. 1
( 12)


cambridge illustrated dictionary of astronomy
This lavishly illustrated new dictionary written by an experienced writer and
consultant on astronomy provides an essential guide to the universe for amateur
astronomers of all ages. It can be used both as a comprehensive reference work, and
as a fascinating compendium of facts to dip into.

Around 1300 carefully selected and cross-referenced entries are complemented by
hundreds of beautiful color illustrations, taken from space missions, the Hubble
Space Telescope, and other major observatories on Earth and in space.
Distinguished stellar illustrator Wil Tirion has drawn 20 new star maps especially
for inclusion here. A myriad of named astronomical objects, constellations,
observatories and space missions are described in detail, as well as biographical
sketches for 70 of the most luminous individuals in the history of astronomy and
space science. Acronyms and specialist terms are clearly explained, making for the
most thorough and carefully assembled reference resource that teachers and
enthusiasts of astronomy will ever need.

j a c qu e l i n e m i t t o n trained as an astronomer at both Oxford and Cambridge
Universities. She is the author or co-author of over 20 astronomy books for both
children and adults, and has also been consultant or contributor to many other
reference books. She has been editor of the Journal of the British Astronomical
Association, and the annual Handbook of the British Astronomical Association. As Press
Of¬cer of the Royal Astronomical Society, she made regular contributions to TV and
radio about astronomical developments. She continues to keep up-to-date with
recent astronomical advances.
cambridge illustrated dictionary


jacqueline mitton
cambridge university press
Cambridge, New York, Melbourne, Madrid, Cape Town, Singapore, Sao Paulo
Cambridge University Press
The Edinburgh Building, Cambridge CB2 8RU, UK
Published in the United States of America by Cambridge University Press, New York
Information on this title: www.cambridge.org/9780521823647

ª J. Mitton 2007
This publication is in copyright. Subject to statutory exception
and to the provisions of relevant collective licensing agreements,
no reproduction of any part may take place without
the written permission of Cambridge University Press.
First published 2007

Printed in the United Kingdom at the University Press, Cambridge
A catalog record for this publication is available from the British Library
Library of Congress Cataloging in Publication data
ISBN 978-0-521-82364-7 hardback

Cambridge University Press has no responsibility for the persistence
or accuracy of URLs for external or third-party internet websites referred to
in this publication, and does not guarantee that any content on such
websites is, or will remain, accurate or appropriate.

There is always something new in astronomy. Exciting discoveries follow one
after another at a dizzying pace, thanks to the batteries of giant telescopes
perched on mountain tops and equipped with the latest technological
innovations, observatories orbiting high above the troublesome atmosphere,
and spacecraft exploring the worlds of the solar system from close quarters.
Keeping abreast of it all can be a challenge!
For this illustrated A-to-Z, I have made an up-to-date selection of 1 800
entries covering hundreds of named astronomical objects as well as the terms
and abbreviations most commonly encountered in astronomy. I have also
included biographical entries on 70 people who have made signi¬cant con-
tributions to the development of astronomy. Three hundred entries are illu-
strated, nearly all in color.
The idea for an illustrated dictionary grew from the dictionary I originally
compiled in 1988“90, the most recent edition of which was published by
Cambridge University Press in 2001. But this is a new book with a fresh style,
which I hope will appeal to a wide range of readers young and old “ not just as
a reference source in which to look things up, but also as a book full of
fascinating facts and beautiful pictures to dip into anytime.

Using the book
The alphabetical order takes no account of word breaks or hyphens. Entries
beginning with a Greek letter or a number are treated as if the number or letter
were spelled out.
Words printed in italics and preceded by the symbol ¤ have their own
entries, but not all possible cross-references are indicated in this way. The
symbol ¤ preceding a word or words in italics means ˜˜see also.™™

I am deeply grateful to the numerous individuals who have provided me with
advice and information and indebted to the countless reference sources I have
consulted since I began to compile my dictionary database in 1988. It is
impossible to list them all but I would particularly like to thank my husband,
Simon, for his support and for his assistance in compiling the biographical
entries. Any errors or omissions, however, I accept as being my own
responsibility. I would like to thank the many organizations that have freely


made their superb pictures available and those who have given me individual
permission to use their copyright images. A full list of credits can be found at
the end of the book. Finally, I would like to thank Cambridge University Press
for their continuing support during the preparation of this book.

Jacqueline Mitton
November 2006

aberration An effect that makes the observed position of a star slightly different
from its true position. It results from a combination of the ¬nite speed of the
starlight and the motion through space of the observer on Earth. Most
aberration is due to Earth™s yearly motion in orbit around the Sun and is called
annual aberration. A much smaller contribution from Earth™s daily rotation is
called diurnal aberration.
absolute magnitude A number that gives the true, relative brightness of an
astronomical body, ignoring the dimming effect of distance. The absolute
magnitude of a star is the ¤ magnitude it would appear to be if it were 10
¤ parsecs away. The absolute magnitude of a planet, asteroid or comet is the
¤ apparent magnitude it would have if it were at a distance of 1 AU from both the
Sun and Earth, with its disk fully illuminated.
absolute zero The lowest possible temperature. It is the zero point of the Kelvin
temperature scale used in science. Its equivalent on the Celsius scale is
À273.16  C.
absorption line A sharp dip in a continuous ¤ spectrum. Absorption lines look like
narrow gaps in a spectrum. They are seen in the spectra of the majority of
stars. In the case of the Sun, they are known as ¤ Fraunhofer lines.
Atoms create these dark lines by absorbing radiation. Each chemical
element creates a unique pattern of lines. By measuring the strengths of
absorption lines it is possible to deduce the abundance of the various elements,
though the lines are also affected by temperature, density and other factors.
¤ emission line.
absorption nebula A dark interstellar cloud that blocks the light from stars and
galaxies lying behind it. Absorption nebulae range in size from small ¤ globules
to large clouds visible to the naked eye. Absorption nebulae contain both dust
and gas, and the temperatures in them are low enough for simple molecules to
form. Much of what is known about these nebulae comes from observing their
infrared and radio radiation, which, unlike visible light, can pass through
them. ¤ molecular cloud.
accelerating universe The concept that the expansion of the universe is speeding
up. Evidence that the universe is now expanding at an ever faster rate ¬rst
came from measurements made in the late 1990s of the distances to very
remote galaxies in which there were ¤ supernova explosions. Astronomers
estimate that the expansion of the universe began to accelerate about 5 billion

accretion disk

An absorption nebula. Lanes of absorbing cold dust obscure the light of
stars in the Milky Way in this infrared image.

years ago when the power of ¤ dark energy to propel the universe apart
became greater than the power of gravity to hold back the expansion.
accretion disk A disk that forms around a spinning object, such as a star or
¤ black hole, when its gravity draws in material from a companion star or from
the ¤ interstellar medium.
Achernar (Alpha Eridani) The brightest star in the constellation Eridanus,
representing the River Eridanus. Its name comes from Arabic and means ˜˜the
end of the river.™™ It marks the extreme southern point of the constellation.
Achernar is a ¤ B star of magnitude 0.5 and is 144 light years away.
588 Achilles The ¬rst of the ¤ Trojan asteroids to be identi¬ed. Discovered by Max
Wolf in 1906, its diameter is about 116 km (72 miles).
achondrite A type of stony ¤ meteorite that crystallized from molten rock. Unlike
¤ chondrites, achondrites do not contain small mineral spheres known as
¤ chondrules.

active region

An artist™s impression of the accretion
disk that forms around a black hole as
it draws material from a companion

