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Astronomy Glossary

The astronomy glossary aims to help you understand some of the terms used in this website. It is by no means a comprehensive astronomy dictionary.
Andromeda Galaxy

The Andromeda Galaxy or MESSIER 31 (M31) is the closest large galaxy to the MILKY WAY. It is 2.5 million LIGHT YEARS away and gets its name from the CONSTELLATION in which it appears in our night sky (see picture below). The star Alpheratz is also used to form the square shape in the constellation Pegasus, even though it is now assigned exclusively to Andromeda and not Pegasus.

The Andromeda Galaxy is one of the brightest MESSIER objects with an apparent MAGNITUDE of 3.4 and is the farthest object that can be seen with the unaided eye. We only see the very bright central region, if we could see the whole of the galaxy it would stretch 6 full Moon widths across the sky (3 degrees).

It is a spiral galaxy 260,000 LIGHT YEARS in diameter containing 1 trillion stars so over twice the size of our own Milky Way galaxy. It is the most massive of the galaxies in the LOCAL CLUSTER which contains the MILKY WAY, the Triangulum galaxy (M33) and 44 other smaller galaxies. It has a double nucleus with at least one supermassive BLACK HOLE hidden at its core. There are at least 450 GLOBULAR CLUSTERS orbiting around the galaxy and some of these are the most densely populated globulars ever seen.

The Andromeda Galaxy is approaching the Milky Way at approximately 100-140 km/s and in about 3.75 billion years the galaxies will collide, merging and evolving into a new type of galaxy, an ELLIPTICAL GALAXY or a LARGE DISC GALAXY.
Asteroid 3200 Phaethon

Discovered in October 1983 this unusual asteroid may be an extinct comet. It measures 5.1 Km in diameter and its orbit crosses the orbits of Mars, Earth, Venus and Mercury. It was the first asteroid to be discovered by a spacecraft.

Phaethon's (pronounced FAY-a-thon) most remarkable distinction is that it approaches the Sun closer than any other numbered asteroid.  The surface temperature at its closest (perihelion) could reach approximately 1025 Kelvin. This is why it was named after the Greek myth of Phaëton, son of the sun god Helios.

Phaethon is the parent body of the Geminids meteor shower observed between the 7th-16th December. Along with the Quadrantids, observed between 1st-6th January, they are the only meteor showers originating from an asteroid rather than a comet. It will approach relatively close to the Earth on December 14, 2093, passing within 0.0198 AU (Astronomical Units).

Astronomical Twilight

Astronomical twilight is defined to begin in the morning, and to end in the evening when the centre of the Sun is geometrically 18 degrees below the horizon. From the end of astronomical twilight in the evening to the beginning of astronomical twilight in the morning, the sky (away from urban light pollution) is dark enough for all astronomical observations. Most casual observers would consider the entire sky fully dark even when astronomical twilight is just beginning in the evening or just ending in the morning, and astronomers can easily make observations of point sources such as stars, but faint diffuse items such as nebulae and galaxies can be properly observed only beyond the limit of astronomical twilight. In some places, especially those with sky glow, astronomical twilight may be almost indistinguishable from night.

Astronomical Unit

The astronomical unit (AU) is a unit of length approximately equal to the distance from the Earth to the Sun. The currently accepted value of the AU is 149 597 870 691 ± 30 metres (about 150 million kilometres or 93 million miles).
Image courtesy NASA Blue Moon

It takes 29.53 days for one lunation i.e. the time taken to go from, for example, New Moon back to New Moon. There are 365.24 days in a year so there are usually 12.37 lunations in one year which equates to roughly 11 days more than the number of days in 12 lunar cycles. The extra days accumulate so every 2-3 years there is an 'extra' 13th full moon. This extra full moon means that one of the seasons ends up having 4 full moons and not 3. Originally the 3rd full moon in the season with 4 full moons was called a Blue Moon. In 1946 an article was printed which misinterpreted this traditional definition; it stated that if there is a second full moon in one month then this second full moon is called a Blue Moon. This misinterpretation seems to have been widely adopted as a current description.  

