The Essential Astronomy Glossary

The Essential Astronomy Glossary

An essential glossary of astronomy terms to help any budding astronomers out in the field. Or just for some light bedtime reading!

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Aperture - The diameter of a telescope’s main lens or mirror. It is a telescope’s most important attribute. The larger the aperture, the more light can be gathered and more objects can be viewed through the telescope.

Asterism - Any prominent star pattern that isn’t a whole constellation, such as the Northern Cross or the Big Dipper.

Asteroid - Also known as a Minor Planet, it is a solid body orbiting the Sun that consists of metal and rock. 

Astronomy - The study of objects and matter outside the Earth’s atmosphere. 

Astrophotography - Taking images of astronomical objects and events.

Averted Vision - Viewing an object by looking slightly to its side. This technique can help you detect faint objects that are invisible when you stare directly at them.

Alt-Azimuth Mount - A telescope mount that is simple to use. It uses two handles to control the telescope’s altitude (up/down) and azimuth (left/right) position. 

Azimuth - A horizontal measurement used to locate the position of an object in the sky. It is measured clockwise from North and spans 360 degrees around you.


Barlow Lens - A lens that is placed into the focusing tube to effectively double or triple a telescope’s focal length and, in turn, the magnification of any eyepiece used with it.

Black Hole - A concentration of mass so dense that nothing, not even light, can escape its gravitational pull. 

Blue Moon - A term used to describe an extra full moon that occurs in a season. The moon does not appear blue in colour. 


Celestial Coordinates - A grid system used for locating objects in the sky. It is anchored to the celestial poles and celestial equator. Declination and right ascension are the celestial equivalents of latitude and longitude.

Collimation - Aligning the optical elements of a telescope so that they all point in the correct direction. Most reflectors and compound telescopes require occasional collimation in order to produce the best possible images. 

Coma - An area of dust or gas surrounding the nucleus of a comet

Comet - A ball of ice and rocky debris, typically a few miles across that orbits the Sun in a long ellipse. When the Sun evaporates the ice and coma and tail is formed. 

Compound Telescope - A telescope constructed with a mirror in the back and a lens in the front. The most popular designs are the Schmidt-Cassegrain telescope and the Maksutov-Cassegrain telescope.

Computerised Mount - A telescope mount that, once aligned, can find and track objects in the sky without the need to study star charts. 

Cassegrain Telescope - A compound telescope that has a paraboloidal primary mirror and hyperboloidal secondary mirror. These telescopes tend to produce the highest quality images.

Conjunction - When a Moon or a planet appears especially close either to another planet or a bright star. 

Constellation - A distinctive pattern of stars used informally to organise a part of the sky. 


Dark Adaptation - The eye’s transition to night vision in order to see faint objects. Dark adaptation is rapid during the first 5 or 10 minutes after leaving a well-lit room but full adaptation requires at least half an hour. It can be ruined by a momentary glance at a bright light which is why many astronomers use red torches whilst observing.

Declination (Dec.) - The celestial equivalent of latitude, denoting how far (in degrees) an object in the sky lies north or south of the celestial equator.

Diagonal - A mirror or prism that attaches to the focuser of a refractor or compound telescope. It allows you to look horizontally into the eyepiece when the telescope is pointed directly overhead.

Dobsonian - A type of Newtonian Reflector that uses a simple but effective wooden mount. Dobsonians provide more aperture per pound than any other telescope design.

Double Asteroid - Two asteroids that revolve around each other and are held together by the gravity between them. Also called a binary asteroid.

Double Star - Also known as a Binary Star. Two stars that lie very close together. Usually, such stars orbit so closely that they appear as a single point of light even when viewed through professional telescopes.

Dwarf Planet - A celestial body orbiting the Sun that is large enough to be rounded by its own gravity but has not cleared its neighbouring region of planetesimals. Pluto has been considered a dwarf planet since 2006.


Earthshine - Sunlight reflected by the Earth that makes the otherwise dark side of the Moon glow faintly. It is especially obvious during the Moon’s thin crescent phase. 

