The Facts On File

DICTIONARYof

ASTRONOMYFifth Edition

The Facts On File

DICTIONARYof

ASTRONOMY

Edited byJohn DaintithWilliam Gould

Fifth Edition

The Facts On File Dictionary of AstronomyFifth Edition

Copyright 1979, 1985, 1994, 2000, 2006 by Market House Books Ltd

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PREFACE

This dictionary is one of a series designed for use in schools. It is intended for stu-dents of astronomy, but we hope that it will also be helpful to other science studentsand to anyone interested in the subject. Facts On File also publishes dictionaries in avariety of disciplines, including chemistry, biology, computer science, Earth science,physics, mathematics, forensic science, weather and climate, and marine science.

This book is based on an edition first published by the Macmillan Press in 1979 andrevised in 1985, 1994, and 2000. This fifth edition has been extensively revised andextended and now contains over 3700 headwords covering the terminology of mod-ern astronomy. A totally new feature of this edition is the inclusion of over 1500 pro-nunciations for terms that are not in everyday use. A number of appendixes havebeen included containing useful astronomical data. There is also a list of Web sitesand a bibliography. A guide to using the dictionary has also been added to this lat-est version of the book.

We would like to thank all the people who have cooperated in producing this book.A list of contributors is given on the acknowledgments page. We are also grateful tothe many people who have given additional help and advice.

v

ACKNOWLEDGMENTS

Editor (previous editions)

Valerie Illingworth M.Phil.

Contributors

Peter H. Cadogan Ph.D.John O. E. Clark B.Sc.Heather Couper B.Sc., F.R.A.S.Carolin Crawford Ph.D., F.R.A.S.Charles A. Cross M.A., F.R.A.S.Tony Dean Ph.D., F.R.A.S.Peter Duffett-Smith Ph.D., F.R.A.S.Andrew Fabian Ph.D., F.R.A.S.Ian Furniss Ph.D., F.R.A.S.Peter B. J. Gill F.R.A.S.Nigel Henbest M.Sc., F.R.A.S.David W. Hughes Ph.D., F.R.A.S.Garry E. Hunt D.Sc., Ph.D., F.I.M.A., F.R.A.S.,

F.Met.S., M.B.C.S.Philip L. Marsden Ph.D., F.I.P., F.R.A.S.Rachael Padman Ph.D., F.R.A.S.Alan Pickup F.R.A.S.Phillipp Podsiadlowski Ph.D., F.R.A.S.Ken Pounds Ph.D., F.R.S., C.B.E.Gareth Rees Ph.D., F.R.A.S.Pam Spence M.Sc.Graham Woan Ph.D., F.R.A.S.Allan J. Willis Ph.D., F.R.A.S.John Zarnecki Ph.D., F.R.A.S.

Pronunciations

William Gould B.A.

vi

CONTENTS

Preface v

Acknowledgments vi

Guide to Using the Dictionary viii

Pronunciation Key x

Entries A to Z 1

Appendixes

Table 1: Planets, Orbital and 535Physical Characteristics

Table 2: Planetary Satellites 536

Table 3: Asteroids 538

Table 4: Meteor Showers 539

Table 5: Constellations 540

Table 6: Brightest Stars 541

Table 7: Nearest Stars 542

Table 8: Messier Numbers plus 543Equivalent NGC, IC Numbers

Table 9: Astronomical 545and Physical Constants

Table 10: Famous People 546in the Field of Astronomy

Table 11: The Greek Alphabet 547

Web Sites 548

Bibliography 550

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GUIDE TO USING THE DICTIONARY

The main features of dictionary entries are as follows.

HeadwordsThe main term being defined is in bold type:

apparent solar time Time measured byreference to the observed (apparent) mo-tion of the Sun, ...

PluralsIrregular plurals are given in brackets after the headword.

catena (plural: catenae) A chain ofcraters. The word is used in the approvedname of such a surface feature on a planetor satellite.

VariantsSometimes a word has a synonym or alternative spelling. This is placed in brackets afterthe headword, and is also in bold type:

sun dogs (mock suns) Two bright, dif-fuse patches of light occasionally seen inthe daytime sky, ...

Here, mock suns is another term for sun dogs. Generally, the entry for the synonymconsists of a simple cross-reference:

mock suns Another name for SUN DOGS.

AbbreviationsAbbreviations for terms are treated in the same way as variants:

Anglo-Australian Observatory (AAO)An observatory at the Siding Spring Obser-vatory site in New South Wales, Australia.

The entry for the abbreviation consists of a simple reference:

AAO Abbrev. for AngloAustralian Ob-servatory.

