like).

Dust Tail - often curved and yellow in colour, dust tails are visible by the reflection of sunlight off solid particles. The tails become curved when distant dust particles thrown off the nucleus lag behind the coma. Tails only form when a comet comes close to the Sun, and they always face in a direction opposite to the Sun.

Fountains - diffuse features observed on the sunward side of the coma (q.v.)

Gas Tail - also called the ion tail, it is usually straight, blue and filamentary. Gas tails form when ultra-violet light ionizes the cometary gases, causing them to fluoresce. Like the dust tail (q.v.), a gas tail only forms when the comet comes close to the Sun and always faces away from it. The tail of a comet may stretch over 93 million miles (150 million km) through space.

Hood - a parabolic-shaped feature enveloping the nucleus, only seen in bright comets near perihelion (q.v.)

Jet - a fine and delicate feature seen emanating from a cometary nucleus (q.v.), always curving and often spiral. They are caused by the outgassing of material from the nucleus, the spiralling effect being caused by the rotation of the nucleus itself, much like a Catherine Wheel.

LPR Filter - abbreviation for Light Pollution Reduction Filter. A special type of filter, attached to a telescope eyepiece, which suppresses the effects of sky-glow (light pollution) from streetlights by only allowing selected wavelengths to pass through the eyepiece. The so-called nebula filters transmit only in the wavelengths at which nebulae shine most brightly (mostly Oxygen III and Hydrogen Alpha), effectively darkening the sky and increasing the contrast of the nebulae against the sky.

Messier Number (M44, M70 etc.) - designations from the Messier Catalogue of nebulous objects compiled by French astronomer Charles Messier and published in 1784. He listed a total of 109 objects, comprising gaseous nebulae, planetary nebulae, galaxies, open clusters and globular clusters. Messier compiled the catalogue in order to prevent confusion with newly discovered comets, for which he was a keen searcher.

Magnitude - a measure of the relative brightness of stars and other celestial objects. The brighter the object, the lower the assigned magnitude value. Because of the logarithmic nature of the magnitude scale, stars of magnitude 2 are 2.5 times fainter than those of magnitude 1, and magnitude 3 stars are (2.5 x 2.5) = 6.3 times fainter than magnitude 1, and so on. Objects brighter than magnitude 1 have a zero or even a negative value. The values given in the current context are apparent magnitudes (sometimes referred to as visual magnitude, symbol v) i.e. the magnitudes as they appear in visible light from the Earth's surface. Sample magnitude values are given below: 


- 26.7
Full Moon
- 12.7
Venus (at brightest)
- 4.4
Sirius (brightest star)
- 1.4
Naked Eye limit (approx)
~ + 6.0 to + 6.5
Neptune (at brightest)
+ 7.7
Pluto (at brightest)
+ 13.6
Faintest objects imaged by the Hubble Space Telescope
~ + 30

Cometary magnitudes are difficult to give precisely because they are extended objects with a bright nucleus, a diffuse coma and often with a tail. In practice, the magnitude given for a comet refers specifically to the coma (q.v.) as it would be if its total light was compressed into a star-like point. The vast majority of comets do not become bright enough to be visible with the naked eye. The brightness of a comet is dependant on many factors; the size of the nucleus, the activity of the nucleus, the distance of the comet from the Sun and the relative geometry of the Sun, Earth and comet.

The twenty-one brightest stars in the night sky are listed in the following table. The letter 'v' in brackets indicates that the star has a variable brightness:-


Star Name



Apparent Magnitude

Right Ascension



(light years)



Alpha CMa

Canis Major

- 1.46

6h 45m

- 16░.7




Alpha Car


- 0.72

6h 24m

- 52░.7



Rigil Kentaurus

Alpha Cen


- 0.27

14h 39m

- 60░.8




Alpha Bo÷


- 0.04

14h 15m

+ 19░.2




Alpha Lyr


+ 0.03

18h 36m

+ 38░.8




Alpha Aur


+ 0.08

5h 16m

+ 46░.0




Beta Ori


+ 0.12 (v)