The Moon of course is not blue. This would be a rare event indeed and only occurs in certain atmospheric conditions; e.g., when there are volcanic eruptions or when exceptionally large fires leave particles in the atmosphere. The image you see here has been taken using a blue filter.

In this example conjunction is the position of the outer planet (in orange) in relation to the Earth when the outer planet is at the opposite side of the Sun to the Earth. At this point in the orbits of the 2 bodies the outer planet is the furthest it will be from the Earth. However because of the elliptical nature of planetary orbits the actual distance between the Earth and the outer planet will be different at each conjunction.
Simplified diagram to show opposition and conjunction. The elliptical orbits are very exagerated.

Imagine the night sky (the celestial sphere to be a bit more technical!) as a huge dot to dot puzzle. If you joined some of the stars by imaginary lines you would make patterns.

In ancient times poets, farmers and astronomers began to pick out patterns in the stars and named them after heroes and fabled animals. In Ptolemy's time 48 constellations were recognised by the Eastern Mediteraneans and many mythological stories were associated with them. Nowadays because we have included the whole of the celestial sphere and not just the ones that Ptolemy could see the tally has gone up to 88.

While the patterns of old seem quite specific to the brightest stars, the constellations of today make up a block of sky encompassing not only the recognised pattern of stars but all the stars within that block which are not necessarily visible to the unaided eye. Each block forms a border with another block much like the county boundaries in the UK. These specific boundaries were put in place by the International Astronomical Union in 1922 and the 88 constellations have remained since then.

We still recognise the patterns that the brighter stars make within the whole of the constellation but assigning a region of sky to one constellation has helped us to locate specific deep sky objects. For example Messier object M13 is located in the constellation Hercules (see below). The green line marks the constellation boundary.
Constellation of Hercules


An eclipse occurs when one celestial object moves into the shadow of another celestial object along the line of sight of an observer. The observer sees either a total eclipse where the first body is completely obscured or a partial eclipse when only a portion of the first body is obscured. Examples of eclipses include a Lunar eclipse and a Solar Eclipse. Satellites of planets can also eclipse each other as they orbit around their parent planet.

First Quarter

M22: Image courtesy of NASA Globular Cluster

A globular cluster ia a nearly symetrical, spherical collection of hundreds of thousands of very old stars held tightly together by gravity. The stars are probably some of the first stars that formed in our Milky Way galaxy. The highest concentration of stars is in the central region making it look like a glowing almost 3 dimensional ball through a telescope.  

The name is derived from the Latin globulus meaning a small sphere and there are currently about 158 known globular clusters in the Milky Way. In larger galaxies there may be many more. In fact every galaxy of sufficient mass has an associated group of globular clusters.

Several globular clusters with extremely massive cores may even have a black hole lurking at their centre but there is no scientific evidence to prove this yet.
 Greatest Elongation

This refers to the position of an inner planet (Mercury or Venus) when it is at maximum angular separation from the Sun as viewed from Earth i.e. how far from the Sun Mercury or Venus appears in our sky (see diagram 1). Mercury and Venus are particularly easy to see when at greatest elongation. When the planet is at maximum EASTERN elongation the planet is seen in the evening close to sunset. When the planet is at maximum WESTERN elongation, the planet is seen in the morning close to sunrise (see diagram 2).

Jupiter's Great Red Spot

Jupiter's most famous feature is its Great Red Spot (GRS). The spot was first seen by the English astronomer Robert Hooke in 1664. This means that it has been raging for at least 349 years. In fact nobody knows how long it has been active. It is huge and can fit at least 3 Earths inside it!

