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Derek Haselden - Astronomical Glossary
Category: Meteorology and astronomy > Astronomy
Date & country: 05/11/2007, UK
Words: 797


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.

Adrastea
Satellite of the planet Jupiter. See TABLE 7. JUPITER - SATELLITE DATA

Aerolite
A meteorite whose main composition is stoney.

Aglaia
One of the `minor planets` of our solar system. Discovered by Luther on 15 Sep 1857. Diameter in km: 158. More data in TABLE 13. THE MINOR PLANETS

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.

Aitne
Satellite of the planet Jupiter. See TABLE 7. JUPITER - SATELLITE DATA

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.

Albiorix
Satellite of the planet Saturn. See TABLE 8. SATURN - SATELLITE DATA

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

alpha Capricornids
Meteor shower. Most active day is Aug.2. Slow, bright, yellow meteors. Fireballs often seen.

alpha Cygnids
Meteor shower. Most active day is Jul.21. 2 radiants, produces steady activity throughout summer.

alpha Scorpiids
Meteor shower. Most active day is Apr.27. Low activity from several radiants.

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°.

Amalthea
Satellite of the planet Jupiter. See TABLE 7. JUPITER - SATELLITE DATA

Amphitrite
One of the `minor planets` of our solar system. Discovered by Marth on 1 Mar 1854. Diameter in km: 200. More data in TABLE 13. THE MINOR PLANETS

Ananke
Satellite of the planet Jupiter. See TABLE 7. JUPITER - SATELLITE DATA

Andromeda
English name: Andromeda (Princess). Genitive name: Andromedae. Visibility: Visible from the UK See TABLE 20: THE CONSTELLATIONS.

Andromeda (Princess)
The constellation Andromeda. Genitive name: Andromedae. Visibility: Visible from the UK See TABLE 20: THE CONSTELLATIONS.

Andromedae
The constellation Andromeda. English name: Andromeda (Princess). Visibility: Visible from the UK See TABLE 20: THE CONSTELLATIONS.

Andromedids
Meteor shower. Most active day is Nov. 20. Shower now thought to be dead. (Comet defunct)

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.

Antlia
English name: The Air Pump. Genitive name: Antliae. Visibility: Parts visible from the UK. See TABLE 20: THE CONSTELLATIONS.

Antliae
The constellation Antlia. English name: The Air Pump. Visibility: Parts visible from the UK. See TABLE 20: THE CONSTELLATIONS.

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.

Apodis
The constellation Apus. English name: The Bird of Paradise. Visibility: Never visible from the UK. See TABLE 20: THE CONSTELLATIONS.

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.

Apus
English name: The Bird of Paradise. Genitive name: Apodis. Visibility: Never visible from the UK. See TABLE 20: THE CONSTELLATIONS.

Aquarii
The constellation Aquarius. English name: The Water Bearer. Visibility: Visible from the UK See TABLE 20: THE CONSTELLATIONS.

Aquarius
English name: The Water Bearer. Genitive name: Aquarii. Visibility: Visible from the UK See TABLE 20: THE CONSTELLATIONS.

Aquila
English name: The Eagle. Genitive name: Aquilae. Visibility: Visible from the UK See TABLE 20: THE CONSTELLATIONS.

Aquilae
The constellation Aquila. English name: The Eagle. Visibility: Visible from the UK See TABLE 20: THE CONSTELLATIONS.

Ara
English name: The Altar. Genitive name: Arae. Visibility: Never visible from the UK. See TABLE 20: THE CONSTELLATIONS.

Arae
The constellation Ara. English name: The Altar. Visibility: Never visible from the UK. See TABLE 20: THE CONSTELLATIONS.

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.

Arend
A comet in our solar system with an orbital period of 7.99years. More data in TABLE 14. NOTABLE PERIODIC COMETS

Arend-Rigaux
A comet in our solar system with an orbital period of 6.82years. More data in TABLE 14. NOTABLE PERIODIC COMETS

Areography
The proper name for the geography of Mars.