5h 14m

- 8░.2




Alpha CMi

Canis Minor

+ 0.38

7h 39m

+ 5░.2




Alpha Eri


+ 0.46

1h 37m

- 57░.2




Alpha Ori


+ 0.5 (v)

5h 55m

+ 7░.4



Agena (Hadar)

Beta Cen


+ 0.61

14h 3m

- 60░.3




Alpha Aql


+ 0.77

19h 50m

+ 8░.9




Alpha Cru

Crux Australis

+ 0.83

12h 26m

- 63░.1




Alpha Tau


+ 0.85

4h 35m

+ 16░.5




Alpha Sco


+ 0.96 (v)

16h 29m

- 26░.4




Alpha Vir


+ 0.98 (v)

13h 25m

- 11░.1




Beta Gem


+ 1.14

7h 45m

+ 28░.0




Alpha PsA

Piscis Australis

+ 1.16

22h 57m

- 29░.6




Alpha Cyg


+ 1.25

20h 41m

+ 45░.3



Becrux (Mimosa)

Beta Cru

Crux Australis

+ 1.25 (v)

12h 47m

- 59░.7




Alpha Leo


+ 1.35

10h 8m

+ 11░.9


Sources: "The Guinness Book of Astronomy Facts & Feats" by Patrick Moore (Guinness, Enfield, 1983) and "Norton's Star Atlas" edited by Ian Ridpath (Longman, London, 1989)

NGC - abbreviation for the New General Catalogue of Nebulae and Clusters of Stars, prepared by Danish astronomer J. L. E. Dreyer in 1888. Each item is assigned a number in the catalogue. Together with the two subsequent Index Catalogues (IC) there are over 13,000 objects listed.

Nucleus - an icy rubble-pile, typically a few kilometres or more across, forming the permanent part of a comet. It comprises mainly water ice and dirt. Whenever the comet comes close to the Sun the nucleus becomes active, outgassing and sublimating material as the comet rotates. A typical cometary nucleus will survive around 200 or so close approaches to the Sun before disintegrating.

Oort Cloud - a theoretical cloud of comets orbiting the solar system at a distance of about a light year (63, 240 AU) from the Sun. First proposed by the Dutch astronomer Jan Oort, it is considered to be the origin of most of the comets which enter the inner solar system. It is thought that the gravitational pull of nearby stars causes the comets in the cloud to be pulled either into the solar system or out towards the distant star. Other comets may originate from a belt of comets beyond Neptune known as the Kuiper Belt.

Opposition - an alignment of celestial bodies such that the Sun, the Earth and another celestial body form a straight line, with the Earth in the centre of the alignment. The celestial body in question will then be at a point opposite the Sun in the sky, crossing the observer's meridian at around local midnight.

Perihelion - the point in the orbit of a planet or comet at which it is closest to the Sun. Its most distant point from the Sun is called the aphelion.  At its aphelion, Comet Hale-Bopp reaches a distance of 360 AU (q.v.) from the Sun - nine times further out than Pluto.

Position Angle (abbrev pa) - the angle at which a feature is orientated relative to the direction of celestial North. It is measured from 0 to 360 degrees from North through East, South and West. For example, a comet whose tail is at pa = 270░ has its tail facing to the West.

Pseudo-Nucleus - a term given for the appearance of the nuclear region of a comet through a telescope. Because of the small size of a cometary nucleus (q.v.), it cannot be seen through even the most powerful telescopes. The bright stellar-like concentration at the centre of the coma is therefore termed the "false", or pseudo-nucleus.

Shells - a series of arcs seen ahead of the coma of a bright comet when it comes close to the Sun. Shells are caused by the outgassing of material from a cometary nucleus. As the nucleus rotates, the material forms a series of spirals around it, much in the manner of a Catherine Wheel. The telescopic view is usually of a series of concentric arcs ahead of the coma.

Spectroscopy - the study of the spectra from astronomical sources of radiation. The lines and bands in stellar spectra are characteristic of the atoms, molecules and ions producing them. They are measured across the entire range of the electromagnetic spectrum, and are the main source of information on the composition and nature of celestial bodies.



Comet Hale-Bopp

The Naked-Eye


Night Sky







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Copyright  Martin J Powell  2001-6