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Published 28th March 2013 by

Use this Astronomical Terms Glossary to look up the many technical terms and definitions that you may be unfamiliar with in astronomy.
Introduction to Astronomy Series
  1. Introduction to Astronomy
  2. The Celestial Sphere - Right Ascension and Declination
  3. What is Angular Size?
  4. What is the Milky Way Galaxy?
  5. The Astronomical Magnitude Scale
  6. Sidereal Time, Civil Time and Solar Time
  7. Equinoxes and Solstices
  8. Parallax, Distance and Parsecs
  9. Luminosity and Flux of Stars
  10. Kepler's Laws of Planetary Motion
  11. What Are Lagrange Points?
  12. Glossary of Astronomy & Photographic Terms
  13. Astronomical Constants - Some Useful Constants for Astronomy

This astronomical terms glossary contains definitions for some of the most common words used in astronomy, cosmology, astrophysics, and space exploration and where possible links you to articles containing an in-depth explanation, pictures and links to related in-depth articles on this site.

TermDescriptionFurther Reading


The original system of rating photographic materials, which was devised by the American standards Association. The ISO rating system is now used in place of the ASA.


The inability of a lens to produce a perfect, sharp image, especially towards the edge of the lens field. This is because each colour in the optical spectrum cannot be focused at a single common point on the optical axis. These faults can be reduced by compound lens constructions, and the use of small apertures.

Absolute Magnitude

Absolute magnitude is the apparent magnitude, m, an object would have if it were at a standard luminosity distance away from us.

See Also: Apparent Magnitude, Standard Luminosity Distance, Magnitude

The Astronomical Magnitude Scale

Adapter Ring

Circular mount, available in several sizes, enabling accessories such as filters to be used with lenses of different diameters.

Adaptive Optics

A technology to improve the performance of optical systems by reducing the effects of rapidly changing optical distortion. Adaptive optics works by measuring the distortion and rapidly compensating for it either using deformable mirrors or material with variable refractive properties.

See Also: Seeing, Transparency

Why do Stars Twinkle?

Alpha Centauri

Alpha Centauri is the closest star system to the Earth. The dimmest star in the system, Proxima Centauri, is the closest star to us, other than our sun.

Alt-Azimuth Mount

A type of telescope mount that only moves "up" and "down" (Altitude and Azimuth).

See Also: Equatorial Telescope Mount

Buying Your First Telescope


Altitude is the angle of the object from the observer's horizon. If an object is on the horizon, its altitude is 0 degrees. If it is at the observer's zenith, its altitude is 90 degrees.

See Also: Zenith

Angular Distance

Angular distance measures the proportion of a circle that the arc in question consists of. Angular distance is expressed in degrees, radians, arc minutes.

See Also: Arc Minute, Arc Second

Parallax, Distance and Parsecs

Angular Momentum

Angular momentum is a momentum-like quantity associated with a circular motion around an axis of rotation.

Annular Eclipse

An eclipse where the Sun looks like an "annulus" or ring. The ring is visible when the Moon does not entirely cover the disc of the Sun during the eclipse.

See Also: Baily's Beads, Eclipse


The aperture is the size of a telescope's main lens (or mirror).

See Also: Focal Length

Buying Your First Telescope

Apparent Magnitude

Apparent magnitude is a measure of the brightness of a celestial object as seen from Earth. The lower the number, the brighter the object. Negative numbers indicate extreme brightness. The full moon has an apparent magnitude of -12.6; the sun's is -26.8. Under ideal conditions, the human eye can see objects up to 6th magnitude.

See Also: Absolute Magnitude, Averted Vision, Limiting Magnitude, Magnitude

The Astronomical Magnitude Scale

Arc Minute

An arc minute is a measure of angularity and is equal to one-sixtieth of a degree (there are 360 degrees in a circle).

See Also: Arc Second

Parallax, Distance and Parsecs

Arc Second

An arc second is equal to one-sixtieth of an arc-minute.

See Also: Arc Minute

Parallax, Distance and Parsecs


Flaws in the image generated by the camera, either by the sensor and camera processing or by the storage method. JPEG is well known for artefacts as it compresses the image. Artefacts can also take the form of "jaggies" on diagonal lines, which make them appear to staircase.


A sub set of a constellation, asterisms consist of a group of stars that are either part of the main constellation or separate.