Multiple definitionsSome terms have two or more distinct senses. These are numbered in bold type

ADS 1. Abbrev. for Astrophysics DataSystem.2. Prefix used to designate an object aslisted in the New General Catalog of Dou-ble Stars,

viii

Cross-referencesThese are references within an entry to other entries that may give additional useful in-formation. Cross-references are indicated in two ways. When the word appears in the de-finition, it is printed in small capitals:

armillary sphere A device, dating backto antiquity, composed of a set of gradu-ated rings representing circles on the CELES-TIAL SPHERE, such as the ecliptic, celestialequator, and colures.

In this case the cross-reference is to the entry for celestial sphere.

Alternatively, a cross-reference may be indicated by See, See also, or Compare, usu-ally at the end of an entry:

C-M diagram Abbrev. for color-magni-tude diagram. See HertzsprungRusselldiagram.

Hidden entriesSometimes it is convenient to define one term within the entry for another term:

Be stars Irregular variables of spectraltype B in which bright emission lines of hy-drogen are superimposed on the normalabsorption spectrum. .... Some Be stars areyoung stars, rather more massive than AESTARS: together they are sometimes classedas Herbig Ae-Be stars, ....

Here, Herbig Ae-Be stars is a hidden entry under Be stars, and is indicated by italic type.The individual entries consists of a simple cross-reference:

Herbig Ae-Be stars See Be stars.

PronunciationsWhere appropriate pronunciations are indicated immediately after the headword, en-closed in forward slashes:

baryons /ba-ree-onz/ A class of ELEMEN-TARY PARTICLES, including the proton andneutron, that take part in strong interac-tions (see fundamental forces).

Note that simple words in everyday language are not given pronunciations. Also head-words that are two-word phrases do not have pronunciations if the component words arepronounced elsewhere in the dictionary.

ix

/a/ as in back /bak/, active /ak-tiv/// as in abduct /b-dukt/, gamma /gam-//ah/ as in palm /pahm/, father /fah-ther/,/air/ as in care /kair/, aerospace /air--

spays//ar/ as in tar /tar/, starfish /star-fish/, heart

/hart//aw/ as in jaw /jaw/, gall /gawl/, taut /tawt//ay/ as in mania /may-ni/ ,grey /gray//b/ as in bed /bed//ch/ as in chin /chin//d/ as in day /day//e/ as in red /red/// as in bowel /bow-l//ee/ as in see /see/, haem /heem/, caffeine//kaf-een/,/ baby /bay-bee//eer/ as in fear /feer/, serum /seer-m//er/ as in dermal /der-ml/, labour /lay-ber//ew/ as in dew /dew/, nucleus /new-klee-s//ewr/ as in epidural /ep-i-dewr-l//f/ as in fat /fat/, phobia /foh-bi/, rough

/ruf//g/ as in gag /gag//h/ as in hip /hip//i/ as in fit /fit/, reduction /ri-duk-shn//j/ as in jaw /jaw/, gene /jeen/, ridge /rij//k/ as in kidney /kid-nee/, chlorine /klor-

een/, crisis /kr-sis//ks/ as in toxic /toks-ik//kw/ as in quadrate /kwod-rayt//l/ as in liver /liv-er/, seal /seel//m/ as in milk /milk//n/ as in nit /nit//ng/ as in sing /sing/

/nk/ as in rank /rank/, bronchus /bronk-s//o/ as in pot /pot/// as in dog /dg//o/ as in buttock /but-k//oh/ as in home /hohm/, post /pohst//oi/ as in boil /boil//oo/ as in food /food/, croup /kroop/, fluke

/flook//oor/ as in pruritus /proor--tis//or/ as in organ /or-gn/, wart /wort//ow/ as in powder /pow-der/, pouch

/powch//p/ as in pill /pil//r/ as in rib /rib//s/ as in skin /skin/, cell /sel//sh/ as in shock /shok/, action /ak-shn//t/ as in tone /tohn//th/ as in thin /thin/, stealth /stelth//th/ as in then /then/, bathe /bayth//u/ as in pulp /pulp/, blood /blud/// as in typhus /t-fs/// as in pull /pl/, hook /hk//v/ as in vein /vayn//w/ as in wind /wind//y/ as in yeast /yeest/// as in bite /bt/, high /h/, hyperfine /h-

per-fn//yoo/ as in unit /yoo-nit/, formula /form-yoo-l//yoor/ as in pure /pyoor/, ureter /yoor-ee-ter//r/ as in fire /fr//z/ as in zinc /zink/, glucose /gloo-kohz//zh/ as in vision /vizh-n/

x

Bold type indicates a stressed syllable. In pronunciations, a consonant is sometimes dou-bled to prevent accidental mispronunciation of a syllable resembling a familiar word; forexample, /ass-id/ (acid), rather than /as-id/ and /ul-tr- sonn-iks/ (ultrasonics), rather than/ul-tr-son-iks/. An apostrophe is used: (a) between two consonants forming a syllable, asin /den-tl/ (dental), and (b) between two letters when the syllable might otherwise be mis-pronounced through resembling a familiar word, as in /the-r-pee/ (therapy) and /talk/(talc). The symbols used are:

Pronunciation Key

A0620-00 A possible low-mass BLACKHOLE in the constellation Monoceros. Seealso X-ray transients.