It was named the Great Red Spot around 1878 when it turned a vivid brick red, but more recently it has faded to a much less conspicuous pale tan. This huge, long-lived storm is spinning like a hurricane. However, unlike hurricanes on Earth, the GRS rotates in an anticlockwise direction in Jupiter's southern hemisphere, showing that it is a high-pressure system. In fact the storm gets weaker and stronger over time and the higher the pressure the redder is the spot.
The image courtesy of NASA/JPL/University of Arizona shows a true-color image of Jupiter taken by NASA's Cassini spacecraft on October 8, 2000. The Great Red Spot is upper left of center. It always stays in the south edge of the brownish South Equatorial Belt. In this image south is up to match the inverted view in many astronomical telescopes.

Lunar Eclipse

An eclipse of the Moon can only occur at FULL MOON and only if the Moon passes through some portion of the Earth's shadow (see diagram 2 under phases of the Moon).

A TOTAL eclipse happens when the entire Moon passes through Earth's umbral shadow (see diagram 1 below).

A partial lunar eclipse occurs when only part of the Moon passes through Earth's umbral shadow.
Image courtesy of Mr Eclipse and Starry Skies

Why do we not have a total lunar eclipse every month? The Moon's orbit around the EARTH is tilted about 5 degrees to the orbit of the Earth around the Sun (see diagram 2). If the Moon's orbit around the Earth were in the same plane as the Earth's around the Sun (the ecliptic,) we would indeed have a monthly eclipse. The Moon passes through the ecliptic only twice a month at a pair of points called the nodes. The rest of the time the Moon is either above or below the plane of the Earth's orbit and does not pass directly through the Earth's shadow.

Magnitude is a measure of how bright a celestial object looks. Those objects that can be seen with the naked eye are ranked in 6 magnitudes from first to sixth magnitude. First magnitude is the brightest and 6th magnitude the faintest, which always seems a little odd! Anyway a sixth magnitude object is exactly 100 times less bright than a first magnitude object. This means that the difference between a first and second magnitude object is approximately 2.51 times. To get the difference between a first and second magnitude object all you do is multiply 2.51 x 2.51 = 6.3.This means that a third magnitude object is about 6.3 times less bright than a first magnitude object.

To make things a little more complicated, an object 2.51 times brighter than magnitude 1 becomes magnitude 0. An object 6.3 times brighter than magnitude 1 becomes magnitude -1.

Sirius is the brightest STAR in the sky and has a magnitude of -1.44. The full Moon has a magnitude of -12.7 and the Sun has a magnitude of -26.7.

New moon

Light Year

A light year is the distance that light travels in one year. Light travels at 186,000 miles per second or 300,000 km per second so the distance is pretty huge. It equates to 5,900,000,000,000 miles (nearly 6 trillion miles) or 9,460,000,000,000 km (just over 9 trillion km).
Meteor Shower

Meteor showers are caused by streams of dust and rock called meteoroids. This debris is usually left behind by comets as they orbit the Sun. When a comet approaches the Sun it will begin to vaporise leaving behind a meteoroid stream also known as a dust trail. As Earth passes through this dust trail every year at the same time, meteoroids will enter the Earth's atmosphere and "burn up" leaving a bright visible streak called a meteor. If meteors occur only seconds or minutes apart then it is known as a meteor shower and it takes its name from the constellation from which the shower appears to originate (the radiant point). For example the Perseids appears to originate in the constellation Perseus and the Taurids appears to originate from the constellation Taurus.  

Most meteoroids that cause meteors are the size of a pebble. Their bright streaks become visible between 40 and 75 miles (65-120 km) above the Earth's atmosphere and they disintegrate at altitudes of around 30-60 miles (50-95 km). At speeds of up to 26 miles per second (42 km/s) they are visible for just fractions of a second. Any large meteoroids that survive and reach the surface of Earth are called meteorites.