See Also: Constellation

A Guide to Asterisms – The Unofficial Constellations

Averted Vision

At night, the outer surface of the retina is more sensitive to light than the centre. When you look slightly to one side of a faint object, the light falls on the more sensitive outer part of the retina, usually revealing faint details, which would not be visible when looking directly at the object. Is this why we only see ghosts and apparitions in the corner of our eye?

See Also: Apparent Magnitude

Dark Eye Adaption – How We See In the Dark


Azimuth is the angle of the object from the observer's north point (projected onto the horizon). If an object is due north, its azimuth is 0 degrees. If it is due east, its azimuth is 90 degrees, etc. To find an object in the sky, two coordinates are needed, its altitude and its azimuth.

Baily's Beads

Baily's beads are bead-like bursts of light that appear about 15 seconds before and after totality during a solar eclipse.

See Also: Eclipse

Barrel Distortion

This is where straight lines appear to bow outwards when near the edge of the frame during wide angle shots.

Black Body

A black body is an idealized body that is a perfect radiator and perfect absorber of electromagnetic radiation. A black body not only absorbs all wavelengths of energy and radiates at all wavelengths, but it does this at the maximum possible intensity for any given temperature. A star is a good approximation to a black body since stellar gases are very good absorbers of energy.

See Also: Spectral Class

What is Black Body Radiation?

Black Hole

A black hole is a super massive object in space that is so dense that within a certain radius, its gravitational field does not let anything escape from it, not even light.

What Is a Black Hole? Black Holes Explained – From Birth to Death

Blue Moon

The second Full Moon in the same calendar month. A rare event.

Blue Moon Calendar – When is the Next Blue Moon?

Blue Shift

Blue shift is the opposite of redshift, the latter being much more noted due to its importance to modern astronomy. It is also used informally to refer to a hypsochromic shift in photochemistry.

See Also: Doppler Shift, Red Shift

Redshift and Blueshift

Cassini Division

The Cassini Division is the main, dark division between Saturn's largest rings, the A and B rings.

See Also: Encke Divsion

Celestial Equator

an imaginary line around the middle of the Celestial Sphere, an imaginary projection of the Earth's equator onto the sky.

The Celestial Sphere – Right Ascension and Declination

Celestial Sphere

A projected sphere centred on the Earth that we can imagine all the stars are painted on.

The Celestial Sphere – Right Ascension and Declination

Circle of the Horizon

The Circle of the Horizon surrounds the Earth-bound observer. The horizon circle is divided into 360 degrees, with 90° between each of the four cardinal directions of North, South, East, and West.

The Celestial Sphere – Right Ascension and Declination

Circumpolar Star

A circumpolar star is one whose apparent path seems to circle a celestial pole. A circumpolar star never sets; it is always above the observer's horizon.


Lining up the optical components in a telescope, e.g. the lenses, mirrors, prisms, and eyepieces, into their proper positions. Collimating maximizes image quality.

Newtonian Telescope Collimation


A constellation is a group of stars that, when seen from Earth, form a pattern.

See Also: Asterism

Guide to the Constellations and Mythology

Coronal Holes

Coronal holes are areas in the coronal where the Sun's magnetic field loops out into space instead of looping back into the Sun.

See Also: Coronal Mass Ejection

Coronal Mass Ejection

Coronal mass ejections (CME's) are huge, balloon-shaped plasma bursts that come from the Sun. As these bursts of solar wind rise above the Sun's corona, they move along the Sun's magnetic field lines and increase in temperature up to tens of millions of degrees.

See Also: Coronal Holes

Cosmic Rays

Cosmic rays are very high energy particles that travel through space near the speed of light.

Dark Matter

Dark matter is unknown matter that may constitute as much as 99 percent of the matter in the universe.

Death Star Theory

The Death Star Theory refers to the fact that mass extinctions are periodic, and may be caused by the Earth's passing through the Oort cloud every 26 million years.


Declination is a celestial coordinate that is used to measure the degrees of latitude above or below the celestial equator on the celestial sphere.

See Also: Right Ascension

The Celestial Sphere – Right Ascension and Declination

Depth of Field

Distance between the nearest point and the farthest point in the subject which is perceived as acceptable sharp along a common image plane.

Depth of Focus

Distance which the film plane can be moved while maintaining an acceptably sharp image without refocusing the lens.


Diffraction is the ability of a wave to bend around corners. The diffraction of light established its wave nature.

Diffuse Nebula

A diffuse nebula is a wide, spread-out, irregularly-shaped cloud of gas (mostly hydrogen gas) in space that can be up to 100 light-years wide. This type of nebula can appear to be light or dark.