AAO Abbrev. for AngloAustralian Ob-servatory.

AAT Abbrev. for Anglo-Australian tele-scope.

Abell Catalog /ay-bl/ The standardcatalog of rich CLUSTERS OF GALAXIES visiblefrom the northern hemisphere. It was pub-lished by George Abell in 1958. A supple-ment for the southern hemisphere waspublished by Abell and others in 1988.

aberration 1. (aberration of starlight).The apparent displacement in the positionof a star because of the finite speed of lightand to the motion of the observer, whichresults primarily from the Earths orbitalmotion around the Sun. It was discoveredin 1729 by the English astronomer JamesBradley. Light appears to approach the ob-server from a point that is displacedslightly in the direction of the Earths mo-tion. The angular displacement, , is givenby the relation tan = v/c, where v is theEarths orbital velocity and c is the speed oflight. Using the Earths mean orbital speedgives the constant of aberration, equal to20.4955 arc seconds. Over the course of ayear, the star appears to move in a small el-lipse around its mean position; the ellipsebecomes a circle for a star at the pole of theECLIPTIC and a straight line for one on theecliptic. The maximum displacement, i.e.the semimajor axis of the ellipse, is 20.5 arcseconds.

The aberration due to the Earths or-bital motion is sometimes termed annualaberration to distinguish it from the very

much smaller diurnal aberration that re-sults from the Earths rotation on its axis.Compare annual parallax.2. A defect in the image formed by a lens orcurved mirror, seen as a blurring and pos-sible false coloration in the image. Aberra-tions occur for all light rays lying off theoptical axis and also for those falling atoblique angles on the lens or mirror sur-face. The four principal aberrations areCHROMATIC ABERRATION (lenses only),SPHERICAL ABERRATION, COMA, and ASTIG-MATISM. CURVATURE OF FIELD and DISTOR-TION are other aberrations. Chromaticaberration occurs when more than onewavelength is present in the incident lightbeam. For light of a single wavelength,only the latter five aberrations occur.These image defects may be reduced butnot completely eliminated in an opticalsystem by a suitable choice of optical ma-terials, surface shape, and relative posi-tions of optical elements and stops. See alsoachromatic lens; correcting plate.3. A defect in the image produced by anelectronic system using magnetic or elec-tronic lenses.

ablation /ab-lay-shn/ The loss of ma-terial from the surface of a moving body asa result of vaporization, friction, etc. Forexample, atmospheric atoms and mol-ecules erode the surface of a meteoroid anddamage the protective heat shield of a re-turning space shuttle.

absolute magnitude See magnitude.

absolute zero The zero value of theTHERMODYNAMIC TEMPERATURE SCALE, i.e.0 K (273.15 C). Absolute zero is the low-est temperature theoretically possible. Atabsolute zero molecular motion almost

1

A

absorption

2

absolute zero molecular motion almostceases.

absorption The conversion of all orpart of the energy incident on a materialmedium into some other form of energywithin the medium. For example, part ofthe energy of incident light or infrared ra-diation may be used in exciting the atomsor molecules of the absorbing substance.

absorption lines, bands See absorp-tion spectrum.

absorption nebula See dark nebula.

absorption spectrum A SPECTRUM thatis produced when ELECTROMAGNETIC RADI-ATION has been absorbed by matter. Typi-cally, absorption spectra are producedwhen radiation from an incandescentsource, i.e. radiation with a CONTINUOUSSPECTRUM, passes through cooler matter.Radiation is absorbed (i.e. its intensity isdiminished) at selective wavelengths sothat a pattern of very narrow dips or ofwider troughs i.e. absorption lines orbands are superimposed on the continu-ous spectrum.

The wavelengths at which absorptionoccurs correspond to the energies requiredto cause transitions of the absorbing atomsor molecules from lower ENERGY LEVELS tohigher levels. In the hydrogen atom, for ex-ample, absorption of a photon with the re-quired energy results in a jump of theelectron from its normal orbit to one ofhigher energy (see hydrogen spectrum).

The absorption lines (or bands) of a starare produced when elements (or com-pounds) in the outermost layers of the starabsorb radiation from a continuous distri-bution of wavelengths generated at a lowerlevel in the star. Basically the same el-ements occur in stars. Since each elementhas a characteristic pattern of absorptionlines for any particular temperature (andpressure) range, there are several differenttypes of stellar spectra depending on thesurface temperature of the star. See spec-tral types. See also emission spectrum.

abundance The relative proportion of

each element, or of each isotope of an el-ement, found in a celestial object or struc-ture. See cosmic abundance.

