Astronomical GLOSSARY A - Z

A

 

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.)

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 good reflector, 0.0 means no reflection at all.

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

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). 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°.

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.

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.

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 for the Solar System.

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). 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). 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. Aurora Borealis are seen above the North Pole, Aurora Australis above the South.

Azimuth (Azi) 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.

B

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 Eath-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.

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.

C

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 variable. 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). The flaring of an object when close to the edge of an optical field of view.

Conjunction. A geometrical arrangement of the Earth, Sun and another planet(s) in a line, with the planet(s) behind the Sun. The complementary arrangement is called opposition, when the planet(s) are behind Earth.
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.

D

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 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 day. 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. One of two measurements of the position of an object in the sky. Declination measures in degrees from 0 to 90 the position of a celestial object north or south of the celestial equator, just like latitude measures positions on the Earth north or south of the equator. The other coordinate is right ascension. 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 in the same sense as that of the 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 would exhibit a frequency shift toward a lower frequency 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).

E

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.

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 Sun's path in the sky. Planets and the Moon closely follow this path, since their orbital planes are nearly aligned with Earth's orbital plane. The ecliptic is tilted 23.5 degrees from the celestial equator.

Electron. An atomic particle that carries a negative charge.

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

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.

F

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.

G

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.

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

Galaxy, The. The system of which the Sun is a member. The Galaxy is also known as the Milky Way galaxy. It is spiral in shape and contains about 100 000 million stars.

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.

H

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.

I

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.

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.

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

J

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.

K

Kelvin (degrees). A measurement of temperature, symbol 'K'. Kelvin is measured is 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.

L

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 curve. Brightness or intensity of light plotted against time on a graph. Astronomers discover dark stellar companions using the light curve of the star. As a dark orbiting object eclipses the star, the brightness falls, producing a dip on the light curve. Careful analysis of the light curve reveals the masses of the star and dark companion plus the distance to this eclipsing binary system.

Light year (l.y.). The distance light travels in one year, about 5.88 trillion miles or almost 800 times the diameter of our solar system. The nearest star is a mere four light-years away, while the nearest galaxy lies 2.2 million light-years from Earth.
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.

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.

Luminosity. A measure of the rate of energy flowing from a source, like a galaxy, star, or light bulb. Luminosity tells astronomers how fast energy "leaks" from the star. Luminosity is a measure of power divided into units called watts. Given the luminosity of a star, an astronomer can calculate the distance to the star by measuring the star's brightness. Temperature and luminosity are related, but dependent on the mass and size of a star.

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

Lunar Eclipse. A spectacular, though relatively common, celestial event that occurs when the Moon, Earth and Sun form a line in space. The Moon travels through Earth's shadow.

Lunar Phases. The Moon passes through its phases every 29.5 days. The same phase will fall on the same date of the year according to the Saros cycle, every 18 years, 11 days, and eight hours.
New Moon. In conjunction with the Sun; rises and sets with it. 0 percent illuminated, 0 degrees elongation.
Crescent Moon. Prominent just after new moon at sunset; less than 50 percent illumination.
First Quarter . In opposition with the Sun; rises at noon and sets at midnight; 50 percent illuminated, 90 degrees east elongation.
Waxing Gibbous Lunar illumination increasing between first quarter and full.
Full Moon Rises at sunset and sets at sunrise; 100 percent illuminated, 180 degrees elongation.
Waning Gibbous Decreasing illumination decreasing between full and last quarter.
Last Quarter Rises at midnight and sets at noon; 50 percent illuminated, 90 degrees west elongation.

 

M

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

Magellanic clouds. Two companion galaxies of our own, called the Large Magellanic Cloud and the Small Magellanic Cloud. They are visible only from the Southern Hemisphere.

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 factors 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.)

The magnitude scale is organized logarithmically because that's the way our eyes perceive brightness, whether it's light bulbs or stars. For instance, your eye would perceive the same brightness difference between a 25- and 50-watt light bulb as it would between a 100- and 200-watt light bulb. Likewise with stars, your eye would detect the same brightness difference between a first- and a second-magnitude star as it would between a second- and third magnitude star. If you repeat this exercise down to a sixth-magnitude star, the first-magnitude star's brightness (amount of light received on Earth) is 100 times the sixth-magnitude star.

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. The total matter content of an object. Also a physical measure of inertia. Newton's law states that mass is related to force and acceleration: m = F/a. Einstein says that mass and space are somehow related, because mass warps space and space directs the motion of 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. Most meteorites are classified as stony because of their mineral composition, which may include olivine, pyroxene, serpentine, sulfates, organic compounds, iron, and nickel.