Acrux (Alpha Crucis) The brightest star in the constellation Crux. To the naked
eye it looks like a single white star of magnitude 0.9, but a telescope shows two
¤ B stars, of magnitudes 1.4 and 1.9, separated by 4.4 arc seconds. The spectrum
of the brighter one shows it has a very close companion so there are at least
three stars in this system, which is 320 light years away.
active galactic nucleus (AGN) A small central region in a galaxy where
exceptionally large amounts of energy are being generated. The only way such
a concentrated source of power can be explained is by matter falling into a
supermassive ¤ black hole. Active galaxies are categorized by their appearance
and the nature of the radiation they emit. ¤ Quasars, ¤ Seyfert galaxies,
¤ radio galaxies, ¤ N galaxies and ¤ blazars are all examples.
AGNs have high-speed jets of material shooting out from them. The black
hole is surrounded by a ring of dust and gas at right angles to the jets. The
differences between the various categories of AGN can be accounted for by the
level of their power output and the angle from which they are viewed. In radio
galaxies, the ring is edge-on, hiding the light from the disk of hot material
swirling into the black hole. In quasars and Seyfert galaxies, the ring is
oriented so we can see the light emitted by the hot, glowing disk. Blazars are
thought to have jets pointing directly at Earth.
active galaxy A galaxy with an ¤ active galactic nucleus at its center.
active optics A method of maintaining the precise shape of the main mirror in a
re¬‚ecting telescope. A computer continually monitors the quality of the image
and feeds the information back to a motorized support system under the
mirror. Using active optics means that mirrors can be thinner and more
light-weight. The mirror™s tendency to change its shape under its own weight
as the telescope moves can be corrected in just a few minutes.
active region A region in the outer layers of the Sun where there is ¤ solar activity.
Active regions develop where strong magnetic ¬elds break through from
below. ¤ Sunspots, ¤ plages and ¤ ¬‚ares are all evidence of an active region. The
radiation given off is normally enhanced across the whole of the
electromagnetic spectrum, from X-rays to radio waves, except in sunspots

Adams, John Couch (1819“92)

The active galaxy Centaurus A. This X-ray image shows a jet being ¬red from the center.

Active optics. The computer-controlled supports under the 3.5-meter primary mirror of
the WIYN Telescope at the Kitt Peak Observatory.

themselves, where the temperature is reduced and less light is emitted. There
is a large variation in the size and duration of active regions: they may last
from several hours up to a few months. Electrically charged particles and
the enhanced ultraviolet and X-radiation from active regions affect the
¤ interplanetary medium and Earth™s upper atmosphere.
Adams, John Couch (1819“92) John Couch Adams is chie¬‚y remembered for
predicting the existence and position of the planet Neptune in 1845 by


analyzing the way Uranus had departed from its expected orbit since it was
discovered in 1781. A brilliant mathematician, he worked on the motion of the
Moon and planets. He became a professor at the University of Cambridge in
1858 and director of the University Observatory in 1860.
Unfortunately for Adams, in 1845 neither George Biddell ¤ Airy, the
Astronomy Royal, nor the Director of the Cambridge Observatory, Professor
James Challis, treated his prediction of a new planet with any seriousness or
real urgency. Unknown to them, Urbain J. J. ¤ Leverrier in France had
independently made the same calculation. Acting on Leverrier™s prediction,
Johann Galle in Berlin discovered Neptune in 1846. Only later, and after a good
deal of controversy, did Adams received credit for his work when Challis and
John ¤ Herschel pointed out that his prediction was made before Leverrier™s.
adaptive optics A technique for improving the image an astronomical telescope
makes by compensating for changes in the quality of ¤ seeing. A small, very
thin, ¬‚exible mirror placed a short distance in front of the focus of the
telescope corrects for the distortion. An image sensor detects the amount of
distortion and feeds the information to a microprocessor, which controls
actuators to bend the corrector mirror. The system has to respond to changes
in less than a hundredth of a second.
Adhara (Epsilon Canis Majoris) The second-brightest star in the constellation
Canis Major. Lying 425 light years away, it is a giant ¤ B star of magnitude 1.5
with an eighth-magnitude companion. Derived from Arabic, Adhara means
˜˜the virgins,™™ which may come from a name given to a triangle of stars of
which Adhara is one.
2101 Adonis A small asteroid, discovered in 1936 by Eugene Delporte, which
came within 2 million km (1.4 million miles) of Earth in 1937. After that it was
lost until 1977 when it was recovered following a recomputation of its orbit. It
is a member of the ¤ Apollo asteroid group and is about 2 km across.
Adrastea A small inner moon of Jupiter discovered by David Jewitt in 1979. It
measures 26 · 20 · 16 km (16 · 12 · 10 miles).
Advanced Electro-Optical System Telescope (AEOS Telescope) A US Air Force
3.67-m telescope at the Air Force Maui Optical Station in Hawaii. It is the
world™s largest telescope capable of tracking rapidly moving objects, such as
satellites. Though built principally for military purposes, it is also used for
astronomical research.
aerobot A scienti¬c experiment package deployed in a planetary atmosphere
using a balloon.
aerobraking The controlled use of atmospheric drag to reduce the speed of a
satellite and modify its orbit.
aeronomy The study of physical and chemical processes in the upper atmosphere
of Earth, or of any planet.


Agena An alternative name for the star ¤ Hadar.
Ahnighito meteorite ¤ Cape York meteorite.
airglow Faint light given out by Earth™s own atmosphere. From space, it appears as
a ring of greenish light around the Earth. It is caused by the Sun™s radiation.
The brightest airglow comes from a layer approximately 10“20 km (about 10
miles) thick at a height of around 100 km (60 miles). Glowing oxygen and
sodium atoms are major contributors. Airglow does not includes
¤ thermal radiation, ¤ auroras, lightning and ¤ meteor trains.
air shower A cascade of high-energy, electrically charged particles in the
atmosphere, triggered by the collision of a ¤ cosmic ray particle with the
nucleus of a gas atom. The nucleus that has been hit emits a number of
fast-moving particles. They in turn strike other nuclei, which eject yet more
Airy, Sir George Biddell (1801“92) Airy was one of the most prominent ¬gures
in British astronomy during the nineteenth century. He began his career at
Cambridge University, where he became a professor of mathematics in 1826,
then professor of astronomy and Director of the Observatory two years later.
He was appointed ¤ Astronomer Royal in 1835 and held that position for 46 years
until he retired at the age of 80. He was both a very able mathematician and a
practical scientist who tackled many varied problems relating to astronomy
and geophysics. He also had exceptional organizing ability. As Astronomer
Royal, he re-equipped the Royal Observatory at Greenwich and improved the
range and ef¬ciency of its work.
Airy published hundreds of papers but he is now remembered most for his
work in optics, his detailed analysis of the orbits of Earth and Venus and
estimating the mean density of Earth in an experiment involving pendulums at
the top and bottom of a mineshaft. He was also responsible for establishing
Greenwich Mean Time as the of¬cial time throughout Britain and the
¤ transit circle he had built at Greenwich in 1850 was later chosen as the zero
point of longitude for the world.
Aitne A small outer moon of Jupiter discovered in 2001. Its diameter is about 3 km
(2 miles).
albedo The proportion of the light falling on a body or surface that is re¬‚ected.
Albedo is given either as a decimal between 0 (perfectly absorbing) and 1
(perfectly re¬‚ecting) or as an equivalent percentage. For a planet or asteroid,
the ratio between the total amount of re¬‚ected light and the incident light is
called the Bond albedo. The re¬‚ecting qualities of planetary bodies are also
measured by their geometric albedo. Geometric albedo is formally de¬ned as
the ratio between the brightness of the body as viewed from the direction of
the Sun and the brightness of a hypothetical white sphere of the same size and
at the same distance that is diffusely re¬‚ecting sunlight.


