Derek Haselden's Astronomical Glossary

DEREK HASELDEN'S ASTRONOMICAL GLOSSARY
Updated 1st January 2004

INTRODUCTION
Below is a list detailing some of the many terms and definitions that are frequently encountered in astronomy. This list is by no means exhaustive but I have attempted to cover the majority of commonly encountered terms. So far, they number around 250, comprising over 8000 words. I know that's not much but I've got to start somewhere!

Dare I say that at point somebody will say "Ah, but what about such and such?" Well, if that's the case, let me know and I'll happily add it to this list. Having recently looked at the index of an astronomy book I shudder to think how many more terms could be added. Anyway, my thanks to Joyce Hamilton-Dyer for suggesting this page - thus condemning me to nearly 30 hours of typing and proof reading.

The list is far too numerous for me to provide a complete set of 'Jump To' links but I have done you the favour of at least giving you an A to Z list, and you can easily get back to the top. Also, where a term is used as part of another term's description I have marked that term in bold text to give you some form of cross referencing. There, aren't I good to you?

GLOSSARY A - Z
A | B | C | D | E | F | G | H | I | J | K | L | M

N | O | P | Q | R | S | T | U | V | W | X | Y | Z

OTHER PAGES BY DEREK HASELDEN

Aberration. A defect in an optical instrument or system. There are several types of abberation, descriptions of a few follow: Chromatic abberation occurs in lenses and is caused by different wavelengths of light focusing at different points and results in coloured fringes around objects; Spherical abberation is caused when the innner and outer parts of a lens or mirror have different focal lengths and results in blurred images; Astigmatism is where an image is focused into a cross or ellipse and is caused by optics that are not a uniform, spherical shape; Coma causes elongated or fanned images toward the edge of the field of view.

Aberration (of starlight). An effect of the Earth's motion around the Sun which slightly alters the true positions of the the stars. Light moves at a speed of almost 300 000km (186 000 miles) per second and the Earth orbits the Sun at an average speed of 28km (17 miles) per second. As a result the stars appear to be shifted from their true poition. Aberration may alter a star's position by up to 20.5 seconds of arc.

Absolute magnitude. The apparent magnitude or brightness that a star or other celestial object would have if it was viewed from a standard distance of 10 parsecs, (32.6 light years). Absolute magnitude is therefore the true or intrinsic brightness of an object. (See also Apparent magnitude and Magnitude.)

Absorption line. A break of depression in a continuous spectrum caused by the absorption of photons within narrow wavelengths by some types of atom, ion, or molecule. Any atom, ion or molecule has its own set of characteristic absorption lines which appear when electrons associated with the atom, ion, or molecule absorb radiation and jump to higher energy levels.

Achromatic. A term referring to a lens that has been manufactured to eliminate the worst effects of chromatic abberation. Such a lens is in fact made up of two separate lenses, known as elements, that together correct the worst effects of chromatic abberation.

Aerolite. A meteorite whose main composition is stoney.

Airglow. The faint background glow in the night sky caused by gas in the ionosphere. Because of airglow the night sky is never completely dark as seen from Earth's surface.

Albedo. The reflecting power of a planet or other non-luminous object. For example, the Moon is a poor reflector of light and its albedo is just 0.12. Albedo is typically expressed by a simple numerical factor, 1.0 means a perfect reflector, 0.0 means no reflection at all.

Almanac. A book of tables detailing the positions, times and other data about celestial objects, usually produced annually.

Altazimuth mount. A type of telescope (or other instrument) mounting where the telescope can move freely in any direction - generally, up and down with respect of the horizon (altitude); and left and right along the horizon (azimuth).

Altitude (Alt). The measurement of an object's angle, in degrees, above the horizon. Part of the horizontal system of co-ordinates. An object directly overhead at the zenith would have an altitude of 90°, an object on the horizon an altitude of 0°.

Angstrom (unit). One hundredth-millionth of a centimetre.

Angular diameter. The apparent size of an object, usually expressed in degrees, minutes, or seconds of arc.

Angular distance. The apparent distance between two objects on the celestial sphere measured in degrees, minutes or seconds of arc.

Aphelion. The point at which an orbiting body is furthest from the Sun in its orbit.

Apochromat. A lens consisting of three or more elements which gives a greater reduction of chromatic abberation than that of a two-element (Achromatic) lens.

Apogee. The furthest point of the Moon from the Earth in its orbit.

Apparent (or Visual) magnitude. The apparent brightness of a celestial object. The lower the magnitude, the less bright the object. Thus, the Sun has an apparent magnitude of -27; the Moon up to -12; Venus up to -4; the brightest stars -1; the faintest stars visible to the naked-eye +6, the faintest objects yet detected about +30. For mainly historical reasons the magnitude scale has the peculiar attribute of having brighter objects at negative values and vice-versa. (See also Absolute magnitude and Magnitude.)

Apparition. The period or time when an object is visible and well placed for observation.

Appulse. The apparent close approach of two celestial bodies as seen from Earth, such as a star and a planet, or two planets for example.

Arc (measurement of). Angles on the celestial sphere, measured in degrees, minutes and seconds. Arc may be an expression of the angular distance between two celestial objects or the angular size of an object.

Areography. The proper name for the geography of Mars.

Aspides. The points in the Moon's orbit where it is either closest to Earth (perigee) or farthest from Earth (apogee).

Asterism. A pattern of stars larger than a cluster but smaller than a constellation. Examples of an asterism would be Orion's Belt, or The Hyades in Taurus.

Asteroids. One of several names for the minor planet swarms of the Solar System. Asteroids are found in many regions of the solar system but most are to be found orbiting the Sun between the orbits of Mars and Jupiter. This region is often refered to as the Asteroid Belt.

Astigmatism. An optical defect where an image is focused into a cross or ellipse and is caused by optics that are not a uniform, spherical shape.

Astrograph. A telescope designed specifically for astrophotography.

Astrolabe. An ancient instrument used for measuring the altitudes of celestial objects.

Astrometry. The branch of astronomy that deals with precisely measuring the positions of objects on the celestial sphere.

Astronomical Unit (AU). The mean distance between Earth and the Sun. A unit of distance, equal to 149 600 000 kilometres (92 900 000 miles).