Ariadne
One of the `minor planets` of our solar system. Discovered by Pogson on 15 Apr 1857. Diameter in km: 84. More data in TABLE 13. THE MINOR PLANETS

Ariel
Satellite of the planet Uranus. See TABLE 9. URANUS - SATELLITE DATA

Aries
English name: The Ram. Genitive name: Arietis. Visibility: Visible from the UK See TABLE 20: THE CONSTELLATIONS.

Arietis
The constellation Aries. English name: The Ram. Visibility: Visible from the UK See TABLE 20: THE CONSTELLATIONS.

Ashbrook-Jackson
A comet in our solar system with an orbital period of 7.47years. More data in TABLE 14. NOTABLE PERIODIC COMETS

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.

Astraea
One of the `minor planets` of our solar system. Discovered by Hencke on 8 Dec 1845. Diameter in km: 120. More data in TABLE 13. THE MINOR PLANETS

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.

Atalante
One of the `minor planets` of our solar system. Discovered by Goldschmidt on 5 Oct 1855. Diameter in km: 120. More data in TABLE 13. THE MINOR PLANETS

Atlas
Satellite of the planet Saturn. See TABLE 8. SATURN - SATELLITE DATA

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.

Auriga
English name: The Charioteer. Genitive name: Aurigae. Visibility: Visible from the UK See TABLE 20: THE CONSTELLATIONS.

Aurigae
The constellation Auriga. English name: The Charioteer. Visibility: Visible from the UK See TABLE 20: THE CONSTELLATIONS.

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.

Autonoe
Satellite of the planet Jupiter. See TABLE 7. JUPITER - SATELLITE DATA

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.

Belinda
Satellite of the planet Uranus. See TABLE 9. URANUS - SATELLITE DATA

Bellona
One of the `minor planets` of our solar system. Discovered by Luther on 1 Mar 1854. Diameter in km: 124. More data in TABLE 13. THE MINOR PLANETS

Berenice's Hair
The constellation Coma Berenices. Genitive name: Comae Berenices. Visibility: Visible from the UK See TABLE 20: THE CONSTELLATIONS.

Bianca
Satellite of the planet Uranus. See TABLE 9. URANUS - SATELLITE DATA

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.

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.

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

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.

Boethin
A comet in our solar system with an orbital period of 11.2years. More data in TABLE 14. NOTABLE PERIODIC COMETS

Bolide
A brilliant exploding meteor.

Bolometer
An instrument used to measure heat radiation.

Bootes
English name: The Herdsman. Genitive name: Bootis. Visibility: Visible from the UK See TABLE 20: THE CONSTELLATIONS.

Bootis
The constellation Bootes. English name: The Herdsman. Visibility: Visible from the UK See TABLE 20: THE CONSTELLATIONS.

Borrely
A comet in our solar system with an orbital period of 6.88years. More data in TABLE 14. NOTABLE PERIODIC COMETS

Brooks 2
A comet in our solar system with an orbital period of 6.89years. More data in TABLE 14. NOTABLE PERIODIC COMETS

Brorsen-Metcalf
A comet in our solar system with an orbital period of 72.0years. More data in TABLE 14. NOTABLE PERIODIC COMETS

Bus
A comet in our solar system with an orbital period of 6.52years. More data in TABLE 14. NOTABLE PERIODIC COMETS

Caeli
The constellation Caelum. English name: The Graving Tool. Visibility: Parts visible from the UK. See TABLE 20: THE CONSTELLATIONS.

Caelum
English name: The Graving Tool. Genitive name: Caeli. Visibility: Parts visible from the UK. See TABLE 20: THE CONSTELLATIONS.

Caliban
Satellite of the planet Uranus. See TABLE 9. URANUS - SATELLITE DATA

Calliope
One of the `minor planets` of our solar system. Discovered by Hind on 16 Nov 1852. Diameter in km: 174. More data in TABLE 13. THE MINOR PLANETS

Callirrhoe
Satellite of the planet Jupiter. See TABLE 7. JUPITER - SATELLITE DATA

Callisto
Satellite of the planet Jupiter. See TABLE 7. JUPITER - SATELLITE DATA

Calypso
Satellite of the planet Saturn. See TABLE 8. SATURN - SATELLITE DATA