719 Albert A small asteroid discovered from the Vienna Observatory in 1911 by
Johann Palisa, who named it after a benefactor of the Observatory, Baron
Albert Freiherr von Rothschild. It was subsequently lost for almost 80 years,
but recovered by chance in May 2000. It is only about 2“4 km (2 miles) across
but makes relatively close approaches to Earth of between 30 and 46 million
km (19 and 29 million miles) every 30 years. 1911, the discovery year, was one
such occasion.
Albiorix A small outer moon of Saturn in a very elliptical orbit. It was discovered
in 2000 and its diameter is estimated to be 26 km (16 miles).
Albireo (Beta Cygni) The second brightest star in the constellation Cygnus. Visual
observers regard it as one of the most beautiful double stars. The brightest of
the pair is a giant, yellow“orange ¤ K star of magnitude 3.2 and is itself a
double too close to be split in a telescope. Its companion is a bluish ¤ B star of
magnitude 5.4. The two stars are separated by 34 arc seconds and are 380 lights
years away.
Alcaid Alternative form of ¤ Alkaid.
Alcor (80 Ursae Majoris) A fourth-magnitude ¤ A star very close to ¤ Mizar, one of
the bright stars forming the ˜˜tail™™ of the Great Bear. The two stars are
separated by 11.5 arc minutes on the sky and both can easily be seen by the
naked eye. Though they look like a double star, their distances are not known
accurately enough to say whether they form a real binary system or not. Alcor
is about 81 light years away.
Alcyone (Eta Tauri) The brightest member of the ¤ Pleiades star cluster in the
constellation Taurus. Alcyone is a ¤ B star of magnitude 2.9.
Aldebaran (Alpha Tauri) The brightest star in the constellation Taurus. Its Arabic
name means ˜˜the follower.™™ Aldebaran is a giant ¤ K star of magnitude 0.9.
Although it appears in the sky to be part of the Hyades star cluster, it is not in
fact a cluster member, lying only half as far away at a distance of 65 light years.
Alderamin (Alpha Cephei) The brightest star in the constellation Cepheus. It is an
¤ A star of magnitude 2.7 lying 49 light years away. The name, which is of
Arabic origin, means ˜˜the right arm.™™
Algenib (Gamma Pegasi) One of the four stars marking the corners of the
¤ Square of Pegasus. It is a ¤ B star of magnitude 2.8 and is 335 light years away.
The name comes from Arabic and means ˜˜the side.™™ The star Alpha Persei,
more usually known as ¤ Mirfak, is also sometime called Algenib.
Algieba (Gamma Leonis) A second-magnitude star, which is the third brightest in
the constellation Leo. Viewed through a telescope, Algieba is a ¤ visual binary,
consisting of two yellowish giant stars separated by 4 arc seconds. Their
individual magnitudes are 2.6 and 3.8, and they take more than 500 years to
complete one orbit around each other. Algieba is 126 light years away. Its
Arabic name means ˜˜the forehead.™™


Algol (The Demon Star; Beta Persei) An ¤ eclipsing binary system in the
constellation Perseus, which is one of the best-known of all variable stars. Algol
varies between magnitudes 2.2 and 3.5 over a period of 2.87 days because the
two stars regularly cross in front of each other as viewed from Earth.
The brighter member of the system is a ¤ B star and the fainter one a much
larger but far cooler ¤ G star. As the G star cuts off light from its more brilliant
companion, their combined brightness declines over 4 hours, reaching a
minimum that lasts only 20 minutes. The eclipse of the dimmer star by its
partner causes a dip in brightness of only 0.06 magnitude, which is not
detectable by eye. Regular variations in the spectrum of Algol over a period of
1.862 years reveal the presence of a third, more distant star in the system.
The spectrum also reveals evidence for ¤ mass transfer between the two
close companions, which are separated by less than one tenth the distance
between the Sun and Earth. Observations that Algol is a radio star erratically
¬‚aring up to 20 times its normal radio brightness support the idea that mass
transfer is taking place.
Alioth (Epsilon Ursae Majoris). The brightest star in the constellation Ursa Major,
the Greek letters in this case being allotted in order of position rather than of
brightness. Alioth is an ¤ A star of magnitude 1.8 and is 81 light years away.
Alkaid (Eta Ursae Majoris) A star in Ursa Major, at the end of the bear™s ˜˜tail.™™ It is
a ¤ B star of magnitude 1.9. The Arabic name means ˜˜chief of the mourners,™™
for the Arabs saw the constellation as a bier rather than a bear. Its distance is
100 light years.
Allan Hills A region in Antarctica from where large numbers of meteorites have
been recovered. The meteorites become concentrated in the area by natural
movements in the ice sheet, and are relatively easy to identify against the ice.
Allen Telescope Array A project of the ¤ SETI Institute and the University of
California, Berkeley, to construct a radio telescope in the form of an array of dish
antennas each 6.1 meters (200 feet) across. The main purpose is to seek signals
from possible extraterrestrial civilizations, but the array will be available for
conventional radio astronomy as well. Construction has begun at Hat Creek
Observatory. The target is a total of 350 dishes sometime between 2015 and 2020.
Allende meteorite A meteorite of the ¤ carbonaceous chondrite type, which fell in
Mexico in 1969. More than 2 tonnes of material was scattered over an area
48 km by 7 km (30 by 4 miles).
ALMA ¤ Atacama Large Millimeter Array.
Almagest A large astronomical work written by the Greek astronomer ¤ Ptolemy
(Claudius Ptolemaeus), who worked in Alexandria between about ad 127 and
151. Almagest is an Arabic corruption of Greek, meaning ˜˜The Greatest,™™
though Ptolemy™s original title was The Mathematical Collection. It is one of the
most important works on astronomy ever written. Ptolemy included a star

Alpha Centauri

The Allen Telescope Array. An artist™s impression of the array when complete.

catalog and dealt with the motion of the Moon and planets. The rules set out
for calculating the future positions of the planets on the basis of an Earth-
centered universe were used for centuries.
almanac A book of tables giving the future positions of the Moon, planets and
other celestial objects. An almanac normally covers one calendar year.
Alnath An alternative spelling of the star name ¤ Elnath.
Alnilam (Epsilon Orionis) One of the three bright stars forming Orion™s belt. It is a
¤ supergiant ¤ B star of magnitude 1.7, estimated to be 1340 light years away.
˜˜Alnilam™™ comes from the Arabic for ˜˜string of pearls.™™
Alnitak (Zeta Orionis) One of the three bright stars forming Orion™s belt. Its Arabic
name means ˜˜the girdle.™™ Alnitak is a ¤ supergiant ¤ O star of magnitude 1.8
and is about 800 light years away.
Alpha Centauri The brightest star in the constellation Centaurus and the nearest
bright star to the Sun, at a distance of 4.36 light years. It is a ¤ visual binary star
with an orbital period of 80 years. It consists of a ¤ G star and a ¤ K star, which
have a combined magnitude of 0.27. The eleventh-magnitude star Proxima
Centauri, though two degrees away on the sky, is thought to be associated
with this star system because it has a similar motion in space. Proxima, a dim
¤ M star, is the nearest star to the Sun at a distance of 4.24 light years. Alpha
Centauri is also called by the Arabic name Rigil Kentaurus (sometimes Rigel, or
shortened to Rigil Kent), which means ˜˜the foot of the Centaur.™™ Another
alternative name is Toliman.