Astrophysics. The study of the physical nature of celestial objects.

Atom. The basic structure of which all matter is made. It is made up of three particles; protons and neutrons (which make up the nucleus) and electrons (which orbit the nucleus). (The one exception to this is hydrogen which has only a proton at its nucleus. In its basic state an atom will form an element, the kind of element being dictated by the structure's atomic number. For example, a hydrogen atom will normally have 1 of each particle and its atomic number is 1. A helium atom, which has the atomic number 2, will have two of each particle, and so on. Atoms may be broken up (fission) or fused with another (fusion). Any combination of atoms will form a molecule.

Aurora. 'Polar lights' which occur in the Earth's upper atmosphere, caused by particles emitted by the Sun causing gas molecules in Earth's atmosphere to glow. Aurora Borealis are seen above the North Pole, Aurora Australis above the South.

Azimuth (Azi). The measurement of an object's angle, in degrees, along the horizon. Usually measured from North (0°) Part of the horizontal system of co-ordinates. For example due south would be 180° azimuth.

Bailly's Beads. Bright points of light seen along the edge of the Moon just before and just after a total eclipse of the Sun. They are caused by sunlight shining though valleys (or between peaks) at the Moon's limb.

Barycentre. The centre of gravity of the Earth-Moon system. The Earth is 81 times the mass of the Moon and the barycentre of the Earth-Moon system actually lies inside the Earth.

Big Bang. The hypothetical event that is thought to have created the universe. Estimates vary but many accounts of this event put it as occurring about 15 billion years ago.

Billion. 1 000 000 000 - one thousand million.

Binary Star. A system of two stars that are genuinely associated with each other and are moving around their common centre of gravity.

Black Hole. A volume of space in which gravity is so intense that nothing can escape, not even light.

BL Lacertæ objects. Objects which are strong emitters of infra-redradiation. They are very luminous and remote and are thought to have similar properties to quasars.

Blue Moon. The second Full Moon of any month. A cycle of Lunar Phases (a Lunation)is completed in approximately 29.5 days so any month except February can have a Blue Moon. Typically, a Blue Moon occurs roughly every two and a half years, hence the expression "Once in a Blue Moon".

Bode's law. A mathematical relationship of uncertain significance that links the distances of the planets from the Sun. Strictly speaking it should be called Titius' Law since it was discovered by J. D. Titius several years before J. E. Bode popularised it in 1772. Some refer to it as the Titius-Bode Law.

Bolide. A brilliant exploding meteor.

Bolometer. An instrument used to measure heat radiation.

CCD. Charge Coupled Device. An electronic device used in place of conventional photographic film. They consist of a silicon chip which is sensitive to light, the chip being divided up into picture elements (pixels). Light falling onto the chip builds up an electrical signal which is passed on to a processing computer in the form of digital data for later storage and processing. In some cases the signal from the chip is passed direct to a monitor for immediate display. CCD's are very sensitive to light, much more so than photographic film, therefore exposures can be much shorter. The downside is that they are smaller in area and often have less resolution than normal film.

Captured rotation. Rotation of an object that spins at the same rate as that object takes to orbit another object. Sometimes referred to as synchronous rotation. The Moon is a good example of an object that has captured rotation.

Carbon stars. Red stars of spectral type 'R' and 'N' which have carbon-rich atmospheres.

Cassegrain reflector. A type of reflecting telescope where the secondary mirror is convex and the reflected light is then passed through a hole in the main or primary mirror. The advantage of this design is that it may be made more compact than a Newtonian reflector of equal focal ratio.

Celestial sphere. An imaginary sphere that surrounds the Earth, the Earth being at the centre of that sphere.

Central meridian. The imaginary north-south line that bisects a planet. It is used as a reference for estimating the longitude of planetary features as it rotates.

Central meridian transit. The passage of a particular feature on a planet across the planet's central meridian.

Cepheid. A short period variable star with very regular variations in magnitude. The name comes from the prototype star Delta Cephei. Cepheids are important astronomically because there is a clear link between their luminosity and their period of variation. Cepheids are also known to be highly luminous, giant stars than can be seen from great distances - distances can then be calculated by sheer observation alone.

Chromatic aberration. A defect present in all single lenses because different wavelengths of light come to a focus at different distances from the lens. This defect will show up as a false colour fringe around a star for example. The defect is corrected by using multiple lens arrangements using different types of glass.

Chromosphere. The part of the Sun's atmosphere that lies above the photosphere of the Sun.

Circumpolar star. A star that never sets as seen from a particular location on Earth. For example, the stars of the Great Bear (Ursa Major) never set as seen from England.

Collimation. The act of aligning the optical components of an instrument such as a telescope so that the instrument performs correctly.

Coma (cometary). The cloud of gas and dust that makes up the head of a comet, the nucleus is at the centre.

Coma (optical). An optical defect where objects close to the edge of a field of view appear to be flared or fanned out.

Conjunction. An instant when two celestial objects appear to lie very close together or in a line as seen from a particular viewpoint.

Conjunction, Inferior. A conjunction when one of the inferior planets (Mercury or Venus) appears to lie very close to the Sun, or in line with the Sun, as seen from Earth, but with the planet between Earth and the Sun. Obviously, a superior planet (Mars, Jupiter, Saturn, Uranus, Neptune or Pluto) cannot be at inferior conjunction!

Conjunction, Superior. A conjunction when one of the inferior planets (Mercury or Venus) appears to lie very close to the Sun, or in line with the Sun, as seen from Earth, but with the planet on the far side of the Sun.

Constellation. An area of the celestial sphere bounded by internationally decreed lines of Right Ascension and Declination. The whole celestial sphere is divided up into a total of 88 areas of varying size, each with its own name. The smallest is Crux (The Southern Cross) and the largest is Hydra (The Watersnake). The most common concept of a constellation is generally the pattern of stars (usually the brightest ones) that make up a familiar figure or pattern. But, strictly speaking, a constellation actually refers to an area of sky rather than the pattern. Without exception, a constellation's area exceeds the area of the pattern that gave that area its name.