Types of Nebula and Nebulae

Doppler Shift

The Doppler shift (or Doppler Effect) is an increase or decrease in wavelength as the object emitting the wave moves relative to the observer.

See Also: Blue Shift, Red Shift

Redshift and Blueshift

Dwarf Planet

A dwarf planet is an object that orbits a sun, has enough mass to give it a nearly round shape, is not a satellite of another object, and has not "cleared the neighbourhood" of its own orbit. This definition was established on Aug. 2006 by the International Astronomical Union.


An eclipse happens when the moon blocks the Sun (Solar Eclipse_ or the Earth's shadow falls on the moon (Lunar Eclipse).

See Also: Baily's Beads


An imaginary circle which all the other planets appear to follow during their orbits around the Sun over a year.

Emission Nebula

An emission nebula is a nebula that glows; it emits light energy. The reddish light is produced when electrons and protons combine, forming hydrogen atoms.

See Also: Diffuse Nebula

Types of Nebula and Nebulae

Encke Divsion

The Encke Division splits the A Ring, the outermost of the major rings of Saturn.

See Also: Cassini Division

Equatorial Telescope Mount

An equatorial telescope mount is a complex device that is aligned parallel to Earth's axis and is pivoted at a right angle in order to follow the apparent motion of celestial objects.

See Also: Alt-Azimuth Mount

Polar Alignment of an Equatorial Telescope Mount


Equinoxes are days in which day and night are of equal duration. The two yearly equinoxes occur when the Sun crosses the celestial equator.


The eyepiece is the part of a telescope that you look into. It is a lens that magnifies the image formed by the main optical system.

F Numbers

Numbers on the lens barrel indicating the size of the aperture relative to the focal length of the lens. f numbers are calculated by dividing the focal length of the lens by the effective diameter of the aperture.

See Also: Aperture

Focal Length

Focal length is the distance from the lens (or mirror) in a telescope to the point at which the object being observed is focused. Generally the longer the focal length, the higher the magnification.

See Also: Aperture

Focal Plane

Imaginary line perpendicular to the optical axis which passes through the focal point. It forms the plane of sharp focus when the lens is set at infinity.

Gamma Ray

Gamma rays are very high energy electromagnetic radiation. Gamma ray bursts are mysterious and powerful astronomical phenomenon that emit short-lived flashes of gamma rays.

See Also: X-Rays

Gamma Ray Bursts

Gravitational Lens

A gravitational lens is a massive object in space (like a galaxy) that warps space and bends light that passes by it, due to the gravitational forces of the massive object.

Guide Number

This is a number that represents the power of a flashgun. You can use it to work out what lens aperture to use for a given subject distance by dividing the guide number by the distance in metres. These days camera can calculate this automatically.

ISO Rating

ISO (International Organization for Standardization) is the measure of a photographic film's sensitivity to light. Film with lower sensitivity (lower ISO speed rating) requires a longer exposure and is thus called a slow film, while film with higher sensitivity (higher ISO speed rating) can shoot the same scene with a shorter exposure and is called a fast film.

Kuiper Belt

The Kuiper belt is a region beyond Neptune in which at least 70,000 small objects orbit, including Quaoar and Sedna.

See Also: Oort Cloud


Latitude is the angular distance north or south of the equator of a celestial object.

See Also: Longitude

Light Pollution

The term light pollution refers to unwanted light reflected (or emitted) up into the sky. This can be anything from an inefficient street light design where light is emitted not only downwards but upwards as well, or a bad security light that shines upwards, building spotlights or sky beams. All these light sources illuminate any particles in the atmosphere and the result is a orange "city glow" in the sky. This greatly reduces the number of stars visible and in the city all but the brightest are invisible. For more information please visit the International Dark-Sky Association website

Light Pollution

Limiting Magnitude

Limiting Magnitude is the magnitude of the faintest star visible. Astronomers use magnitude to describe the apparent brightness of a star in the sky. Under ideal conditions the naked eye limiting magnitude is around 6, however at my location it is closer to 3.5 (the lower the magnitude, the brighter the star is). It must be noted that the limiting magnitude quoted by manufacturers assume experience observers under perfect conditions, and is rarely obtainable under normal use.