Alphard (Alpha Hydrae) The brightest star in the constellation Hydra. Its Arabic
name means ˜˜the solitary one of the serpent.™™ It is a ¤ K star of magnitude 2.0
lying 175 light years away.
Alphekka (Gemma; Alpha Coronae Borealis) The brightest star in the constellation
Corona Borealis. It is an ¤ A star of magnitude 2.2. The Arabic name, also spelt
Alphecca, means ˜˜bright one.™™ This star is sometimes called by the Latin name
Gemma, the ˜˜jewel™™ in the crown. Its distance is 75 light years.
Alpheratz (Sirrah; Alpha Andromedae) The brightest star in the constellation
Andromeda, also marking one corner of the ¤ Square of Pegasus. It was formerly
considered to belong to Pegasus and was designated Delta Pegasi. Alpheratz is
an ¤ A star of magnitude 2.1 and is 97 light years away.
Alphonsus A lunar crater, 118 km (73 miles) in diameter. A prominent ridge runs
across the center, almost along a north“south line, through a central peak
about 1 km high. Temporary reddish clouds were observed there in 1958 and
1959, possibly due to the release of gas from the rocks.
Alpine Valley (Vallis Alpes) A ¬‚at-bottomed valley on the Moon, running for
150 km (95 miles). It crosses the lunar Alps and connects the Mare Frigoris with
the Mare Imbrium.
ALSEP Abbreviation for Apollo Lunar Science Experiment Package, an
experimental set-up deployed on the Moon by astronauts during the manned
¤ Apollo program (1969“72). One was left by every mission except the ¬rst. All
the experiments were turned off in 1978.

The Apollo 16 ALSEP with astronaut John Young.


Al Sufi, Abd Al-Rahman (903“986) Al Su¬ worked in Persia and Baghdad. He was
the ¬rst person to revise Ptolemy™s ¤ Almagest and to relate Greek constellation
and star names to traditional Arabic ones. His Book on the Constellations of Fixed
Stars was published in 964 and included drawings of each constellation. He was
the ¬rst person to record seeing the ¤ Andromeda Galaxy, describing it as a
˜˜small cloud.™™ He also wrote on the use of ¤ astrolabes. In the West, he has
sometimes been known as Azophi, which is the name given to the crater on
the Moon named in his honor.
Altair (Alpha Aquilae) The brightest star in the constellation Aquila. It is an
¤ A star of magnitude 0.8 and is one of the closest bright stars at a distance of only
17 light years. Derived from Arabic, the word Altair means ˜˜the ¬‚ying eagle.™™
altazimuth mount A form of telescope mount that allows the telescope to rotate
about two axes “ one horizontal and one vertical. It is the simplest type of
mount to construct but the telescope must be turned about both axes
simultaneously in order to track the motion of celestial objects across the sky.
However, computers that can control the motion of a large telescope mean
that altazimuth mount are now used for all new professional instruments.
altitude The height of a celestial object, measured upwards as an angle from the
aluminizing The process that deposits a thin re¬‚ecting layer of aluminum on the
glass surface of a telescope mirror.
Amalthea A small inner moon of Jupiter discovered by Edward E. Barnard in 1892.
Images obtained by the ¤ Voyager 1 mission showed it as a red-colored,
potato-shaped object. The surface is heavily cratered, the largest depression,
Pan, being 90 km (56 miles) across. The red color is thought to be due to sulfur
compounds blown off the moon ¤ Io. Data from the ¤ Galileo spacecraft show
that it is like an icy rubble pile with a density less than that of water. Amalthea
measures 262 · 146 · 134 km (163 · 91 · 83 miles).

A stereo pair of images of Amalthea taken by the Galileo spacecraft in
August and November 1999.


AMANDA A detector for cosmic neutrinos, built into the Antarctic ice at the South
Pole. AMANDA stands for ˜˜Antarctic Muon And Neutrino Detector Array.™™ The
detector consists of photomultiplier tubes arranged in concentric rings
between 1300 and 2400 meters (0.8“1.5 miles) below the surface. The
photomultipliers detect light ¬‚ashes created when neutrinos occasionally
interact with atomic nuclei in the ice, causing then to emit muons. AMANDA
began operation in 1999 and in 2005 was incorporated into a successor project
called ¤ IceCube. ¤ neutrino astronomy.
1221 Amor A small asteroid, about 1“2 km (1 mile) across, which is the prototype
of a group of Earth-approaching asteroids. The Amor group travel on orbits
that bring them closer than the main ¤ asteroid belt with ¤ perihelion between
the orbits of Mars and Earth. They can cross Mars™s orbit but not Earth™s. Amor
was discovered by Eugene J. Delporte in 1932.
29 Amphitrite A large asteroid discovered from London in 1854 by Albert Marth.
It is estimated to be 200 km (125 miles) across.
analemma The ¬gure-of-eight obtained if the Sun™s position in the sky is recorded
at the same time of day throughout the year. The position of the Sun varies
from day to day because Earth™s axis is tilted to its orbit around the Sun and
because Earth™s orbit is elliptical rather than circular.
Ananke A small moon of Jupiter discovered from Mount Wilson Observatory in 1951
by Seth B. Nicholson. It is about 28 km (17 miles) in diameter. It belongs to a family
of jovian moons on highly tilted, ¤ retrograde orbits around their parent planet.
Andromeda A large but not very conspicuous northern constellation. In classical
mythology, Andromeda was the daughter of King Cepheus and Queen
Cassiopeia. She was condemned to be sacri¬ced to a sea monster but rescued
by Perseus. In old star atlases, Andromeda was usually shown as a chained
woman. The three brightest stars, Alpha (Alpheratz or Sirrah), Beta (Mirach)
and Gamma (Alamak) represent her head, hip and foot. Andromeda is most
famous for the ¤ Andromeda Galaxy.
Andromeda Galaxy (M31; NGC 224) A large ¤ spiral galaxy, visible to the unaided
eye as a misty patch in the constellation Andromeda. It belongs to the
¤ Local Group of galaxies and is similar to our own ¤ Galaxy but its disk appears
to be at least about half as large again as the Milky Way™s and more luminous.
Its mass is estimated to be 300“400 billion Suns. Spiral features are not easy to
see since we view its disk tilted by only 13 away from edge-on. In a small
telescope, only the small central nucleus is visible though the fainter outer
parts extend over 3 degrees of sky “ more than six times the apparent diameter
of the Moon. Several dwarf galaxies belonging to the Local Group are in orbit
around the Andromeda Galaxy, notably M32 and NGC 205.
The Andromeda Galaxy was the ¬rst object to be recognized as lying
beyond the Milky Way when ¤ Edwin Hubble estimated its distance in 1923.

Anglo-Australian Telescope

3h 2h 1h 0h 23h



ω º
χ ι 7662
Andromeda Galaxy
… 205 (M110)
„ M32
µ ρ
± Alpheratz

ζ +20º

2h 1h 0h WIL TIRION

Magnitudes: 5 4 3 2 1 0 brighter than 0 Variable stars

Open clusters Globular clusters Planetary nebulae Bright nebulae Galaxies

A map of the constellation Andromeda.

Lying 2.4“2.9 million light years away, it is the most distant object visible to
the naked eye.
Andromedids A ¤ meteor shower associated with Comet ¤ Biela. The ¬rst recorded
appearance of the shower was in 1741. Spectacular meteor storms were
observed radiating from near the star Gamma Andromedae on November 27 in
both 1872 and 1885, following the break-up of Comet Biela. After a moderate
shower in 1904, the Andromedids were not recorded again until a few were
identi¬ed in 1940. In recent years, about three Andromedids an hour have
been detected, around November 14. The shower is also known as the Bielids.
Anglo-Australian Observatory (AAO) An observatory at the Siding Spring site of
the ¤ Mount Stromlo and Siding Spring Observatories in New South Wales, Australia.
It is funded jointly by the governments of Australia to operate the 3.9-m (150-
inch) ¤ Anglo-Australian Telescope and the 1.2-m (48-inch) Schmidt telescope.
Anglo-Australian Telescope (AAT) A 3.9-m (150-inch) re¬‚ecting telescope, owned
and funded jointly by the governments of Australia and the UK. It is situated
at the ¤ Siding Spring Observatory site in New South Wales, Australia. The
telescope was constructed in the early 1970s and started scheduled observing
in 1975. It was the ¬rst telescope to be fully computer controlled.

angular diameter

The Andromeda Galaxy™s central region.