Corona. The tenuous, outermost part of the Sun's atmosphere. It is visible to the naked-eye, but only at the time of a total solar eclipse or with the use of specialised instruments.

Coronagraph. An instrument that allows study of the Sun's inner corona at times of non-eclipse.

Cosmic rays. High velocity particles reaching Earth from outer space. The heavier cosmic ray particles are broken up when they enter Earth's upper atmosphere.

Cosmogony. The study of the origin and evolution of the universe.

Cosmology. The study of the universe considered as a whole.

Counterglow. The English name for the skyglow opposite the Sun, very difficult to observe, caused by very thinly spread interplanetary material. Usually known by its German name, the Gegenschein.

Culmination. The maximum altitude that a celestial object attains above the horizon.

Dawes limit. The maximum practical limit for the resolving power of a telescope. It is 4.56 / d, where d is the aperture in the instrument in inches. For example, the Dawes limit of a of a four inch telescope is 1.1 seconds of arc. A twelve inch instrument would have a Dawes limit of 0.4 seconds of arc. In other words, the Dawes limit is the theoretical point at which the telescope is able to separate two objects that distance apart. In practice this is seldom achieved.

Day, sidereal. The interval between two successive meridian passages (or culminations) of the same star. A sidereal day is equal to 23 hours, 56 minutes and 4.091 seconds. This is the true rotational period of the Earth.

Day, solar. The mean interval between successive meridian passages of the Sun. It is equal to 24 hours, 3 minutes and 56.55 seconds. The solar day is slightly longer than the sidereal day because the Sun seems to moves eastward against the stars, on average at roughly one degree per day.

Declination. The angular distance of a celestial object north or south of the celestial equator. It corresponds to latitude on Earth.

Deep-Sky. The part of space that lies beyond the Solar System. Star clusters, galaxies and nebulae are frequently termed as 'Deep-sky objects'.

Degree (Deg). Symbol °, 1/360th of a circle.

Density. The mass of an object per unit of its volume. Density is usually expressed as a numerical ratio to that of water which has a density of 1. An object denser than water would have a figure of greater than 1, and vice-versa.

Dichotomy. The instant when an object is exactly half lit as seen from Earth. It is usually applied to the Moon or an inferior planet.

Differential rotation. The rotation of a body such as a gaseous planet or the Sun so that different parts are rotating at different speeds. For example, a star or planet which rotates faster at its equator than it does at its poles.

Direct motion. Movement of revolution or rotation of a celestial object in the same sense as that of the Earth. For example, the Sun moves across the sky each day from east to west, an effect of the Earth's rotation on its axis. But, against the background of stars the Sun actually moves from west to east over the course of a year, the effect of Earth's orbit around the Sun. This west to east movement is direct motion. The Moon behave in exactly the same way. The planets also exhibit this west to east motion most of the time but owing to our perspective of the planets they occasionally appear to move in the reverse sense, known as retrograde motion.

Disk. The appearance or face of a planet, star or moon as seen from Earth.

Diurnal. Another name for daily.

Doppler effect. The change in frequency (or wavelength) of light (or other radiation) caused by the motion of an object or the observer. An object receding from an observer would exhibit a frequency shift toward a lower frequency (red shift) and vice-versa.

Double star. A star made up of two components. They are either genuinely associated (binary stars) or they appear close by chance (optical pair or binary).

Early-type star. Any hot star of spectral class O, B or A. Sometimes referred to as 'early spectral-type.

Earthshine. The faint luminosity seen of the night side of the Moon, especially when the Moon appears at a crescent phase. It is caused by light reflected from the Earth onto the Moon.

Eccentricity. The measure of how non-circular an object's orbit is. 0 = a perfect circle; any figure between 0 and 1 = an ellipse; 1 = a parabola; any figure greater than 1 = a hyperbola. Eccentricity may sometimes be expressed as a percentage. Eccentricity may be calculated by dividing the distance between the two foci of the ellipse and the length of the major axis of the ellipse.

Eclipse, lunar. The passage of the Moon through Earth's shadow. Lunar eclipses may be either total or partial. Totality may last up to one and three quarter hours although the period of totality is on average shorter.

Eclipse, solar. The passage of the Moon in front of the Sun so that the Moon is directly in front of the Sun. Totality may last for a little over 7 minutes under favourable conditions. Partial eclipses occur when the Sun is incompletely covered. Annular eclipses occur when the Moon it near the farthest part of its orbit and hence appears smaller. In this case a bright ring of sunlight is seen around the dark body of the Moon. Technically speaking, a solar eclipse could be termed as an occultation of the Sun by the Moon.

Eclipsing binary (or eclipsing variable). A binary star system where one star passes in front of the other causing a variation in the overall brightness of the star system.

Ecliptic. The apparent yearly path of the Sun among the stars.

Electron. An atomic particle that carries a negative charge. It orbits the nucleus of an atom.

Element, optical. Any single part of an optical train such as a mirror or lens. In the case of lenses or eyepieces they may comprise several pieces of glass, each one known as an element. The combination of elements is used to correct the faults that would often be present in a single element lens or eyepiece.

Elongation. The angular distance between the Sun and a planet, or between a planet and a satellite, as seen from Earth.

Emersion. A term used to describe when an object re-emerges after an occultation or eclipse.

Emission lines. Specific wavelengths of light that are brighter than adjoining wavelengths seen in spectra.

Ephemeris. A table or list of the predicted position of an object such as a planet.

Epoch. An instant in time for which the positions of celestial objects are given.

Equator, celestial. The projection of the Earth's equator onto the celestial sphere.

Equatorial mounting. A type of mounting for a telescope (or other instrument) which is set up so that one axis of motion is parallel to the Earth's axis. This arrangement means that only one axis is required to be driven to keep an object in the field of view.

Equinox. The equinoxes are the two points at which the ecliptic intersects with the celestial equator. The Vernal equinox (or First Point of Aries) is where the ecliptic crosses the celestial equator from south to north, the Sun reaches this point around the 21st March. The opposite equinox, the autumnal equinox, is where the ecliptic crosses the celestial equator from north to south. The Sun reaches this point around the 22nd September. The term equinox stems from Latin, equi = equal, nox = night. At the equinoxes the day and night are of equal duration.