Eclipse - An event when the shadow of a planet or moon falls upon a second body. A solar eclipse occurs when the Moon’s shadow falls on the Earth which we see as the Moon blocking the Sun. 

Equatorial Mount - A telescope mount that can be distinguished by its counterweights which allow the telescope to move in line with the Earth’s curvature. It is advantageous for tracking objects as they move across the sky. Some Equatorial Mounts have motors that allow them to automatically move with the Earth.

Eyepiece - The part of the telescope you look into. The shorter the focal length the higher the magnification.


Field of View - The circle of sky that you see when you look through a telescope. Generally, the lower the magnification, the wide the field of view.

Finderscope - A small telescope used to aim your main scope at an object in the sky. They have low magnifications and wide fields of view.

Filter - A piece of glass placed over the eyepiece to improve the view of particular objects. Filters come in different colours, that have varying effects. The most common use of a filter is to improve the contrast of an object in order to see details clearer. 

Flare Star - A faint red star that appears to change in brightness due to explosions on its surface.


Galaxy - A vast collection of stars, gas, and dust, typically 10,000 to 100,000 light-years in diameter and containing billions of stars. 


Light Pollution - A glow in the night sky caused by artificial light. It generally reduces the number of stars you can see. Specific light-pollution filters can be used to improve the visibility of celestial objects in light-polluted areas.

Light-Year - An astronomical unit of measure equal to the distance light travels in a year, approximately 5.8 trillion miles.

Limb - The edge of a celestial object’s visible disk.


Magnification - The amount that a telescope enlarges its subject. It is equal to the telescope’s focal length divided by the eyepiece’s focal length.

Magnitude - A number denoting the brightness of a celestial object. The higher the magnitude the fainter the object.

Maksutov-Cassegrain Telescope - A telescope that has a deeply curved meniscus lens as a correcting plate and incorporates a small secondary mirror that reflects light back through a hole in the primary mirror. 

Meridian - The imaginary north-south line that passes directly overhead (through the zenith).

Messier Object - An entry in a catalogue of 103 star clusters, nebulas, and galaxies, compiled by French astronomer Charles Messier. The modern-day Messier catalogue contains 109 objects.

Meteor - A brief streak of light caused by a small piece of solid matter entering the Earth’s atmosphere at tremendous speed. It is also known as a “shooting star”.

Meteor Shower - An occasion where a collection of meteors move fast across the night sky.

Milky Way - A broad, faintly glowing band stretching across the night sky, composed of billions of stars in our galaxy that are too faint to be seen individuality. The Milky Way galaxy is invisible when the sky is lit up by artificial light.

Moon Filter - The first accessory that any astronomer should buy. The filter acts as “sunglasses” for the Moon, reducing the brightness and increasing the contrast allowing you to see more details in the lunar surface.

Mount - The device that supports your telescope and allows it to point at different parts of the sky. A sturdy, vibration-free mount is every bit as important as the telescope’s optics. A mount’s head can be either alt-azimuth (turning side to side and up and down) or equatorial (turning parallel to the celestial coordinate system). Mounts can also be computerised to find and track objects automatically. 


Nebula - Latin for “cloud”. Bright nebulas are great clouds of glowing gas, lit up by stars inside or nearby. Dark nebulas are not lit up and are visible because they block the light of stars behind them.


Objective - A telescope’s main light-gathering lens or mirror.

Occultation- When the Moon or planet passes directly in front of a more distant planet or star.

Opposition - When a planet or asteroid is opposite the Sun in the sky and is visible all night (rising at sunset, and setting at sunrise).

Orbit - The path a celestial body takes as it moves through space.


Parallax - The apparent change in position of two objects viewed from different locations.

Phase - The fraction of the Moon or other body that we see illuminated by sunlight.

Planet- A celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity (although not massive enough to cause thermonuclear fusion) and has cleared its neighbouring region of planetesimals.