angular diameter The apparent diameter of an object measured as an angle in
degrees, arc minutes or arc seconds. An object™s angular diameter depends on
both its true diameter and its distance.
5535 Annefrank A small asteroid in the ¤ asteroid belt, which was imaged by the
¤ Stardust spacecraft in November 2002. It was discovered in 1942 by
Karl Reinmuth in Heidelberg. Stardust passed by Annefrank at a distance of
3300 km (2050 miles) and found that it is about 6 km (4 miles) across.
annular eclipse A solar ¤ eclipse when a ring of the Sun™s bright disk remains
visible. Since the orbits of Earth around the Sun and of the Moon around Earth
are elliptical, the distances of the Sun and Moon vary slightly so their apparent
sizes vary too. A solar eclipse that would otherwise have been total is annular if
the Moon appears smaller than the Sun at the time.

anthropic principle

Antares (Alpha Scorpii) The brightest star in the constellation Scorpius. This red
¤ supergiant ¤ M star is a ¤ semiregular variable, ¬‚uctuating between magnitudes
0.9 and 1.1 over a ¬ve-year timescale. It is rapidly blowing off gas, which
has formed a small nebula around the star. The name Antares is derived from
Greek and means ˜˜rival of Mars.™™ It has a sixth-magnitude blue companion
3 arc seconds away and lies at a distance of 600 light years.
antenna (aerial) A device for collecting or transmitting radio signals. The design of
an antenna depends on the wavelength at which it is intended to operate
and the strength of the signal. The simplest antenna is a straight rod, or dipole;
the commonest type used in radio astronomy is a paraboloid dish.
Antennae Galaxies The popular name of a pair of colliding galaxies, NGC 4038
and 4039. Two long, curved streamers of stars were pulled out of the galaxies
by the collision. The galaxies are 48 million light years away and the streamers
about a 100 000 light years long.
anthropic principle The idea that the universe must have certain properties to
favor the emergence of life. In theory, a large range of universes with different
physical properties could exist. The anthropic principle states that only a

The Antennae Galaxies. This image is a composite of infrared data
(shown as red) and visible-light data (shown in blue and green).


proportion of them can have intelligent observers. Since we exist, the universe
we inhabit must have characteristics that have permitted us to evolve. This
basic form of the anthropic principle is not generally regarded as controversial,
and is sometimes called the weak anthropic principle.
The so-called strong anthropic principle is more speculative. This asserts
that, because there are so many apparently unconnected coincidences in
nature, which together have made it possible for life to develop, the universe
must give rise to intelligent observers at some stage in its existence.
antitail Part of a comet™s ¤ dust tail that appears to protrude forwards towards the
Sun from the comet™s head, sometimes like a spike. The effect is an illusion
caused by the way the comet™s tail curves and the direction from which we are
seeing it.
Antlia (The Air Pump) A small, faint, southern constellation introduced in the
mid-eighteenth century by Nicolas L. de Lacaille. It was originally called Antlia
Antoniadi, Eugene Michel (1870“1944) Antoniadi was a Turkish-born
Greek“French astronomer who became famous particularly for his
observations of Mars. He worked in Paris from 1893 and became a French
citizen in 1928. His books The Planet Mars, published in 1930, and The Planet
Mercury, which followed in 1934, included the best maps of the planets made
before the space age. He concluded that the ˜˜canals™™ Percival ¤ Lowell had
claimed to see on Mars were an optical illusion.
Antoniadi scale A scale of ¬ve points, devised by the Greek“French astronomer
Euge ¤ Antoniadi that is used by amateur astronomers for describing the
quality of ¤ seeing. The points on the scale are: I, perfect; II, slight undulations
with periods of virtually perfect seeing lasting for several seconds; III, moderately
good seeing, though with noticeable air movements; IV, poor seeing making
observations dif¬cult; V, very bad seeing that permits no useful observation.
Aoede A small outer moon of Jupiter discovered in 2003. Its diameter is about 4 km
(2.5 miles).
Apache Point Observatory An observatory in New Mexico, USA, owned and
operated by a consortium of American universities. The principal instrument is
a 3.5-m (138-inch) telescope for both optical and infrared observations. A 2.5-m
(98-inch) telescope for the ¤ Sloan Digital Sky Survey, together with a 0.6-m
support telescope for the project, opened in 1997.
apastron The positions of the two members of a ¤ binary star system when they
are furthest apart in their orbit.
Apennines (Montes Apenninus) A range of mountains on the Moon, forming part
of the eastern boundary of the Mare Imbrium.
aperture (symbol D) The diameter of the main collecting mirror or lens in a
¤ telescope. For a radio telescope, it is the size of the ¤ antenna. The aperture

Apollo program

is one of the most important characteristics of a telescope since the
ability to detect faint objects and resolving power both increase with larger
aperture synthesis A technique developed by radio astronomers to make maps or
images with the ¤ resolving power of a very large aperture by combining the
observations from a number of smaller ¤ antennas. More recently it has also
been applied to optical and infrared observations.
In the simplest version, two antennas form a ¤ radio interferometer. As the
Earth rotates in the course of a day, one antenna is automatically carried right
around the other. The effect is like sweeping out a ring. On successive days the
separation between the two antennas is changed, so that a different ring is
swept out and a large elliptical area is gradually covered. The records are
combined in a computer to produce a radio map of the section of sky under
observation with the detail resolved as if the telescope aperture were the size
of the total area swept out.
In practice, more than two antennas are normally used to speed up the
process and give greater ¬‚exibility. It is also possible to combine observations
made at different sites, separated by thousands of kilometers, to obtain even
better resolution.
Beginning in the 1990s, the same principle has been used to obtain high-
resolution optical and infrared images. Pioneering instruments include the
Cambridge Optical Aperture Synthesis Telescope (COAST) in the UK and the
Navy Prototype Optical Interferometer (NPOI) at the US Naval Observatory™s
site near Flagstaff, Arizona. ¤ interferometer.
aphelion (pl. aphelia) The point farthest from the Sun in the orbit of a body, such
as a planet or comet, that is traveling around the Sun.
apogee The point furthest from Earth in the orbit of the Moon or of an arti¬cial
Earth satellite.
1862 Apollo A small asteroid, about1.4 km (0.9 mile) across, that was discovered in
1932 by Karl W. Reinmuth. It is the prototype of the Apollo group of asteroids
with orbits that cross Earth™s. Apollo™s orbit also crosses that of Venus.
Apollo program An American space program, which in 1969 successfully
achieved a manned landing on the Moon. The program consisted of 17
missions in all. Numbers 1 to 6 were unmanned test ¬‚ights and Apollo 13 was
aborted following an explosion on board, though the astronauts were returned
safely to Earth. Six Moon landings took place between July 20, 1969 and
December 11, 1972. The astronauts collected samples of lunar rocks and soils
weighing a total of nearly 400 kilograms (nearly 900 pounds), and took many
photographs both on the surface and from lunar orbit. A variety of scienti¬c
experiments were carried out on the surface of the Moon, including ones to
detect ¤ cosmic rays and the ¤ solar wind.

Apollo program

The crew of the Apollo program™s ¬rst mission to land astronauts on the Moon, Apollo 11.
Left to right are Neil A. Armstrong, commander; Michael Collins, command module pilot;
and Edwin E. Aldrin Jr., lunar module pilot.

The Apollo craft consisted of three modules: the Command Module (CM),
the Service Module (SM) and the Lunar Module (LM). The Command and Service
Modules (CSM) remained in lunar orbit with one astronaut on board while the
other two astronauts made the descent to the Moon™s surface in the Lunar
Module. The descent stage was left on the Moon when the astronauts returned
to lunar orbit by means of the ascent stage, and rejoined the Command and
Service Modules. The Service Module was jettisoned shortly before re-entry
into the Earth™s atmosphere.