Escape velocity. The minimum speed that an object must attain to escape from the surface of the planet or other body without being given any extra impetus. The escape velocity of Earth is 11.2 kilometres per second.

Exosphere. The outermost part of the Earth's atmosphere. It is an ill-defined, highly rareified zone which starts at a height of about 700km (435 miles) and extends into the vacuum of space.

Extinction. The apparent reduction in brightness of a celestial object when it is low in the sky and much of its light is absorbed by Earth's atmosphere.

Eyepiece (or occular) . The lens, or lens combination, at the eye end of a telescope. It is responsible for the magnification of the object under scrutiny. There are a variety of eyepiece types with different characteristics. Eyepieces will be stated as having a certain focal length and it is this figure, usually in millimetres, which when divided into the focal length of the telescope, dictates the magnifying factor that a given eyepiece will yield.

Faculae. Bright patches seen of the photosphere during solar observation.

Fireball. A very brilliant meteor.

Flares, solar. Brilliant eruptions in the outer part of the Sun's atmosphere. They can usually only be seen with the aid of special filters and instruments. They are often associated with areas of activity on the Sun such as sunspots. Occasionally, these eruptions may cause charged particles to reach the Earth which may in turn cause auroral displays.

Flocculi. Patches on the Sun's surface visible with spectroscopic equipment.

Focal length. The distance between a lens or mirror and the point where it brings light to a focus.

Focal ratio. The focal length of a telescope divided by its aperture (opening) or primary mirror diameter.

Fraunhofer lines. Dark absorption lines seen in the spectrum of the Sun.

Frequency. The number of waves passing a point in a given time, usually 1 second. Frequency is measured in hertz and is equal to the speed of the wave divided by their wavelength. Longer wavelengths have lower frequency and vice-versa.

Galactic Cluster. Another name for an open star cluster. They are often termed Galactic Clusters because they are found mainly in the plane of our galaxy (The Milky Way). If you were to view our galaxy from afar, you would find that all the open/galactic clusters lie within the spiral arms of the galaxy. See also Open Cluster.

Galaxy. A system made up of stars, nebulae and interstellar matter. Many galaxies, but not all, are spiral in form.

Galaxy, The. The star system of which the Sun is a member. The Galaxy is also known as the Milky Way galaxy. It is spiral in shape, contains about 100 000 million stars and is approximately 100 000 light years across. The nucleus of the Galaxy is roughly 30 000 light years thick tapering away to about 10 000 light years thickness in the spiral arms. The whole system is slowly spinning taking about 200 million years to rotate once. The Galaxy is the second largest member of the Local Group of galaxies, a loose cluster of galaxies roughly dumbbell in shape and about 4 million light years long.

Gamma-rays. Electromagnetic radiation of extremely short wavelength, shorter than X-rays.

Gauss. A unit of measurement of a magnetic field. Earth has a surface magnetic field strength of between 0.3 and 0.6 Gauss.

Gegenschein. A very faint skyglow opposite the Sun, very difficult to observe, caused by very thinly spread interplanetary material. Often known by its English name, Counterglow.

Geocentric. Meaning: As seen from the centre of Earth. A system of co-ordinates.

Geodesy. The study of the shape, mass, size and other features of the Earth.

Giant stars. Stars that are swelling in size as they approach the end of their lives. Giant stars are often no more massive than the Sun but they have expanded to great size and are therefore less dense but highly luminous.

Gibbous. The phase of the Moon or a planet when it is between half and fully illuminated.

Gravity. The force of attraction between two or more masses. This force is dependant on both the masses themselves as well as the distance between them. Being a force, gravity is strictly speaking measured in Newton's but is commonly measured in Kilogrammes.

Great circle. A circle on the surface of a sphere whose plane passes through the centre of that sphere.

Halo, galactic. A roughly spherical shaped region around the main part of the Galaxy.

Heliocentric. Meaning: As seen from the centre of the Sun. A system of co-ordinates.

Hertzsprung-Russell diagram. A diagram in which stars are plotted according to their spectral type and their absolute magnitude.

Horizon. The great circle on the celestial sphere which is everywhere 90 degrees from the observers zenith, the point directly overhead the observer.

Hours, Minutes, Seconds (h, m, s). Measure of time or right ascension.

Hubble's constant. The rate of increase of the recession of a galaxy with increased distance from the Earth. This figure varies depending on which observational data one uses but is often around 50 miles per second per Megaparsec.

Immersion. The entry of an object into shadow during an eclipse, or the covering of an object during an occultation.

Inclination. The angle at which an orbit is tilted with respect of a plane of reference, symbol ' i '.

Inertia. The ability of an object to resist acceleration or deceleration. Inertia can be broadly equated with mass.

Inferior planets. The planets (Mercury and Venus) that lie closer to the Sun than Earth.

Infra-red radiation. Electromagnetic radiation with a wavelength longer than that of visible light but shorter than radio waves.

Ion. An atom or molecule that has; lost one or more of its electrons (a positive ion) or gained one or more electron (a negative ion).

Ionisation. The process where electrons are added to or stripped from an atom or molecule turning it into an ion.

Ionosphere. The region of the Earth's atmosphere that lies above the stratosphere and below the exosphere. The ionosphere is an ionised (charged) layer of the atmosphere (hence the name) that lies at a height of 60 to 1000km (38 to 620 miles) above the Earth. The layer is thought to be a product of the absorbtion of the Sun's ultraviolet radiation. The ionosphere lies below the exosphere and above the stratosphere.

Jovian. Pertaining to the planet Jupiter.

Julian day. A system of counting days from noon 1st January 4713 BC. The name has nothing to do with Julius Caesar but was invented by the mathematician Scaliger who named it in honour of his father, Julius Scaliger. So, 30th August 2000 is Julian Day J2 451 786.0. Figures may be added after the decimal point and they will represent the decimal fraction of the day. So, 6pm, 30th August 2000 will be J2 451 786.25.

Kelvin (degrees). A measurement of temperature, symbol 'K'. Kelvin is measured in degrees from absolute zero. So, 0 degrees Kelvin equals minus -273.16 degrees Centigrade.