Meteoroids vary in size from a dust particle to a small boulder. The number of meteoroids appears inversely proportional to their size for instance there are more meteoroids the size of a dust particle than the size of a grain of sand and there are more meteoroids the size of a grain of sand than the size of a pebble etc. Millions of meteors "burn up" in the Earth's atmosphere every day but most of them are so small that they are invisible. Also many enter the atmosphere during the day.

The reason why meteors glow as they pass through the atmosphere is not because of friction. As the meteoroid leaves the vacuum of space and enters the Earth's atmosphere at high speed there is a rapid compression of the air in front of it, which becomes superheated so much so that it ionises. This is what causes the glowing trail behind the meteor. The meteor itself disintegrates at such a high temperature, which can reach 1650 C.

An occultation is an event that occurs when one object is hidden by another object that passes between it and the observer. The hidden object is either smaller or appears smaller to the observer than the object passing in front of it. In the example shown here there is an occultation of Jupiter by the Moon.

A transit is an astronomical event that occurs when, as seen from an observers viewpoint, one celestial body appears to move across the face of another celestial body, hiding a small part of it. Examples of this would be Venus crossing the face of the Sun, satellites crossing the face of the planet that they are orbiting (see picture of Io transiting across the face of Jupiter).


In this example opposition is the time when a celestial body is on the opposite side of the Earth to the Sun (see diagram below). At this point in the orbits of the 2 bodies the outer planet (shown in orange) is the closest it will be to the Earth. However because of the elliptical nature of planetary orbits the distance between the Earth and the outer planet will be different at each opposition.
Simplified diagram to show opposition and conjunction. The elliptical orbits are very exagerated.
Orion Nebula

The Orion Nebula is also known as M42. The M refers to Charles Messier an 18th century French astronomer and comet hunter. He compiled a list of deep sky fuzzy looking objects so they would not be mistaken for comets. Not all Messier objects were actually discovered by Charles Messier himself.

Nebula (plural nebulae)
is Latin for mist and they are vast areas of cloud and dust between the stars. The Orion Nebula is so huge it is visible to the naked eye even though it is 1,344 light years away. It appears to cover 1 degree of the sky, an area twice the size of the full Moon, it is actually 24 light years across. A light year is roughly equivalent to 9.5 million million Km! It is a region where new stars are formed. These new stars at the centre of the dust cloud light up the surrounding gas making it visible through a telescope.

The Orion Nebula is in the constellation of Orion, which is a very prominent constellation in the winter sky. The nebula is located in the "sword" of Orion which hangs below the 3 stars that depict his belt (see diagram on right)

One of the new stars at the centre of the Orion Nebula is Theta-1 Orionis. It is easy to understand why it is called the Trapezium because, through the telescope, you should see 4 prominent stars in the shape of a trapezium (see Diagram below).

This false colour mosaic was made by combining several exposures from the Hubble Space Telescope Image credit:  NASA Picture of the day

Perigee Moon

Apogee and perigee refer to the distance from the Earth to the moon. The Moon's orbit around the Earth is not circular, it is eliptical so the distance is not always the same (see perihelion below for an example of an eliptical orbit). Apogee is the farthest point from Earth. Perigee is the closest point to Earth and it is in this stage that the moon appears larger. Looking at the moon in the sky without anything to compare it to, you wouldn't notice any size difference. But the difference in size can in fact be quite significant. A perigee Moon is often referred to (in non astronomical terms) as a Supermoon. If you were to photograph a full moon at apogee and perigee (using the same lens), here's how the two sizes would compare with an apogee Moon on the left and a perigee Moon on the right:


All of the planets in our Solar System move around the Sun in elliptical orbits. An ellipse is a shape that can be thought of as a "stretched out" circle or an oval as in the diagram below (this is an exagerated version of the orbital path of the Earth around the Sun). The Sun is not at the centre of the ellipse, as it would be if the orbit were circular. Instead, the Sun is at one of two points called "foci" (which is the plural form of "focus") that are offset from the centre. This means that each planet moves closer towards and further away from the Sun during the course of each orbit. The point in the orbit where the planet is closest to the Sun is called "perihelion".  The point in the orbit where the planet is furthest away from the Sun is called "aphelion".
Phases of the Moon

As the Moon orbits the Earth it shows different phases to observers on Earth. The reason for this is explained pictorially in the diagrams below.