Apollo manned Moon landings

Apollo Astronauts Landing date Landing site

11 Armstrong, Aldrin, Collins July 20, 1969 Mare Tranquillitatis
12 Conrad, Bean, Gordon Gordon November Oceanus Procellarum
19, 1969
14 Shepard, Mitchell, Roosa February 5, 1971 Fra Mauro
15 Scott, Irwin, Worden July 30, 1971 Hadley Rille
16 Young, Duke, Mattingly April 21, 1972 Cayley“Descartes
highland region
17 Cernan, Schmitt, Evans December 11, 1972 Taurus“Littrow region

Arecibo Observatory

Apollo“Soyuz project A joint US“Soviet space project in July 1975 in which an
¤ Apollo program Command and Service Module docked with a Soviet Soyuz
space station in Earth orbit at an altitude of 225 km (140 miles). The two teams
of astronauts were able to visit each others™ craft and they performed a number
of experiments jointly.
apparent magnitude The relative brightness of a star (or other celestial object) as
perceived by an observer. Apparent magnitude depends on both the actual
amount of light the body is emitting or re¬‚ecting and the distance to the
object. The smallest magnitudes correspond to the greatest brightness.
¤ magnitude, absolute magnitude.
appulse A very close approach of one celestial object to another on the sky, so they
seem just to touch without an ¤ occultation taking place.
Apus (The Bird of Paradise) A faint constellation near the south celestial pole,
probably introduced by sixteenth century navigators. It was included by
Johann Bayer in his 1603 star atlas ¤ Uranometria.
Aquarius (The Water Carrier) One of the twelve traditional constellations of the
¤ zodiac. It is one of the larger constellations but contains no very bright stars.
Aquila (The Eagle) A small but prominent northern constellation. It is said to
represent the eagle of classical mythology sent by Jupiter to carry Ganymede to
Olympus. It contains one of the brightest stars, ¤ Altair.
Ara (The Altar) A small, southerly constellation. Its stars are all faint but it is
among the 48 ancient constellations listed by ¤ Ptolemy.
arachnoid An informal term for a type of volcanic feature on ¤ Venus resembling
spiders connected by a web of fractures.
archaeoastronomy The study of how astronomy was done in civilizations and
societies of prehistory. Archaeoastronomy is particularly concerned with
archaeological evidence for astronomical knowledge rather than written
records. Sites that are studied include the stone-age remains in western
Europe, ancient meso-America and the classical Mediterranean civilizations.
Arche A small outer moon of Jupiter discovered in 2002. Its diameter is 3 km
(2 miles).
Arcturus (Alpha Bootis) The brightest star in the constellation Bootes and
¨ ¨
the fourth-brightest star in the sky. It is an orange giant ¤ K star of
magnitude À0.04 lying 37 light years away. The name Arcturus is Greek in
origin, and means ˜˜bear-watcher.™™ It refers to the fact tht Arcturus seems to
follow the Great Bear around the north celestial pole.
Arecibo Observatory A radio astronomy observatory in Puerto Rico, where a dish
305 m (1000 feet) across has been built into a natural bowl shape in hills south
of the city of Arecibo. Completed in 1963, the telescope is operated by the
National Ionospheric and Astronomy Center of Cornell University in the USA.
The re¬‚ecting surface cannot be moved, but radio sources can be tracked by

95 Arethusa

The Arecibo Observatory viewed from the air.

moving the receiver at the focus along a specially designed support. A major
refurbishment was completed in 1997. The telescope is larger in area than all
the other radio telescopes in the world combined. It is used for radar studies of
planets, observing ¤ pulsars and the study of hydrogen in distant galaxies.
Because of its large collecting area, it can pick up fainter signals than any
other dish.
95 Arethusa One of the darkest known asteroids, with an albedo of only a few
percent. Its diameter is about 230 km (140 miles). It was discovered in 1867 by
Robert Luther working in Dusseldorf.
Argo Navis A very large constellation of the southern sky listed by ¤ Ptolemy but
no longer recognized of¬cially. It represented the ship of Jason and the
Argonauts from Greek mythology. It was so large that astronomers in the
nineteenth century started to refer to different parts of the ship. Since 1930,

Aristotle (384“322 bc)

Ariel. A Voyager 2 image made in 1986.

stars that formerly made up Argo have been of¬cially assigned to three
separate constellations: ¤ Carina (The Keel), ¤ Puppis (The Stern or Poop) and
¤ Vela (The Sails).
Ariel One of the larger moons of Uranus, with a diameter of 1158 km (720 miles).
It was discovered by William Lassell in 1851. Images obtained by the ¤ Voyager 2
mission in 1986 showed the surface to be heavily cratered and crossed by fault
scarps and valleys. Its appearance suggests that there has been considerable
geological activity in the past.
Aries (The Ram) A small constellation in the traditional ¤ zodiac. It is said to
represent the ram with the golden ¬‚eece sought by Jason and the Argonauts
in classical mythology. Its brightest star is the second-magnitude ¤ Hamal.
¤ First Point of Aries.
Aristarchus A very bright lunar crater surrounded by a pattern of rays. It is 45 km
(28 miles) across and has terraces on its inner walls. Temporary reddish glows
in Aristarchus have occasionally been reported, perhaps caused by gas being
released from the surface rocks. ¤ lunar transient phenomenon.
Aristarchus of Samos (c. 310“230 bc) The Greek astronomer Aristarchus was the
first person known to have put forward the idea that Earth and the other
planets orbit around the Sun but his theory was rejected by his contemporaries
and not resurrected again until the sixteenth century, when it was again
proposed by Nicolaus ¤ Copernicus. Aristarchus also attempted to measure the
size and distance of the Sun and Moon. Though his results were inaccurate
because he was unable to make precise enough observations, his methods
were correct. He also concluded that the stars were infinitely far away because
he could never detect any changes in their positions.
Aristotle (384“322 bc) Aristotle was one of the greatest of the Greek philosophers.
He developed a new style of philosophy which he applied to all aspects of the
material world, including biology. His philosophy was so influential and
respected, it dominated the way people thought about the universe and the
movements of celestial bodies until the sixteenth and seventeenth centuries.

armillary sphere

Aristotle was born in Stagira, a Greek colony in Macedonia. At the age of
18, he went to Athens to study under the philosopher ¤ Plato. When in his
thirties, he returned to his native city and became tutor and advisor to the
young prince who would become Alexander the Great. Then in 335 bc he
returned to Athens and set up his own school of philosophy.
According to Aristotle, Earth was the center of the universe and the stars
and the Sun, Moon and planets went around it in circular orbits. He believed
that earthly matter was made of four ˜˜elements™™ “ earth, water, ¬re and air.
Earth and water naturally fell down while air and ¬re rose up. He thought of
heavenly bodies as perfect and unchanging and to explain their motion he said
they were made of a ¬fth element, aether, that naturally moved in circles.
Aristotle attempted to explain things by thought and logic, based on simple
every-day experience, rather than conducting what today we would think of as
scienti¬c experiments.
armillary sphere A type of celestial globe that represents the sphere of the sky by
a framework of intersecting rings, with the Earth at the center. The rings
correspond to important circles on the celestial sphere, such as the

A sixteenth century engraving of an
armillary sphere.


