Astronomical Dictionary


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This is by no means a comprehensive glossary of astronomical terms however it should provide some assistance to novices in particular. Click on the symbol for a diagram or picture associated with that word. Cross references within the glossary are indicated by hyperlink text. Click on the letters below to go to the first entry of alphabetical sections.

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Faculae These are brighter patches of the Sun's surface. They are often seen near sunspots. They are best seen near the edge of the Sun because they contrast better with the apparently darker limb. They often herald the appearance of sunspots and may be present after a sunspot has subsided. Some faculae are seen in high Solar latitudes and are not associated with sunspots at all. Also associated with faculae are plages or flocculi - clouds that are high in the chromosphere. These are apparently best observed in light emitted by ionised calcium.
Fireball An extremely bright meteor, also called a bolide.
First Contact The moment at which an eclipse begins. In the case of a Solar eclipse, it is the moment that the Moon starts to move across the face of the Sun. In the case of a Lunar eclipse, it is when the Moon first enters the shadow of the Earth.
Flare Flares are created in the active regions of the Sun by a sudden release of energy. This accelerates charges particles in the region which, in turn, causes radiation to be emitted across the whole spectrum from X-rays to radio waves. Particles are also ejected from the Sun and travel through the Solar System. These cause magnetic disturbances on the Earth. This can cause blackouts and loss of quality in radio communications and aurorae. If the blast of radiation from the flare is poerful enough, it can cause systems failures in spacecraft and has been know to shut down regional power systems on the surface.
Fourth Contact The moment at which an eclipse ends. In the case of a Solar eclipse, it is the moment that the Moon is seen to move completely off the face of the Sun. In the case of a Lunar eclipse, it is when the Moon finally leaves the shadow of the Earth.
Galactic Cluster This is an alternative name for an open star cluster in our Galaxy. It indicates that the cluster lies within our Galaxy rather than in the halo round our Galaxy, where globular clusters lie.
Galaxy A collection of stars gas and dust. There are many different types but they can be broadly grouped into 3 main categories - spiral galaxies, elliptical galaxies and irregular galaxies. When written with a capital 'G', it refers to our own galaxy, often called "the Milky Way". The Universe is populated with galaxies. There is very little intergalactic (between galaxies) matter.
Gegenschein Gegenschein is also known as the counterglow. It is a faint oval patch of light that is very difficult to see, you need a clear and moonless night. It is at the antisolar point i.e. exactly opposite the Sun. It is apparently best seen when the ecliptic is at its highest above the horizon (midwinter from the northern hemisphere and vice versa for the southern hemisphere). It is not well understood but is thought to be caused by the scattering of sunlight from dust in the main plane of the Solar System. Sometimes it can be seen to be joined to the zodiacal light by a parallel sided beam of light. I think that this beam is called the zodiacal band but cannot be 100% sure. I also believe that it is larger in the tropics than in the temperate zones. Click here to visit the NASA website and view a picture of the gegenschein.
Giant Star As stars approach the end of their lives, the nuclear reactions that power them, change. This causes the star to greatly increase in size in a big way. A giant star has a similar mass to our Sun but is much larger and more luminous. The word giant is usually prefixed with the colour of the star e.g. red giant star.
Gibbous The phase of the Moon (or planet) between half and full.
Globular Cluster Unlike open clusters which are found within our Galaxy, globular clusters are external to our Galaxy. They are situated within a spherical volume of space surrounding our Galaxy. A globular cluster is a huge (tens to hundreds of light years in diameter) spherical concentration of stars. The density of stars is much higher than in the spiral arms of the Galaxy. This high density means that the gravitational forces within a globular cluster holds the cluster together, against the gravitational force of the Galaxy. They are amongst the oldest objects in the Universe, 10,000 million years or more in age. They therefore contain stars that are generally much older than those in the open clusters of the Galaxy. Some globulars are bright enough to be seen without optical aid e.g. omega Centauri and 47 Tucanae in the southern hemisphere skies. Probably the best known of the northern hemisphere is M13 in Hercules. This is a fuzzy blob in small telescopes and binoculars but with a moderate telescope of 180 mm aperture or more, together with a medium to high magnification individual stars can be seen. M13 globular cluster
Gould's Belt This is a band of bright young stars that stretches round the sky from Perseus, Taurus and Orion to Centaurus and Scorpius via Carina. It is thought to be a spur off the local spiral arm of our Galaxy.
Granulation In close up, the surface of the Sun appears to be mottled. This mottling of the surface is called granulation. It is thought to be caused by convection currents in the Sun.
Green Flash This is a very rare and difficult to observe atmospheric phenomenon. Blink and you miss it! Literally. The Green Flash lasts only for a few seconds and is best seen as the Sun sets over the sea when the air is very clear. The last segment of the setting Sun is seen to turn green, sometimes it is followed by a green ray rather like a flame.