Kepler's laws of planetary motion. Set of laws laid down between 1609 and 1618 by Johannes Kepler. These laws are: (1). The planets move in elliptical orbits with the Sun at one focus of the orbit. (2). The line joining the centre of the Sun and the centre of the planet sweeps out an equal area in equal times. (3). In the case of a planet, the square of the sidereal period is proportional to the cube of the mean distance from the Sun.

Kiloparsec. One thousand parsecs, equal to 3260 light years.

Kirkwood gaps. Regions in the asteroid belt where very few asteroid are found. The gaps are caused by Jupiter's gravitational influence which shifts asteroids out of orbit if their orbital period is a precise fraction of Jupiter's orbit.

Lagrangian points. Five places where small bodies can exist in stable orbits in the plane of two larger bodies. Three of these points lie in a line joining the two large bodies; one point between the two larger bodies (L1); the other two points either side of them (L2 & L3). The remaining points lie 60° ahead of and behind one of the larger bodies in its orbit around the other (L4 & L5).

Latitude, celestial. The angular distance of a celestial object from the nearest point on the ecliptic.

Libration. The apparent tilting of the Moon as seen from Earth. The result is that over a period of time it is possible to see 59% of the surface of the Moon from Earth, though of course, only 50% at any one time.

Light year (l.y.). The unit of distance in which light travels in one year - 9 464 566 100 km (5 878 612 500 miles).

Light, speed of. 299 792.5 km (186 291 miles) per second in a vacuum, the fastest speed in the Universe.

Limb. The apparent edge of a body such as the Moon as seen from Earth.

Local group. A term used to describe the local cluster of galaxies of which the Milky Way galaxy is part. The largest member of the Local Group is the Andromeda Galaxy, Messier 31.

Lunation. One complete cycle of phases by the Moon, 29.53 days. A lunation is also known as a Synodic month.

M. The prefix used for the 109 objects that were catalogued by Charles Messier.

Magnetosphere. The region of the magnetic field of a planet or other solar system body. Only Mercury, Earth, Jupiter, Saturn, Uranus and Neptune are known to have a magnetosphere.

Magnitude (Brightness). The brightness of a celestial object. The lower the magnitude, the less bright the object and vice-versa. For mainly historical reasons magnitude has the peculiar attribute of having brighter objects at negative values and vice-versa. Each whole number of magnitude is equal to a factor of 2.5:1. So, an object of magnitude +1 is 2.5 times brighter than an object of magnitude +2 but is 2.5 times fainter than an object of magnitude 0. Five whole units of magnitude are equal to a factor of approximately 100 times. So, a magnitude +6 object is 100 times fainter than an object of magnitude +1. (See also Absolute magnitude and Apparent magnitude.)

Main Sequence. A band within the Hertzsprung-Russell diagram that contains the majority of normal stars except for giant stars and white dwarfs.

Maksutov telescope. A type of telescope that uses mirrors and lenses. Maksutov's are in fact Cassegrain telescopes: In the case of a Maksutov the light enters the front of the telescope via a curved corrector plate or lens (curved towards the interior of the telescope) which directs the light onto the concave primary mirror (which has a hole at its centre) which lies at the rear of the telescope. The primary mirror then reflects the light back toward the corrector plate which has a small convex secondary mirror coated onto the centre of the corrector plate. This secondary mirror then directs the light through the hole in the centre of the primary mirror and on into the eyepiece which is attached to the rear of the telescope. Like all Cassegrain telescopes the Maksutov has the advantage to being able to have a longer focal ratio than a Newtonian reflector of equal optical size but at the expence of a more complicated optical train.

Mass. A measure of a body's inertia (resistance to acceleration), the amount of matter that a body contains. Strictly speaking, mass is not the same as weight or gravity, although on Earth they are often regarded as the same thing. Mass is measured in Kilogrammes. Apart from speeds approaching that of light, the mass of a body remains constant whereas weight or gravity is dependant on the masses of two or more bodies and the distance between them.

Mean Sun. An imaginary Sun travelling at a speed equal to the average rate that the real Sun travels along the ecliptic.

Mean. The average of a series of values.

Megaparsec. One million parsecs, a distance equal to 3 260 000 light years.

Meridian. The imaginary line that passes from north to south horizons via the zenith.

Meteor. A streak of light, lasting a few seconds at most, produced when a meteoroid enters the Earth's atmosphere. Most meteoroids are barely the size of a grain of sand and are thought to be cometary debris. In most cases the object is destroyed by friction in Earth's ionosphere at a height of about 100 to 160 km (60 to 100 miles).

Meteorite. Any meteoroid which strikes the surface of the Earth or another body. They are generally larger objects closely associated with asteroid type debris.

Meteoroid. Any small solid object in space. Most meteoroids are barely bigger than a grain of sand.

Metonic cycle. The period of 6 939.6 days, or 19 calendar years, after which the Moon's phases recur on the same day of the year. This period is also equal to 253 lunations.

Micrometeorite. A very small particle of interplanetary debris, too small to cause the luminous flash associated with meteors.

Micron. One thousandth of a millimetre. The symbol used is µ (mu).

Milky Way, The. (1) Another name for The Galaxy. (2) The broad band of faint light that stretches across the sky (when the time is favourable) as seen from a dark site. It is the result of countless millions of stars of The Galaxy seen edge on from the viewpoint of Earth. The Milky Way is brightest in the Northern hemisphere in the constellation of Cygnus, and in the Southern hemisphere in the constellation of Crux.

Million. 1 000 000 - one thousand, thousand.

Mock Sun. An effect caused by ice crystals in Earth's atmosphere which refract sunlight and cause the appearance of two diffuse patches of light 22° either side of the Sun. These patches are termed Parhelia or Sundogs and often appear on the rim of a halo surrounding the Sun. The patches often have coloured fringes, red on one side, blue on the other.

Month, Anomalistic. The interval between two successive perigee passages of the Moon, equal to 27.55 days.

Month, Nodical or Draconic. The interval between successive passages of the Moon through one of its nodes, equal to 27.21 days.

Month, Sidereal. The revolution period of the Moon relative to the stars, equal to 27.32 days.

Month, Synodic. The interval between two successive New Moon's (a lunation), equal to 29.53 days.