The whole of the Moon is only ever half illuminated - the half that faces towards the Sun (diagram 1). However, looking from Earth, we only see a portion of this illuminated face because as the moon orbits the Earth it changes position in space relative to the Sun and Earth. (diagram 2).

Diagrams: Courtesy of NASA (
New Moon is considered as the beginning of the cycle, hence the term New Moon. However, because the moon is between Earth and the Sun the Sun only illuminates the face that we do not see, the ‘Far Side of the Moon'. The face that we normally look at is in total darkness and therefore we cannot see it (see diagram 2 where the white part of the Moon is the only portion lit by the Sun that we can see clearly).

The next phase is a crescent Moon but because more and more of the illuminated face of the Moon will become visible to observers from Earth in the following days it is called a waxing crescent.

The next phase is called First Quarter. The reason why we call this phase of the moon first quarter is because it is ONE QUARTER of the way through the lunar cycle. The lunar cycle is the number of days it takes to go from one New Moon to the next and is 29.53 days. First quarter is often referred to as half moon. This is because only half of the face of the moon that we see is lit by the Sun. Halfway through the lunar cycle is Full Moon when we can see the whole of the illuminated face because it is opposite the Sun with respect to the Earth.

As the illuminated face becomes less the Moon is said to be waning and when it has orbited three quarters of the way around the Earth it is said to be a Last (or Third) Quarter Moon. The next time the moon is a crescent it is a waning crescent and the edge that is illuminated is the opposite edge that is illuminated when it is a waxing crescent. Remembering this becomes easy by saying "if the Light is coming from the Left the Moon is getting Less."

The Moon orbits the Earth in 27.3 days but because the Earth orbits the Sun as well (and therefore has moved a little way around in its orbit), it takes an extra 2.5 days for the Moon to get back to the same position in the Earth-Moon-Sun system to complete one lunation (from one New Moon to the next New Moon). Because it takes 27.3 days to orbit Earth the Moon rises at different times each night. The New Moon rises as the Sun rises, First Quarter Moon rises around mid-day, the Full Moon rises as the Sun sets and the Last Quarter Moon rises around midnight. At certain times therefore the Moon is up during the day. Because it is big, close to the Earth and very reflective it appears 100,000 times brighter than the brightest star in the night sky, this means that we are able to see it during the day.
Saturn's Rings

The rings of Saturn are made up of icy particles ranging in size from micrometres to metres. Almost entirely water ice, the particles are contaminated with some dust and other chemicals. Reflected sunlight from these particles, contribute a great deal to the brightness of Saturn as viewed from Earth and this brightness appears to change over time. This is due not only to the change in the distance from Earth to Saturn as both planets independently orbit the Sun, it is also due to the changing aspect of the rings (see diagram below), which in turn depends on where Saturn is in its orbit around the Sun. This year the rings are closing up and by September 2009 they will appear edge on to our line of sight. This means that they will be practically invisible since the thickness of the ring system is estimated at only 10 metres deep.  
The orbit of Saturn shown at two/three year intervals between the years 1993 and 2020 AD. The orbit of the Earth is seen close to the centre, marked at various dates by a blue-green globe (the orbits are not shown to scale). The dates in blue are the dates of Saturn's opposition to the Sun, i.e. when the planet is closest to the Earth and appears at its brightest for the year. The images in the grey circles show how the planet appears from the Earth (orientated with Celestial North at the top). The points of Saturn's perihelion (i.e. its closest point to the Sun) and aphelion (its most distant point from the Sun) are also marked. The constellation in which Saturn appears, as seen from the Earth, is shown in green. The First Point of Aries is the 'zero point' from which the longitudes of the planets are measured (diagram based on a graphic by space artist David A Hardy).