Meteoroid. A tiny grain of silicate or metal between 1 and 10 millimeters across (barely bigger than a grain of sand) that orbits the Sun. Sometimes meteoroids fall into Earth's atmosphere and become meteors.

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. Our galaxy. The Milky Way is a spiral galaxy with four major arms containing young bright stars, gas and dust. The mass of the Milky Way is estimated to be between 4 x 10^11 and 10^12 solar masses. The luminous diameter of our galaxy is some 80,000 light-years with our Sun orbiting about 25,000 light-years from the galactic center. However, the large-scale rotation of galactic matter suggests an even larger diameter extending beyond 120,000 light-years. This data indicates that galactic mass is not centrally located in the core; rather, the bulk is spread out beyond the Sun's galactic orbit. Perhaps only 10 percent of the galaxy glows as stars; the remainder is nearly invisible. (see dark matter and galaxy)

Million. 1 000 000 - one thousand, thousand.

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).

N

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

Nebula. Any cloud of gas and dust in space.

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 terms 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.

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

O

Objective lens. The primary light gathering optic of a refraction telescope, located opposite of the eyepiece.

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

Opposition. A planet's position when it is 180 degrees east or west of 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. The motion of a massive body around another body, governed by the force of gravity. Planets in our solar system follow an orbit around the Sun, as first noted by Johannes Kepler, in the shape of an ellipse. An orbit is actually composed of two motions: one directly toward the other body (planet or star...) and the another that points away. In the case of a circular orbit, these two component motions are orthogonal (90 degrees apart, or at right angles). Einstein would say that an orbit is actually a geodisc in space -- time, formed by the interaction between space and the mass of the two bodies.

Orbital period. The duration of one orbit. For instance, Earth's orbital period around the Sun is one year, and the Moon orbits Earth in 27.3 days.

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.

P

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.

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).

Q

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 quasi-stellar radio source or quasi-stellar object radiating extraordinary power from a small region in space, having the following characteristics: super galactic luminosity of 10 to 100,000 times that of the Milky Way; compact power-source dimensions, on the order of a galactic core; extreme distance from Earth; red shift indicating a velocity approaching 90 percent of the speed of light; residence inside the core of a galaxies.
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.

R

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.

The position of a celestial object in the sky east of the vernal equinox along the celestial equator. Sidereal hours, minutes and seconds divide the celestial equator from 0 to 24 sidereal hours of right ascension. Keep in mind that the celestial equator is a projection of Earth's equator onto the sky, and Earth's rotation period is 24 sidereal hours (23.9345 solar mean hours). Thus dividing the celestial equator into sidereal hours, minutes and seconds instead of 360 degrees makes some intuitive sense. Set 1 sidereal hour equals 15 degrees to convert between right ascension and degrees.

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.

S

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.Measurement of time based on the position of the vernal equinox instead of the Sun. A sidereal clock does not suffer the irregularities of a solar clock caused by Earth's elliptical orbit. A sidereal clock tracks sidereal time from 0 to 24 sidereal hours. The time reading on the sidereal clock equals the right ascension meridian that lies at the zenith, directly overhead. Astronomers prefer this time keeping method, because sidereal time is fixed to stars. An astronomer compares the right ascension of an object and sidereal time to figure out the optimum observing 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 Eclipse. An arrangement (in order) of the Earth, Moon, and Sun where the Moon blocks the light from the Sun to an observer on Earth. The Moon's shadow does not completely contain the Earth. Only a narrow shadow cone of totality sweeps across the Earth's surface (umbra) surrounded by a partial shadow (penumbra). The solar eclipse alignment of Earth, Moon and Sun does not occur every month, because the Moon's orbit is tilted five degrees from a plane containing the Earth and Sun.

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.

Speed of Light. The maximum velocity for everything in the universe; 186,000 miles a second or fast enough to go to the Moon and back in less than three seconds. See also: light-year

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. The 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.

T

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.

U

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.

V

Variable stars. A star whose brightness changes periodically. These changes may relate to several different situations, from a luminosity change to an eclipsing dark companion. Luminosity changes relate to internal structure and stellar processes. . For instance, the period of luminosity change of a Cepheid variable star is directly related to the Cepheid's luminosity. Thus, all the astronomer has to do is record the Cepheid's light curve, measure the period, match the period to the luminosity, then calculate the distance to the Cepheid. When Cepheids are located inside another galaxy, the astronomer can measure the distance to the galaxy.

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.

W

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.

X

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

Y

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.

Z

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.

The atmosphere is thinnest at the zenith, and thickest at the horizon. Thus astronomers prefer to observe an object when it is closest to the zenith.

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

Zodiac. A ring of constellations that lie along the ecliptic: Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpius, Ophiuchus, Sagittarius, Capricornus, Aquarius, and Pisces in order east of the vernal equinox. 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 when 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.