Month, Tropical. The time taken for the Moon to return to the same celestial longitude (7 seconds shorter than the sidereal month).

Moon, Blue. The second Full Moon that occurs during a calendar month. Since the Moon takes 29.53 days to complete a cycle of phases a Blue Moon cannot occur in February. The last Blue Moon was on 30th November 2001, there are none during 2002 and 2003. The next Blue Moon occurs on 31st July 2004. Blue Moon's occur roughly every two and a half years so are not very common. Hence the term "Once in a Blue Moon".

Moon, Harvest. The Full Moon that occurs nearest the time of the autumnal equinox. So-called because this will occur around the time of harvest.

Moon, Hunter's. The Full Moon that occurs after Harvest Moon. So-called bacause this will occur around the start of the hunting season.

Nadir. The point on the celestial sphere directly below the observer. The nadir is directly opposite the zenith.

Nebula, Nebulae. Any cloud of gas and dust in space. The three nebulae types listed below may appear on their own but are frequently seen in combination.

Nebula, Dark. A dense cloud of gas and dust in space that appears as a dark silhouette against a brighter background. In many cases dark nebulae are part of a larger nebulae complex which provides the light background against which the dark nebulae appears. At infra-red wavelengths young stars not visible at visual wavelengths are often found to be forming. Dark nebulae sometimes appears as a dark feature against a starry background but without any other associated nebulae.

Nebula, Emission. A gas cloud which is close to a star or stars. The radiation from the stars excites the gas so that it emits light of its own. Most of this gas is hydrogen which emits the red/orange light which is typical of this type of nebulae.

Nebula, Reflection. A cloud of gas and dust which lies close to a star or stars which reflects light from those stars. This type of nebulae is often part of a larger nebulae complex (although it is sometimes seen without the other nebulae types above) and the reflection is typically blue in colour due to the dust within the nebulae.

Neutrino. A particle that has no charge, and little or no mass.

Neutron star. The remnant of a very massive star that has undergone a supernova explosion. Neutron stars send out rapidly changing radio emissions and are frequently termed as pulsars.

Neutron. An atomic particle with no charge and a mass almost equal to a proton.

Newtonian reflector. A reflecting telescope in which the light is collected by a concave primary mirror at the rear of the telescope and then reflected back to a flat secondary mirror angled at 45 degrees mounted near the front of the telescope. The light is then reflected through the side of the telescope tube and on into an eyepiece. It is the simplest type of reflecting telescope.

NGC. New General Catalogue. A list of some 7000 plus deep-sky objects.

Nodes. The points at which the orbit of the Moon, a planet or a comet intersect with the plane of the ecliptic; south to north (Ascending Node), north to south (Descending Node).

Nova. A star that suddenly flares up to several times its normal brightness. Typically they remain bright for a short period before fading back into obscurity.

Nucleus. The central part of an atom containing two particles; a proton and (with the exception of hydrogen) and a neutron.

Nutation. A slow 'nodding' of the Earth's axis due to the gravitational tug of the Moon on the Earth's protuberant equator.

Oblateness. The measure of how much a rotating object deviates from being a perfect sphere. A perfect sphere would have an oblateness of 0.0 (0%). The Earth for example is oblate to a factor of 0.0034 (0.34%), while Saturn, the most oblate of the planets has a figure of 0.108 (10.8%). Generally, the planets have an equatorial bulge and in the case of Saturn, its equatorial diameter is some 10.8% greater than its polar diameter.

Obliquity of the ecliptic. The angle between the ecliptic and the celestial equator: 23 degrees, 26 minutes and 45 seconds. This angle is also an expression of the tilt of Earth's axis.

Occultation. The covering-up of one celestial object by another. For example, a total solar eclipse is an occultation of the Sun by the Moon.

Open Cluster. A loose association of stars with no clearly defined shape. Often referred to as 'Galactic Clusters' since they are all found close to the galactic plane (of our galaxy). There are a little over 1000 of these clusters and they may contain just a dozen stars or as many as 500. The clusters occupy a region of space of only a few tens of light years across and are formed in the same region of space from a nebula, only in time do the stars disperse. In some instances traces of the original nebula can still be seen.

Opposition. The position of a planet when it is opposite the Sun in the sky. At this point the Sun, Earth and the planet are roughly in line and the planet will rise at about the same time that the Sun sets: the planet is thus visible virtually all night. A good example is when the Moon is Full, it could be said to be in opposition.

Orbit. The path of a celestial body around its parent body.

Orrery. A model showing the Sun and planets. The model is usually capable of being moved mechanically so that the planets move at the correct speed around the Sun relative to each other. Some versions of an Orrery have just the Sun, Earth and Moon.

Parallax, trigonometric. The apparent shift of an object when viewed from two different directions. This shift may be used to gauge the distance of an object out to distances of about 1 000 light years or more. The object is observed six months apart and the amount it shifts relative to the background stars is measured in arcseconds. No star subtends an angle of one arcsecond, so the angles measured this way are extremely small. However, if the angle is measurable it is possible, by using trigonometry, to measure the distance to a star.

Parsec (pc). Parallax second. The distance at which one astronomical unit subtends an angle of one arcsecond. One parsec is equal to 3.26 light years, or 206 265 astronomical units, or 30.86 million million kilometres.

Penumbra. (1). The lighter part of a sunspot. (2). The area of partial shadow either side of the main umbra cone of shadow cast by the Earth.

Perigee. The position of the Moon in its orbit when it is closest to the Earth.

Perihelion. The position of the Earth (or other planet) in its orbit when it is closest to the Sun.

Period. The interval between successive occurrences of a cyclical event.

Perturbations. The disturbances in the orbit of a celestial object produced by the gravitational effects of other bodies.

Phases. The apparent change in shape of the Moon and inferior planets from Full to New. Mars sometimes displays a gibbous phase but no other planets show a significant phase as seen from Earth.

Photometer. A device that measures the intensity of light from a particular source.

Photon. The smallest unit of light.

Photosphere. The bright visible surface of the Sun.

Planetary nebula. A shell of gas that surrounds a small, dense, hot star. The name is misleading because planetary nebulae have no connection with planets. They probably got their name because some of these nebulae appear disk-like and may have been initially mistaken for planets.