Greenhouse Effect This is the effect of warming the average temperature of a planet by trapping heat from the Sun in the atmosphere. The only reason that water on Earth stays in the liquid form is because we have greenhouse gases in our atmosphere. Without these, the Earth would have cooled below freezing point and life would not have existed. Important greenhouse gases are, carbon dioxide, methane and water vapour. CFC's also have a greenhouse effect but are regarded more as the culprits of destruction of the ozone layer of the atmosphere. Current problems with global warming are thought to be occurring because of the burning of fossil fuels (coal, oil and gas) by humans. There is a major scientific debate about this - some scientists believe that the current warming is a natural phenomenon - but can we take the risk? On Venus, where the atmosphere is largely carbon dioxide, a runaway greenhouse effect has raised temperatures to rather inhospitable levels, around 400 deg. C.
Hour angle The time that has elapsed since an object was on the meridian or the time that will be needed to wait for an object to reach the meridian. West of the meridian - hour angle is positive, east it is negative.
Immersion The entry of an astronomical body into the shadow at the time of an eclipse or the moment that an object is covered during an occultation.
Inferior A planet or other body that orbits closer to the Sun than does the Earth. The inferior planets (Mercury and Venus) exhibit phases. Inferior can also be used to describe a conjunction of these two planets.
Infrared This is radiation with wavelengths longer that red in the spectrum but shorter than radio waves. The range of infrared wavelengths is about 700 nanometres to 1mm. Infrared is radiation is also known as heat radiation. Infrared radiation from the stars and other bodies is absorbed by water molecules in the atmosphere and so it is best observed by orbiting telescopes. Click for a diagram of the electromagnetic spectrum
Interferometer Interferometers can be radio or optical. They use the principle of interference of electromagnetic waves to enable the combination of light or radio waves from two apertures to be combined. This greatly increases the resolution of an instrument. The star Capella was resolved into two stars by a team using optical interferometry whilst many of the worlds radio astronomers link dishes in different countries to get better pictures of radio galaxies.
Inverse Square Law The energy that is received from a source diminishes with distance in accordance with this law. What it means is that for two identical sources e.g. stars of the same colour and brightness, if one is twice as far away, it will appear four times as faint or if it was three times further away it would be nine times fainter. This law also works with gravity.
Kepler Johannes Kepler was born on Dec 27th 1571. At university, he was so inspired by Copernicus' theory of the Solar System that he chose to devote his life's work to astronomy. He devised a theory of his own of regular solids alternating with spheres, to describe how the planets moved. Based on the observations of Tycho Brahe in particular, he realised that his theory was flawed since it could not be used to accurately predict the positions of the planets. During the 10 years between 1609 and 1619, he formulated his three Laws of planetary motion. He even attempted to explain how the planets might move. He imagined some sort of force radiating from the Sun in the same way that the spokes of a wheel radiate from the hub. These "force spokes" pushed the planets round. Newton's theory of gravity led to the correct solution that the planets orbit the Sun because they are moving forwards at high speed and at the same time, being pulled in towards the Sun by its gravitation.
Kirkwood Gaps These are regions of the asteroid (minor planet) belt in which few asteroids are found. The reason for this is because of the gravitational influence of Jupiter. This disturbs the orbits of asteroids whose orbital periods are exact fractions of that of Jupiter itself.
Lens A carefully shaped piece of glas that is used in telescopes to bend rays of light in order to bring them to a focus and to magnify images. Simple lenses will focus different colours at different points, leading to a defect called chromatic aberration. The construction of good quality lenses requires the use of different types of glass, glued together. .
Light year The distance that light travels in a time period of one year, approximately 6,000,000,000,000 miles!
Limb The very edge of the visible disc of the Sun, Moon or a planet.
Lunar Pertaining to the Moon e.g. lunar regolith or lunar eclipse.
Lunation One cycle of lunar phases.
Magnetosphere The area of space around an object influenced by the magnetic field of that object. The shape of magnetosphere is influenced bu any stellar wind in the vicinity.
Magnitude The brightness of an astronomical object. On a clear, Moonless night in a location away from any light pollution, the faintest star visible to the naked eye will be about magnitude 6. In a city, keen eyed observers may spot stars brighter than about magnitude 3.
Major axis The longer axis of an ellipse.
Mercury The planet that is closest to the Sun. For detailed information, including images, click here.
Meridian An imaginary line joining the north and south points of the horizon, passing directly through the zenith. The reference line for a transit.
Meteor Commonly called a shooting star. It is a piece of fast moving debris e.g. dust or fragment of rock that burns up in the Earth's atmosphere because of friction with the air. A meteor shower is a display of meteors that seem to come from the same point (more or less) in the sky. They do so because they are travelling in similar paths through space. They are believed to be debris from the wake of comets travelling through the Solar System.