¤ celestial equator and the ¤ ecliptic. Some of the rings may be movable so that
the sky™s appearance at different times and at different latitudes can be
reproduced. On some armillary spheres, the positions of the brighter stars are
shown by small pointers attached to the ¬xed rings. The use of armillary
spheres dates from at least the third century bc.
array An arrangement of linked radio ¤ antennas to make a ¤ radio telescope.
Arsia Mons A large ¤ shield volcano on Mars. It is about 350 km (220 miles) across at
its base and rises to a height of 27 km (17 miles), 17 km above the level of the
surrounding ridge.
Ascraeus Mons A prominent ¤ shield volcano in the ¤ Tharsis Ridge region of Mars.
It is about 250 km (150 miles) across at its base and rises to a height of
27 km (17 miles), 17 km above the level of the surrounding ridge.
ashen light A dim glow that visual observers occasionally claim to see on the dark
part of Venus when its phase is a very thin crescent. Its origin is unknown but,
if it is a real physical effect in the atmosphere of Venus and not just an optical
illusion, it may be similar to the ¤ airglow in Earth™s atmosphere.
aspect The position of a planet or the Moon, relative to the Sun, as viewed from
association A loose grouping of young stars, typically with between 10 and 100
members. Stellar associations are found along the spiral arms of the ¤ Galaxy.
They contain stars that were born together relatively recently in the same
star-forming cloud and are always found along with interstellar matter.
Associations are not held together very strongly and the stars will disperse
within a few million years.
There are three main types of stellar associations. O or OB associations are
made up of massive, luminous ¤ O stars and ¤ B stars scattered through a region up
to several hundred light years across. T associations contain numerous, low-mass
¤ T Tauri stars. In R associations, the stars are embedded in a ¤ re¬‚ection nebula.
A star A star of ¤ spectral type A. A stars have surface temperatures in the range
7500“11 000 K and are white in color. The most prominent features in their
spectra are the strong absorption lines due to hydrogen atoms. Lines of heavier
elements, such as iron, are also noticeable at the cooler end of their
temperature range. Examples of A stars are ¤ Sirius and ¤ Vega.
asterism A prominent pattern of stars, usually with a popular name, that is
not a complete ¤ constellation. Well-known examples of asterisms are the
¤ Big Dipper (Plough) in Ursa Major and the ¤ Sickle in Leo.
asteroid (minor planet) A miniature planet composed of rock and/or metal.
Asteroids range in size from the largest, ¤ Ceres, which is nearly 1000 km
(600 miles) across, down to about 100 m (300 feet). Smaller objects are more
often called meteoroids. Many thousands of asteroids have been individually
identi¬ed and there could be half a million with diameters larger than 1.6 km

asteroid belt

(1 mile). However, the total mass of all the asteroids put together is less than
one-thousandth the mass of Earth. Most asteroid orbits are concentrated in the
¤ asteroid belt between Mars and Jupiter at distances ranging from 2.0 to 3.3
AU from the Sun. However, some asteroids follow orbits that bring them
nearer to the Sun, such as the ¤ Amor group, the ¤ Apollo group and the ¤ Aten
group, and some are more distant from the Sun, such as the ¤ Centaurs.
The ¤ Trojan asteroids share Jupiter™s orbit.
Asteroids are classi¬ed according to how they re¬‚ect sunlight: 75 percent
are very dark, carbonaceous C-types, 15 percent are grayish, stony S-types and
the remaining 10 percent are metallic M-types plus a number of very rare
varieties. The darkest asteroids re¬‚ect only 3 or 4 percent of the sunlight falling
on them, while the brightest re¬‚ect up to 40 percent. The brightness of many
asteroids varies regularly as they rotate. Nearly all have irregular shapes; the
smallest asteroids rotate the most rapidly and are the most irregular in shape.
The ¤ Galileo spacecraft, on its way to Jupiter, ¬‚ew by two asteroids,
¤ Gaspra (on October 29, 1991) and ¤ Ida (on August 28, 1993). Detailed images
showed their rocky surfaces to be pitted with numerous craters, and that Ida
has a small satellite. The ¤ NEAR Shoemaker spacecraft ¬‚ew by ¤ Mathilde in
1997 and went into orbit around ¤ Eros in 2000. The asteroid ¤ Itokawa was
visited by the Japanese spacecraft ¤ Hayabusa in 2005. From the ground, it is
possible to obtain information about the shape of asteroids using radar.
Asteroids are believed to be the remnants of the material from which the
solar system formed. They are gradually disintegrating because of collisions
between them. Most meteorites reaching Earth are small pieces broken off the
asteroids. ¤ near-Earth asteroid, potentially hazardous asteroid.
asteroid belt The region of the solar system, between 2.0 and 3.3 AU from the Sun,
where the orbits of the vast majority of ¤ asteroids are. Within the
belt, there are certain rings where the orbits of groups and families are
concentrated and others, known as ¤ Kirkwood gaps, where there are very
few asteroids,. The proportions of the different types of asteroid change
markedly through the belt. At the inner edge, 60 percent are S-types and 10
percent C-types; at the outer edge the situation is reversed with 80 percent
being C-types and 15 percent S-types.
asteroseismology The study of global oscillations of stars. Asteroseismology
reveals details about the internal structure of stars in the same way that
seismology uncovers information about Earth™s interior. ¤ helioseismology.
5 Astraea An asteroid measuring about 120 km (75 miles) across When it was
discovered in 1845 by the German amateur astronomer Karl L. Hencke, it was
only the ¬fth asteroid to be found and was the ¬rst for 38 years.
astration The cyclic process in which interstellar matter is incorporated into
newly formed stars, where its chemical composition is altered by nuclear


processes, and is then expelled again into the interstellar medium to be used in
the next generation of stars. Astration results in a steady increase in the
proportion of heavier elements in a galaxy.
astrobiology The science concerned with the possibility of living organisms
originating in space or on bodies other than Earth.
astrograph A historical astronomical telescope designed to take wide-angle
photographs of the sky for measuring the positions of stars. In particular, the
refracting telescopes constructed for the ¤ Carte du Ciel project were called
astrolabe An ancient instrument for showing the positions of the Sun and bright
stars at any time and date. Its invention is credited to Greek astronomers who
worked in the second century bc.
A basic astrolabe consists of a circular star map (the ˜˜tablet™™ or ˜˜tympan™™) with
a graticule (the ˜˜rete™™) over the top. These two parts are joined at their common
center so that the rete can rotate over the tablet. Typically, it would be made of
brass. Various engraved scales enable the user to display the positions of the stars
and the Sun for any time and date, though any particular astrolabe is only useful
within a narrow range of latitudes. Astrolabes were often ¬tted with a sight on a
movable arm so that they could be used to estimate the ¤ altitudes of stars.
astrology An ancient tradition that claims to connect human traits and the course
of events with the positions of the Sun, Moon and planets in relation to the
stars. Before the seventeenth century, there was less of a clear distinction
between astrology and the science of astronomy; many scienti¬cally useful
astronomical observations were originally made for astrological purposes.

A typical astrolabe.


astrometry The branch of astronomy concerned with the measurement of the
positions and apparent motions of celestial objects in the sky and the factors
that can affect them.
astronautics The science and technology concerned with all aspects of space travel.

Astronomers Royal

John Flamsteed 1675“1719
Edmond Halley 1720“42
James Bradley 1742“62
Nathaniel Bliss 1762“64
Nevil Maskelyne 1765“1811
John Pond 1811“35
Sir George Biddell Airy 1835“81
Sir William Christie 1881“1910
Sir Frank Watson Dyson 1910“33
Sir Harold Spencer Jones 1933“55
Sir Richard Woolley 1956“71
Sir Martin Ryle 1972“82
Sir Francis Graham-Smith 1982“90
Sir Arnold Wolfendale 1991“95
Sir Martin Rees 1995“

Astronomer Royal Formerly the title of the director of the Royal Observatory
in the UK but, since 1972, an honorary title bestowed on a distinguished
astronomer in the UK.
astronomical unit (AU or a.u.) A unit of measurement used mainly for distances
within the solar system. It was originally based on the average distance
between Earth and the Sun, though it now has a formal de¬nition independent
of Earth™s orbit. Its value is 149 597 870 km (92 955 730 miles). There are about
63 240 astronomical units in a light year.
astronomy The study of the universe and everything in it beyond the bounds of
the Earth™s atmosphere. ¤ astrophysics.
astrophysics The physical theory of astronomical objects and phenomena. This
term was introduced in the nineteenth century to distinguish between using
physics to understand astronomical observations and the mere recording of
positions, movements and phenomena. Though ˜˜astrophysics™™ keeps its
original meaning, ˜˜astronomy™™ is generally considered to encompass all
aspects of the study of the universe, including astrophysics.
Atacama Large Millimeter Array (ALMA) An array of 64 individual dish
antennas, each 12 m in diameter, on an area 10 km square at Llano de

atmospheric window

The Atacama Large Millimeter Array
(ALMA). An artist™s concept of the
compact array.