Planet. Any large body orbiting a star. A somewhat arbitrary term since there appears to be no defining size that clearly differentiates between a planet or asteriod. For example, Pluto is widely regarded as the ninth planet of the Solar System yet its parameters do not clearly conform to the other eight planets of the Solar System. Although it seems unlikely to lose its planetary status, Pluto does appear to be an oddity when you consider such things as its orbit, size and other known data.

Planetoid. Another name for an asteroid or minor planet.

Planisphere. A circular map of the stars covered by a rotating mask that has a window or opening in it. The map and mask are the same size and have time and date markings on the outside. Lining up the time/date marks then expose a given area of the map through the mask so that the stars visible for a given time can then be seen. Planispheres are usually devised so that they can only be used within a certain latitude range.

Poles, celestial. The north and south points of the celestial sphere.

Populations, stellar. There are essentially two types of star regions. I - in which the brightest stars are hot and white; and II - in which the brightest stars are old Red Giants. This is an odd distinction because the first stars to have populated a given region would now be classified as Population II! In our own galaxy Population II stars are more numerous toward the centre of the galaxy while Population I stars are more numerous in the outer regions of the galaxy and spiral arms.

Precession. This is the slow movement of the celestial poles tracing out large circles on the celestial sphere. It is caused by a slow wobble in the Earth's axis due to the gravitational effects of the Sun and Moon on the Earth's equatorial bulge. A reasonable analogy is that of a spinning top - As the top slows down it will start to wobble. This 'Earth wobble' is very slow, one 'wobble' takes about 26 000 years to complete. The other effect is that the equinoxes are slowly moving or precessing, at the rate of about 50 arcseconds a year. The Vernal Equinox is also called the First Point of Aries but due to the effects of precession the Vernal Equinox is now in the constellation of Pisces and moving towards Aquarius.

Primary. A term used to describe the larger or brighter member of a pair of celestial objects.

Prime Meridian. The meridian on Earth's surface which passes through Greenwich Observatory. It is taken to be the dividing line between the east and west hemispheres, 0 degrees longitude.

Prominences. A mass of glowing gas, mainly hydrogen, that rises from the surface of the Sun.

Proper motion. The movement of an individual star on the celestial sphere.

Proton. An atomic particle, part of the nucleus, that has a positive electrical charge.

Pulsar. A neutron star that is rapidly rotating and emitting radio waves. Pulsars are believed to vector the radio emissions so that if Earth lies in the line of sight they appear like a lighthouse (when seen by a radio telescope, that is).

Quadrature. The position of the Moon or another planet when it is at right angles to the Sun as seen from Earth. An example would be when the Moon is seen at First or Last Quarter phases.

Quasar. A very remote, highly luminous body. They are now known to be the cores of very energetic galaxies although precisely what makes them so luminous is uncertain.

Radial velocity. The movement of a celestial body either away from (a positive value) or toward (a negative value) the observer.

Radiant. The point in the sky where meteors of a given shower seem to originate or radiate from.

Radio waves. Electromagnetic radiation with a wavelength longer than infrared.

Red giant. A star that is large and cool compared with the Sun but is many times the diameter of the Sun. These are stars that are nearing the end of their lives.

Red dwarf. A star that is smaller and cooler than the Sun. Most red dwarfs are about one-tenth the mass and diameter of the Sun.

Regression of the nodes. This is a slow movement of the lunar nodes caused by the gravitational pull of the Sun. The lunar nodes move slowly westward, taking 18.6 years to complete one revolution.

Retardation. This is the time difference between Moon rise one day, and Moon rise the following day.

Retrograde motion. Orbital or rotational movement in the opposite sense to that of Earth's motion.

Reversing layer. The layer of the Sun's atmosphere above the photosphere.

Right ascension (R.A.). The angular distance of a celestial body from the Vernal Equinox, measured eastwards. This is one of the equatorial system of co-ordinates and it is measured in hours, minutes and seconds. Right ascension is also the difference in time between the culmination of the Vernal Equinox and the culmination of that object.

Roche limit. The distance from the centre of a planet that another body would have to be within to be broken up by the gravitational pull of the planet. This would only be true for an object that is not gravitationally cohesive.

Saros. The period after which the Sun, Earth and Moon return to approximately the same positions, this period being 18 years and 11.3 days. This period is very useful for eclipse prediction since one eclipse will be repeated by an almost identical event precisely one Saros later.

Schwarzchild radius. The radius that a body must have if its escape velocity is to be equal to the velocity of light. This term is frequently encountered when discussing the properties of black holes.

Scintillation. Another term used to describe the twinkling of a star. It is caused by the Earth's atmosphere distorting the light from a star, mainly when the star is at a low altitude. Planets may also scintillate when close to the horizon but much less so than stars.

Secondary. The smaller or dimmer component of a pair of celestial objects that orbits the primary object.

Selenography. The study of the Moon's surface.

Seyfert galaxies. A kind of so-called 'active' galaxy. Seyfert galaxies have fairly small but bright nuclei and weak spiral arms. These galaxies are also strong emitters of radio wave energy.

Sidereal period. The revolution period of a planet round the Sun. Also the revolution period of a moon around a planet.

Sidereal time. The local time measured according to the apparent rotation of the celestial sphere. When the Vernal Equinox crosses the observer's meridian the sidereal time is 0 hours.

Sidereal. With respect of the stars.

Solar wind. The flow of particles from the Sun in every direction. The 'wind' is an ever present feature of the Sun but the intensity of the wind is dependant on Solar activity.

Solstices. The time when the Sun is at its maximum declination either north or south of the celestial equator. There are two solstices; summer solstice (around 22nd June each year) when the Sun is 23 and a half degrees north of the celestial equator. At this point the Earth's northern hemisphere will have its longest day and the Sun's noon altitude will be at its highest; and winter solstice (around 22nd December each year) when the Sun is 23 and a half degrees south of the celestial equator. At this point the Earth's southern hemisphere will have its longest day and the Sun's noon altitude will be at its highest.