See Also: Magnitude

The Astronomical Magnitude Scale

Local Sidereal Time

Local sidereal time (abbreviated LST) is local time measured by the apparent motion of the stars (instead of the motion of the Sun). LST is measured by the right ascension that is currently at the observer's meridian of the local sky. Astronomers use LST to aim telescopes at astromonical objects.

See Also: Sidereal Time

Sidereal Time, Civil Time and Solar Time


Longitude is the angular distance east or west from the north-south line that passes through Greenwich, England, to a particular location.

See Also: Latitude


Luminosity is the total amount of energy that a star radiates each second.

See Also: Magnitude

Luminosity and Flux of Stars


Magnitude is a measure of brightness of celestial objects. Lower numbers represent brighter objects than higher numbers; very bright star are 1st magnitude, less bright stars are 2nd magnitude, etc. The magnitude scale is logarithmic; a difference in magnitude of 5 is a 100-fold increase in brightness.

See Also: Absolute Magnitude, Apparent Magnitude

The Astronomical Magnitude Scale

Maksutov-Cassegrain Telescope

A Maksutov-Cassegrain telescope is a wide-angle reflecting telescope with a curved correcting lens (called a Meniscus Corrector Shell) that minimizes spherical aberration and a concave mirror that receives light and focuses an image.

See Also: Schmidt-Cassegrain Telescope, Reflecting Telescope, Refracting Telescope


The meridian is an imaginary north-south line in the sky that passes through the observer's zenith.

See Also: Zenith


An imaginary circle passing through the Zenith and North and South Celestial Poles, and is always perpendicular to the horizon.

The Celestial Sphere – Right Ascension and Declination

Messier Objects

Charles Messier made a list of 103 fuzzy objects in space in order not to mistake star clusters, galaxies, and nebulae for comets.

List of Astronomy Math Equations with Workings

Milky Way

The Milky Way is the galaxy our solar system is part of. It can be seen as a bright line of stars stretching across the night sky. It is easier to see when you are far from bright city lights.

What is the Milky Way Galaxy?

New General Catalog

The New General Catalog (NGC) is a list of over 13,000 deep-sky celestial objects. It was developed in 1888.

Newtonian Telescope

The Newtonian telescope is a type of reflecting telescope invented by the British scientist Sir Isaac Newton, which uses a parabolic primary mirror and a flat diagonal secondary mirror.

See Also: Reflecting Telescope, Refracting Telescope

History of the Newtonian Reflector

Noctilucent Cloud

Noctilucent clouds are very high-altitude clouds that are visible at night in June and July from the latitudes 50 to 65 degrees. These clouds are at roughly 82-85 km altitude, a dry part of the atmosphere. Noctilucent means "night shine."


Occultation is when a smaller astronomical body passes behind a larger astronomical body.

See Also: Transit

Oort Cloud

The Oort Cloud is a cloud of rocks and dust that may surround our solar system.

See Also: Kuiper Belt


An astronomical body is said to be at opposition when it makes its closest approach to the point directly opposite to the Sun in the night sky. This means that the object will appear highest in the sky at around midnight, local time, and will be above the horizon for much of the night. For objects which orbit further out in the Solar System than the Earth – almost all bodies other than Mercury and Venus – this configuration happens at around the same time that the object's orbit carries it to its closest approach to the Earth, making it appear at its largest and brightest in the night sky. When an object is at opposition, the Solar System is aligned such that the object lies in a straight line with the Earth and the Sun, the Earth being in the middle, and so the object and the Earth are at almost exactly the same positions around their respective orbits relative to the Sun.

Optical Tube Assembly

The optical tube assembly (OTA) is the main body or tube of a telescope. This optical tube holds the objective.


The length of the parsec is based on the method of trigonometric parallax, one of the oldest methods for measuring the distances to stars. The name parsec stands for "parallax of one second of arc", and one parsec is defined to be the distance from the Earth to a star that has a parallax of 1 arcsecond.

Parallax, Distance and Parsecs

Pincushion Distortion

This is the opposite of Barrel distortion. Straight lines appear to bow inwards when near the edge of the frame. You often see this with zoom lenses at or near there maximum focal length.

Planetary Nebula

A planetary nebula is a nebula formed from by a shell of gas which was ejected from a certain kind of extremely hot star. As the giant star explodes, the core of the star is exposed.

See Also: Emission Nebula

Types of Nebula and Nebulae


Polaris is the current pole star for the Northern Hemisphere; it is 1 degree from the exact Northern celestial pole.

Proper Motion

Proper motion is the actual motion of a star across the sky (not toward or away from the Earth). This motion is due to the orbit of the star in the Milky Way Galaxy.