Planetesimal - A solid object that is believed to exist in protoplanetary disks and in debris disks. They are formed from small dust grains that collide and stick together and are the building blocks that eventually form planets in new planetary systems.

Planisphere- A device that can be adjusted to show the appearance of the night sky for any time or date on a round star map. They can be used to identify stars and constellations but not planets.


Quasar - An unusually bright object found in remote areas of the universe.


Red Dot Finderscope - See “Finderscope”

Red Giant - A stage in the evolution of a star when the fuels begin to exhaust and the star expands to about fifty times its normal size. The temperature cools which gives the star a reddish appearance.

Red Light Torch - A torch with a red LED light typically used during a star party or observation. The red light allows you to see your belongings without breaking your eyes dark adaptation.

Reflector - A telescope that gathers light with a mirror. The most common design is the Newtonian Reflector, design by Issac Newton. These can be used to view Moons, Planets, Nebula and Galaxies.

Refactor - A telescope that gathers light with a lens. Refractor telescopes are best for viewing Moons and Planets and typically are the cheapest option. 

Right Ascension (R.A.) - The celestial equivalent of longitude, denoting how far an object lies east of the Sun’s location during the March equinox. 


Schmidt-Cassegrain Telescope - An astronomical telescope that employs a concave primary mirror, a convex secondary mirror, and a Schmidt corrector plate, so as to eliminate spherical aberrations and achieve a long focal length within a relatively compact housing.

Seeing - The measure of the atmosphere’s stability. Poor seeing makes objects waver and blur when viewed in a telescope at high magnification. 

Shooting Star - See “Meteor”.

Solar Filter - Material that allows safe viewing of the Sun by blocking nearly all of its light. Proper filters should completely cover the front aperture of a telescope and should never be attached to the eyepiece. Viewing the Sun with the naked eye can be highly dangerous, always use a Solar Filter. 

Star - A massive ball of gas that generates huge amounts of energy (including light) from nuclear fusion in its hot, dense core. The Sun is a star.

Star Cluster - A collection of stars orbiting a common centre of mass. Open clusters typically contain a few hundred stars and may only be 100 million years old or less. Globular clusters may contain up to a million stars, and most are at least 10 billion years old (almost as old as the universe itself).

Star Diagonal - See “Diagonal”.

Star Party - A group of people who get together to view the night sky. 

Sunspot - A temporary dar blemish on the surface of the Sun that is a planet-sized region of cooler gas. Sunspots can be viewed safely by using a Solar Filter.

Supergiant - A stage in a star’s evolution following being a red giant, where the core contracts and the star swells to about five hundred times its original size. The star’s temperature drops, giving it a red colour.

Supernova - A star ending its life in a huge explosion.


T-Adapter - A piece of kit that connects to the telescope’s focuser where an eyepiece would usually go. They are specific to each telescope and, when used with a T-Ring, allow a DSLR camera to be attached for astrophotography. 

T-Ring - A piece of kit that connects to a T-Adapter and DSLR camera to be used for astrophotography. T-Rings are specific to the camera model used.

Telescope - An instrument that uses lenses and sometimes mirrors to collect a large amount of light from distant objects to enable direct observation and photography. 

Terrestrial - A term used to describe anything originating on the planet Earth. 

Terminator - The line on the Moon or planet that divides the bright, sunlit part from the part in shadow. It is usually the most exciting and detailed region of the Moon to view through a telescope.

Transparency - A measure of the atmosphere’s clarity. When transparency is high you can see most stars. 


Variable Star - A star whose brightness changes over the course of days, weeks, months or years. 


Waning - The changing illumination of the Moon (or other body) over time. Whilst the Moon wanes it is becoming less illuminated.

Waxing - The changing illumination of the Moon (or other body) over time. Whilst the Moon is waxing it is growing more illuminated.


Zenith - The point in the sky that is directly overhead.

Zodiac - Greek for “circle of animals”. It is the set of constellations situated along the ecliptic in the sky through which the Sun, Moon and planets move.

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  • By Beatrice Debney
  • 10 Aug 2021

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