Specific gravity. The density of a substance taking water to equal 1. For example, Saturn has a specify gravity, or density, of 0.71. Therefore an equal volume of water would weigh more than the planet - if you had a big enough bowl of water, Saturn would float in it! On the other hand, Earth has a specific gravity of 5.5, thus Earth would weigh 5.5 times as much as an equal volume of water.

Spectroscope. A device that splits the light coming from an object. This enables the study of the properties of that light source as well as the detection of elements and molecules present.

Spectroscopic binary. A binary star system where the two (or more) components are too close to be separated visually. Observation by spectroscopic means however can reveal the presence of other components of the binary system that normal optical means cannot.

Spherical abberation. An optical defect caused when the inner and outer parts of a lens or mirror have different focal lengths and results in blurred images.

Stratosphere. The layer of the Earth's atmosphere that lies 10km to 40km (6 to 25 miles) above the surface of the Earth. Within the layer the temperature is almost constant at -55°C (-67°F). The stratosphere lies above the troposphere, the lowest layer of Earth's atmosphere.

Superior planets. The planets that lie at a greater distance from the Sun than that of Earth.

Supernova. The outburst of a star on a tremendous scale. There are broadly 2 types of supernova. (1). Where a white dwarf member of a binary system explodes, or matter falls onto the white dwarf and explodes. (2). The collapse and then chaotic explosion of a single massive star.

Synodic period. The interval between successive oppositions of a superior planet.

Synodic. With respect of the Earth.

Syzygy. The position of the Moon in its orbit when it is either Full or New.

Terminator. The dividing line between light and dark on a moon or planet.

Topocentric. Meaning: As seen from the surface of the Earth. Most celestial co-ordinates used are topocentric.

Transit. (1). The passage of a body across the observer's meridian. (2). The passage of Mercury or Venus across the face of the Sun.

Troposphere. The lowest layer of the Earth's atmosphere, lying at an average height of up to 11km (6.5 miles). Above the troposphere lies the stratosphere, above that the ionosphere, above that the exosphere. In the troposphere the temperature drops steadily except for localised layers of temperture inversion.

Twilight. By astronomical definition, the state of the sky when the Sun is below the horizon but by no more than 18 degrees.

Ultraviolet. Electromagnetic radiation with a wavelength shorter than visible light but longer than X-rays.

Umbra. (1). The darkest part of the shadow cast by Earth into space. (2). The darker portion of a sunspot.

Universal Time (UT, UTC). Co-ordinated Universal Time. The time standard by which Greenwich Mean Time became known as for scientific purposes in 1928. UTC is the time given by broadcast time since 1972. The time-scale is widely known as Greenwich Mean Time but astronomically speaking the term GMT is no longer used.

Variable stars. A star that varies in brightness over a period of time. There are many types of variable stars, some vary over hundreds of days while other display minute variation over a matter of minutes. Another demarkation of variable stars is extrinsic, and intrinsic. Extrinsic variable are not true variables, their fluctuations are caused by eclipse events as in an eclipsing variable. Intrinsic variable are true variables. Their fluctuations are due to physical processes taking place in the stars themselves.

Visual (or Apparent) magnitude. The apparent brightness of a celestial object. The lower the magnitude, the less bright the object. Thus, the Sun has an apparent magnitude of -27; the Moon up to -12; Venus up to -4; the brightest stars -1; the faintest stars visible to the naked-eye +6, the faintest objects yet detected about +30. For mainly historical reasons the magnitude scale has the peculiar attribute of having brighter objects at negative values and vice-versa. (See also Absolute magnitude and Magnitude.)

Volume. The amount of space occupied by a body or fluid.

Wavelength. The distance between a given point on one wave to the same point on the next wave.

Weight. The 'heaviness' of an object, the amount of attraction between two or more masses. Weight is often measured in Kilogrammes but the proper (SI) unit of weight is the Newton. Weight is not the same as mass which is a measure of how much matter or inertia an object has. Weight on the other hand is dependant on two or more masses and is a measure of the force of gravity acting on those two objects. For example, an astronaut will experience weightlessness in space because he is distant (though not entirely free) from the Earth's gravitational influence. His mass however will still be the same as it would be on Earth. He is just as massive as he was before - but he weighs little or nothing!

White dwarf. A very small, dense star that has used up its nuclear energy. Stars of this kind are at the end of their evolution.

Wolf-Rayet stars. Stars that are very hot and are surrounded by an expanding gaseous envelope. They appear greenish-white in colour and their spectra show distinctive bright emmision lines.

X-rays. Electromagnetic radiation with a wavelength shorter than ultraviolet light but longer than gamma rays.

Year, anomalistic. The period for successive perihelion passages of the Earth, a little less than 5 minutes longer than the sidereal year.

Year, calendar. The mean length of the year according to the Gregorian calendar, 365 days, 5 hours, 49 minutes and 12 seconds.

Year, sidereal. The period taken by the Earth to complete one orbit of the Sun, 365.26 days.

Year, tropical. The period taken for successive passages of the Sun across the Vernal Equinox, 365.24 days.

Zenith. The point on the celestial sphere that lies directly overhead an observer, exactly 90 degrees away from every part of the observer's horizon.

Zenith distance. The angular distance of an object from the zenith.

Zodiac. The band of constellations through which the Sun travels each year. The Zodiac is actually a band across the sky, 8 degrees either side of the ecliptic. With the exception of Pluto, all the planets and the Sun and Moon will be found within the Zodiac.

Zodiacal light. A cone of light stretching from the horizon along the ecliptic. It is only seen during good sky conditions when the Sun is a few degrees below the horizon. It is caused by fine, thinly spread interplanetary material lying close to the plane of the solar system.

ABOUT THIS DOCUMENT...
This glossary was written by Derek Haselden and is the result of many, many hours of digging through books, magazines, journals and other reading matter, and as such I cannot claim it to be an 'original' work. I have put a lot of work into this but I am more than happy for others to make use of this page, the only condition I would make is that you ask for permission first. Otherwise, please feel free to edit, extract, uproot and quote as you see fit, but please, do not distort or mangle it too much. Feel free to copy, distribute, paraphrase, et cetera, et cetera, but do me the honour of quoting the writer - me! Thanks.
Derek Haselden

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