If any portion of a meteor survives its passage through the atmosphere and lands on the surface of the Earth, it is then called a meteorite. See also achondrite.

Book: Meteorites and Their Parent Planets

Meteoroid It is a piece of fast moving debris e.g. dust or fragment of rock that has not entered the Earth's atmosphere but has the potential to do so.
Metonic cycle Every 6939.6 days (19 years), the same lunar phase recurs on the same day of the year. This time period is known as the Metonic cycle. During each cycle, there are exactly 235 lunations.
Minor axis The shorter axis of an ellipse.
Moon A smaller body orbiting a planet. Also referred to as a natural satellite.
Nebula A cloud of gas and dust lying between the stars. Some are bright whilst others are dark. Bright ones shining because of reflected light are called reflection nebulae. Bright ones shining because the gas is exited by radiation from nearby stars are called emission nebulae. Dark nebulae appear like holes in the starry background because they are not illuminated by nearby stars. Nebulae are often the birthplaces of stars.
Newtonian A type of reflecting telescope.
NGC The New General Catalogue of Nebulae and Clusters of Stars produced by the astronomer Dreyer.
Node The points at which the orbit of a planet intersect the ecliptic. The ascending node is where the planet crosses the ecliptic from S to N and vice-versa for the descending node.
North Celestial Pole From any place in the Northern Hemisphere, there is a point in the northern sky around which the stars appear to rotate. It is currently near Polaris, the Pole Star. There is an equivalent point in the southern hemisphere, although there is no significantly bright star to mark it.
Nova A star that suddenly and temporarily increases its brightness to perhaps 100,000 times normal. There are a variety of mechanisms that cause this.

This is a wobble of the Earth's axis.

Objective The light collecting part of a telescope i.e. the main mirror or the lens at the front of a refracting telescope.
Occultation When a small astronomical body is hidden from view by a larger one, e.g. the Moon hiding a star or planet from view as is moves through space, it is called an occultation rather than an eclipse.
Open Cluster Open clusters are internal to our Galaxy. One particularly well known open cluster is the Pleiades (also known as the seven sisters) in the constellation of Taurus. It is said that some observers with exceptionally good eyesight can se up to 11 members of the cluster. Others were observed by Charles Messier, who catalogued objects whilst searching for comets. These are given "M" numbers e.g. M36. The ones that were not catalogued by Messier appear in other catalogues of deep sky objects such as the NGC (New General Catalogue). To the naked eye, under dark skies, the brighter open clusters appear as faint fuzzy patches but in binoculars or telescopes, their starry nature can be seen. picture of the Pleiades open cluster
Opposition A planet is said to be in opposition when it is 180 degrees from the Sun in the sky.
Orbit The path of one astronomical body round another, usually elliptical in shape.
Parallax The apparent change in position of a star caused by the annual motion of the Earth round its orbit. When viewed from one position, a nearby object will appear in one place relative to the background. If the observer moves position, the nearby object will appear to be in a slightly different position. Try it using your thumb. Close one eye and line up your thumb with something a bit further away. Swap eyes and you will see your thumb jump to a different position, it will no longer lined up. This is because the background is further away and so appears to move less (like the trees at the side of the road seem to rush 'past' but a town in the distance moves 'past' more slowly). Nearer stars will therefore seem to be in different positions at different times of the year, when compared to the background stars.
Parsec Comes from the two words "parallax" and "second". A Parsec is a distance in space, equal to about 3.26 light years. In terms of angles, a circle is divided into 360 degrees, a degree is divided into 60 minutes and each minute is divided into 60 seconds. A parsec is the distance that a star would need to be from the Earth in order to give a parallax of one second of angular movement against the background sky.
Penumbra The lighter part of a shadow during an eclipse. An observer standing in the penumbra of a Solar eclipse will see a partial eclipse. This word can also mean the lighter part of a sunspot. See also umbra
Perigee The point in the orbit of an artificial satellite or the Moon that is closest to the Earth. See also apogee.
Perihelion The point on an orbit that is nearest the Sun. See also aphelion.
Phase The phase of a moon or planet is the proportion of the sunlit side that is visible to an observer. From the Earth, the Moon, Mercury and Venus go through a complete range of phases from 0% of the surface visible to 100% visible. The outer planets show only the gibbous phase. The phase of an astronomical object depends upon the angle between the observer, the Sun and the object being observed.
Phase angle The angle between the object, Sun and observer. A phase angle of 180 degrees means that the Sun and the object lie in opposite directions from the observer and therefore the whole of the sunlit side is visible. A phase angle of 0 degrees means that the Sun and object ar in the same direction, as seen by the observer thus the unlit side will be presented to the observer, rendering the object effectively invisible. A phase angle of 90 or 270 degrees means that half of the sunlit side will be visible to the observer so a half phase will be visible. Between 0 and 90 degrees, the phases are waxing crescents.Between 90 and 180 degrees, waxing gibbous, between 180 and 270 the phases are waning gibbous and greater than 270 degrees - waning crescent.

A large object, more or less spherical, that shines only by reflecting light from the star around which it orbits. Planets of our Solar System are classed as Rocky (Mercury, Venus, Earth, Mars, Pluto) or Gas Giants (Jupiter, Saturn, Uranus, Neptune). Several Extrasolar (outside the Solar System) planets have been announced by various teams from observatories around the world. Many of these seem to be larger than any in out Solar System and orbit closer to the parent star. These have been detected by careful observations of wobbles in a star's motion. See also Kepler.


The Cambridge Planetary Handbook

The Planet Observer's Handbook

Discovering the Solar System

Puzzling Questions About the Solar System

Our Worlds


Planitia A term that is applied to a smooth low-lying plain on a planet other than Earth. Click here for a basic table of selected Planitia of the Solar System

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