Chajnantor in the Atacama desert in northern Chile. ALMA operates in the
millimeter and submillimeter regions of the spectrum. Contruction of this
joint US/European project started in 2003 and the telescope was expected to be
fully completed and operational by 2009.
2062 Aten The prototype of the Aten group of Earth-approaching asteroids that
have orbits lying mainly closer to the Sun than Earth. Aten was discovered
in 1976 by Eleanor Helin and is only about 0.8 km (0.5 mile) across.
Atlas (1) The innermost small satellite of Saturn, discovered in 1980 by Richard
Terrile during the ¤ Voyager 1 mission. It measures 37 · 34 · 27 km (23 · 21 · 17
miles) and orbits Saturn at a distance of 137 670 km (85 544 miles).
Atlas (2) A third-magnitude star in the ¤ Pleiades cluster.
atmosphere The gaseous outermost layer of a planet, moon or star. Since gas has a
natural tendency to expand into space, only bodies with gravity strong enough can
retain atmospheres. Mercury and the Moon, for example, are not massive enough
to hold on to atmospheric gases. Earth, Venus, Mars and Titan are examples of
rocky bodies with substantial atmospheres. In the giant planets, Jupiter, Saturn,
Uranus and Neptune, there is no clear boundary between the gaseous layers on the
outside and the liquid below; their ˜˜atmosphere™™ is the top layer of gas.
The more transparent outermost layers of a star are also described as an
atmospheric extinction A reduction in the brightness of an astronomical object
caused by absorption and scattering in Earth™s atmosphere. Extinction is worse
the nearer the object is to the horizon and more blue light is cut out than red,
which makes objects look redder than they really are.
atmospheric refraction A small deviation in the direction of light rays passing
through Earth™s atmosphere. Refraction makes objects seem slightly higher up
than they really are. Atmospheric refraction is greatest near to the horizon.
atmospheric window A range of wavelengths of ¤ electromagnetic radiation that
can pass through Earth™s atmosphere without much absorption, scattering or
re¬‚ection. There are two main windows: the optical window and the radio


Atmospheric windows: the colored vertical lines show how far different
kinds of radiation penetrate through Earth™s atmosphere.

In the optical (or visible) region of the spectrum, wavelengths between about
300 and 900 nm can pass through the atmosphere. This range includes some near-
ultraviolet and infrared radiation, invisible to the human eye. The radio window
covers a range of wavelengths between a few millimeters and about 30 m,
equivalent to frequencies from 100 GHz to 10 MHz. There are also some narrow
windows in the infrared (micrometer wavelengths) and submillimeter parts of the
spectrum where the atmosphere is moderately transparent to the radiation,
particularly at locations such as deserts where the atmosphere is very dry.
AU (a.u.) Abbreviation for ¤ astronomical unit.
Auriga (The Charioteer) A large and prominent northerly constellation, described
from ancient times as representing a charioteer. Its brightest star is ¤ Capella,
associated by the Greeks with the mythological she-goat Amalthea, who
nurtured the infant Zeus. The nearby triangle of fainter stars, Epsilon, Zeta and
Eta, is called ˜˜the Kids.™™ The star Elnath, formerly designated Gamma Aurigae
and shared with the neighboring constellation of Taurus, now of¬cially
belongs to Taurus as Beta Tauri.
aurora (pl. auroras or aurorae) A display of luminous colors in the night sky. The
lights often take the form of rays, moving curtains or a band in the east“west
direction. Auroras are most often seen from high-latitude regions of Earth and
the popular name for them is the northern or southern lights.
The usual cause of an aurora is a ¤ magnetic substorm, a disturbance in
Earth™s magnetic ¬eld triggered by particles from the Sun. At a height of

Australia Telescope National Facility

8h 7h 6h 5h 4h

+50 º

ψ6 ψ1 ο
π 1664
ψ4 Capella
ρ · ζ
„ µ

θ M38


Alnath β (Tau)

7h 6h 5h WIL TIRION

Magnitudes: 5 4 3 2 1 0 brighter than 0 Variable stars

Open clusters Globular clusters Planetary nebulae Bright nebulae Galaxies

A map of the constellation Auriga.

around 100 km (60 miles), high-energy particles from Earth™s ¤ magnetosphere
collide with oxygen atoms and nitrogen molecules, which then emit mainly
green and red light.
A large number of phenomena take place in the ¤ ionosphere during
auroras, such as pulsations of the ¤ geomagnetic ¬eld, electric currents in the
ionosphere and the emission of X-rays. Far more energy is emitted in the
invisible parts of the spectrum than as visible light.
Strong auroras of ultraviolet light have been observed on Jupiter and Saturn.
auroral oval An oval-shaped belt on Earth where auroras are most likely to be
seen. The two ovals lie asymmetrically around Earth™s north and south
magnetic poles. During the day, they are about 15 of latitude from the poles,
increasing to 23 during the night. When Earth™s magnetic ¬eld is disturbed,
the ovals become wider and extend farther towards the equator.
auroral zone The zones on Earth™s surface where most night-time auroras are
seen. The zones are located at latitudes of about 67 north and south, and are
about 6 wide.
Australia Telescope National Facility An Australian radio astronomy facility
opened in 1988. It consists of a number of antennas at three separate sites in New
South Wales and uses ¤ aperture synthesis to map astronomical radio sources.

Australia Telescope National Facility

The aurora (right) is triggered by charged particles blasted from the Sun (left). The center
image, taken from space, shows charged particles spreading down over the US during a large
solar storm in July 2000.

Part of the Australia Telescope National Facility, showing ¬ve of the six antennas making
up the compact array.


The Compact Array, located at the Paul Wild Observatory at Culgoora near
Narrabri, consists of six antennas, each 22 m in diameter. Five can be moved
along an east“west track, 3 km (2 miles) long. The sixth is on its own track 3 km
farther to the west.
Greater resolving power is achieved by linking one or more of the
antennas in the Compact Array with a 22-m (72-foot) dish, 100 km (60 miles) to
the south at Mopra, near the Siding Spring optical observatory, and the 64-m
(210-foot) dish at Parkes, which was completed in 1961 and is 200 km (120
miles) farther to the south. Together, these antennas form the Long Baseline
Autonoe A small outer moon of Jupiter discovered in 2001. Its diameter is about
4 km (2.5 miles).
autoguider An electronic device for guiding a telescope automatically while it is
making an observation. Even though a telescope may be driven by electric
motors to follow the stars across the sky, further minor corrections are normally
necessary to prevent the ¬eld of view drifting during a long observation. An
autoguider detects drift and activates the drive motors to compensate.
axis The imaginary line through a body about which it rotates or is symmetrical.
azimuth The bearing of an object measured as an angle around the horizon
eastwards, starting from north.

Baade, (Wilhelm Heinrich) Walter (1893“1960) Walter Baade was born in
Germany and began his astronomical career at the Bergedorf Observatory of
the University of Hamburg. While there, he discovered the unusual asteroid
¤ Hidalgo. In 1931 he moved to the ¤ Mount Wilson Observatory in California
where he worked until his retirement. He discovered a total of 10 asteroids,
including ¤ Icarus.
In the 1940s, he used the 100-inch telescope at Mount Wilson to resolve
individual stars in the ¤ Andromeda Galaxy and two of its small companion
galaxies. This led him to divide stars into two broad groups, ¤ Population I and
¤ Population II. He realized that Population I had the characteristics of young
stars while population II was older. He also found that each population had its

. 1
( 12)