Reflecting Telescope

A reflecting (or Newtonian) telescope uses two mirrors which magnify what is viewed.

See Also: Refracting Telescope

Refracting Telescope

A refracting telescope uses two lenses which magnify what is viewed; the large primary lens does most of the magnification.

See Also: Reflecting Telescope

Right Ascension

Right ascension is a celestial coordinate that is used to measure the degrees of longitude on the on the celestial sphere. Zero degrees of right ascension is the position of the Sun during the vernal (spring) equinox (March 21).

See Also: Declination

The Celestial Sphere – Right Ascension and Declination

Saber's Beads

Lunar phenomenon seen on extremely young and old crescents. The striking resemblance to 2nd and 3rd contacts during a total solar eclipse was first noted by American astronomer Stephen Saber.

Schmidt-Cassegrain Telescope

A Schmidt-Cassegrain telescope is a wide-angle reflecting telescope with a correcting lens that minimizes spherical aberration and a concave mirror that receives light and focuses an image. A second mirror reflects the light through a gap in the primary mirror, allowing the eyepiece or camera to be mounted at the back end of the tube.

See Also: Maksutov-Cassegrain Telescope


Seeing and Transparency

Seeing makes the moon wobble and the stars twinkle

Seeing is a measure of atmospheric disturbance caused by strong winds at high altitudes and temperature differentials. The visible result of poor seeing is that stars appear to twinkle at night. Good seeing results when the air is still and the stars do not appear to twinkle. Though a telescope the image will appear sharp and steady. Poor seeing will limit the resolution of the telescope and limit the highest useful magnification. High-pressure systems generally create good seeing, while low pressure creates poor seeing. The image right show the effect of atmospheric disturbance on an image of the moon. This is a animation of five frames taken with a Canon EOS 350d mounted at Prime Focus on a Skywatcher Explorer 200. You can see how the disturbance makes the image appear to bubble and warp.

See Also: Transparency


SETI Search for Extra-Terrestrial Intelligence) is the name for a number of organized efforts to detect Extraterrestrial life. A number of efforts with SETI have been organized, including projects funded by the United States Government. The general approach of SETI projects is to survey the sky to detect the existence of transmissions from a civilization on a distant planet, an approach widely endorsed by the scientific community as hard science.

Sidereal Time

Sidereal time is time measured relative to the stars (the period between successive conjunctions with any star) instead of relative to the motion of the Sun). One sidereal day, equal to 23 hours and 56 minutes, is the period during which the earth completes one rotation on its axis.

See Also: Local Sidereal Time

Sidereal Time, Civil Time and Solar Time

Space-Time Curvature

The curvature of space-time is a distortion of space-time that is caused by the gravitational field of matter. The degree of curvature depends on the strength of the gravitational field.

Spectral Class

Spectral classes are groups of stars that have similar characteristic emission lines in their spectra (indicating that they have similar compositions).

Standard Luminosity Distance

Used for Absolute magnitude calculations, standard luminosity distance is equal to 10 parsecs.

See Also: Absolute Magnitude, Parsec

Luminosity and Flux of Stars


The terminator is the day-night line on a planet or moon.


Transit is when a smaller body passes in front of a larger one.

See Also: Ocultation


Transparency is a measure of how dark the sky is and is caused by the humidity, dust and light pollution in the atmosphere. Transparency will affect the limiting magnitude (see below) of the naked eye or telescope, as well as the telescopes ability to resolve objects (reduces resolution)

See Also: Seeing

Variable Star

A variable star is one whose brightness changes regularly. They can have periods ranging from minutes to years. The apparent changes in brightness are caused by different phenomena; some change in size, some eject material, and others are in pairs that periodically obscure and enhance each other.

Variable Stars

White Hole

A white hole is the time reversal of a black hole, another singularity in space-time. Matter emerges unpredictably from a white hole (unlike a black hole, into which matter is drawn).

See Also: Black Hole


X-rays are a type of electromagnetic radiation. Each photon of X-ray radiation has a lot of energy. X-rays can go through most solid objects. X-ray images of celestial objects are one way of learning about their high-energy properties.

See Also: Gamma Ray


An observer's zenith is the point directly overhead.

See Also: Meridian

Tutorial Series

This post is part of the series Introduction to Astronomy. Use the links below to advance to the next tutorial in the couse, or go back and see the previous in the tutorial series.

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