Diagram and caption courtesy of:

It takes 29.457 Earth years for Saturn to orbit the Sun. During this time we see the rings from different angles. Previously the South Pole of Saturn has appeared tilted towards Earth and we have been looking at the underside of the rings. When the rings start to slowly open up again we will begin to see the top side of the rings as the North Pole appears tilted in our direction. By the time Saturn has completed one orbit the ring cycle, from our point of view will start all over again.

Another ring around Saturn has recently been discovered (6th October 2009) using the Spitzer Space Telescope, which revealed an infrared glow thought to come from sun-warmed dust in a tenuous ring. The ring spans from 128 to 207 times the radius of Saturn - or further - and is 2.4 million kilometres deep. It is the largest planetary ring in the solar system but is quite diffuse making it very difficult to detect using visible light. The source of the ring's material seems to be Saturn's outer moon Phoebe, which orbits the planet at an average distance of 215 times the radius of Saturn. If space rock hits Phoebe the impact may generate the debris which has made the ring. 

Solar Eclipse

A total Solar eclipse occurs when the Moon passes directly in front of the Sun from the observers line of sight. It can only occur when the phase of the Moon is at  NEW MOON. A total eclipse will be visible if you are located in the umbral shadow (see diagram below). A partial solar eclipse of varying degrees will occur if you are located in the penumbral shadow.
Image courtesy of Mr Eclipse

So why does it happen? It is a quirky fact of nature really. The Sun is 400 times lrger than the Moon but the Moon is 400 times closer to the Earth than the Sun. This ratio means that every 18 months or so, somewhere on Earth the moon can be seen to totally cover the disc of the Sun. Try an experiment. Hold up your thumb at arms length and try to cover something in the background with your thumb (it is better if you close one eye). How much can you cover? Now bring your thumb closer to your face. How much can you cover now? The closer your thumb gets to your face, the more background you cover. 

This ratio is not always the same however because the orbits of the Earth and the Moon are not circular, they are elliptical. For example if, during an eclipse, the Moon is further away from Earth in its elliptical orbit it appears smaller. At this point the distance and size ratio of the Sun and Moon is not sufficient for the Moon to be able to totally block out the Sun. You see an annular eclipse the so called Ring of Fire. Looking down from space, we would see that the Moon's umbral shadow is not long enough to reach Earth.  Instead, the antumbra shadow reaches Earth.  The track of the antumbra is called the path of annularity.  If you are within this path, you will see an eclipse where a ring or annulus of bright sunlight surrounds the Moon at the maximum phase.


Spectroscopy is a powerful tool in astronomy. Spectroscopes attached to the reflecting telescopes turned the light from individual stars into spectra - miniature rainbows that can be used like fingerprints. By analysing their patterns, researchers can extract a wealth of information including what stars are made of, how hot they are and how fast they are moving towards or away from Earth.
This is a detailed spectrum of the star Arcturus - a red giant. It was taken from an American Observatory and shows a continuous spectrum displayed over 50 strips. Within each strip the vertical black bars are absorption lines which give information about the chemical composition of the star.

A transit is an astronomical event that occurs when, as seen from an observers viewpoint, one celestial body appears to move across the face of another celestial body, hiding a small part of it. Examples of this would be Venus crossing the face of the Sun, satellites crossing the face of the planet that they are orbiting (see picture of Io transiting across the face of Jupiter).

If the larger celestial body hides a major part, or all of, the smaller celestial body (i.e from an observers point of view the smaller celestial body moves behind the larger body), then it is an occultation rather than a transit.
Image courtesy NASA Type 1a Supernova

This type of supernova occurs when a tiny WHITE DWARF star acquires additional mass by siphoning matter from a companion star. When it reaches a critical mass, the heat and pressure in the centre of the star sparks a runaway nuclear fusion reaction and the white dwarf star explodes.