Acamar /ay-k-mar/ See Eridanus.

acceleration 1. (linear acceleration)Symbol: a. The rate of increase of velocitywith time, measured in meters per secondper second (m s2), etc.2. (angular acceleration) Symbol: . Therate of increase of ANGULAR VELOCITY withtime, usually measured in radians per sec-ond per second.

acceleration of gravity Symbol: g. Theacceleration to the center of a planet (orother massive body such as a natural satel-lite) of an object falling freely without airresistance, i.e. acceleration due to down-ward motion in a gravitational field. It isequal to GM/R2, where G is the gravita-tional constant and M and R are the massand radius of the planet. The accelerationis thus independent of the mass of the ac-celerated object, i.e. it is the same for allbodies (neglecting air resistance) falling atthe same point on the surface of the planet,satellite, etc.

On Earth the value of the accelerationof gravity is about 9.81 meters per secondper second. The value varies from place toplace on the Earths surface because of dif-ferent distances to the Earths center andgreater acceleration toward the equator. Inaddition, it is affected by local deposits oflight or heavy materials. See also micro-gravity; weight; weightlessness.

accretion (aggregation) The increase inmass of a body by the addition of smallerbodies that collide and stick to it. The rela-tive velocity of any two colliding bodiesmust be low enough for them to coalesceon impact rather than fly apart. Once alarge enough body forms, its gravitationalattraction accelerates the accretionprocess. Accreting objects in the Universeare numerous and diverse. They includeprotoplanets, protostars, black holes, andX-ray binaries. The accretion process isthought to occur generally in the form of adisk. Accretion is now assumed to have

had an important role in the formation ofthe planets from swarms of dust grains. Inthe outer Solar System the grains were likedirty snowflakes and thus accretion wasaccelerated. See Solar System, origin.

accretion disk See black hole; masstransfer; quasar.

ACE Abbrev. for Advanced Composi-tion Explorer.

Achernar /ay-ker-nar/ ( Eri) A conspic-uous bluish-white star that is the brightestin the constellation Eridanus. mv: 0.5; Mv:1.3; spectral type: B3 Vnp; distance: 22pc.

Achilles ((588) Achilles) The first mem-ber of the TROJAN GROUP of ASTEROIDS to bediscovered, by Maximilian Wolf, in 1906.It lies east of Jupiter and precedes the latterin its orbit of the Sun by 60. See TABLE 3,BACKMATTER.

achondrite /ay-kon-drt/ Any of a classof STONY METEORITES that lack the charac-teristic CHONDRULES of the CHONDRITES.They are usually more coarsely crystallizedthan the chondritic stones and are moresimilar chemically and mineralogically tosome terrestrial rocks. They contain verylittle iron and nickel. Achondrites resemblevolcanic rocks and are thought to be prod-ucts of large-scale melting on their parentbodies, i.e. they are differentiated or re-processed matter. See also HED mete-orites; SNC meteorites.

achromatic lens /ak-r-mat-ik/ (achro-mat) A two-element lens a doublet that greatly reduces CHROMATIC ABERRA-TION in an optical system. The compo-nents, one converging and the otherdiverging in action, are of different types ofglass (i.e. they have different REFRACTIVEINDICES); the combination focuses two se-lected colors, say red and blue, at a com-mon image plane with a small spread infocal length for other colors. The differ-ence in optical power (reciprocal of focallength) for the two colors in one elementmust cancel that in the other element. By a

suitable choice of glasses and surface cur-vatures, the doublet can be aplanatic aswell as achromatic, so that three majoraberrations are minimized (see aplanaticsystem).

Residual color effects in an achromatcan be further reduced by using a com-pound lens of three or more elements anapochromatic lens; each element has an ap-propriate shape and dispersive power sothat three or more colors can be focused inthe same image plane.

Acidalia Planitia /ass--day-lee-/ (for-merly Mare Acidalium) The most promi-nent dark marking in the northernhemisphere of MARS; it is a low-lying areamore than 2615 km in diameter locatedjust below Mars north pole and centeredon the areographic coordinates 47 N lati-tude and 22 W longitude (see areogra-phy). Much of the region seems to bevolcanic in origin and is thought to becovered by black sand resulting from theerosion of dark basalts. There is a prepon-derance of rampart craters (see Craters) inthe area, and ice lies just below the surface.In Mars distant past, Acidalia Planitiamay have been the receptacle for water dis-charged from adjacent major outflowchannels such as Ares Valles. See alsoMars, surface features.

acoustic wave A pressure wave trans-mitted through a gas, liquid, or solid as aresult of small displacements of the parti-cles in the medium. The medium has elas-ticity and inertia; when set into motion itundergoes alternate compressions and rar-efactions that travel through the mediumwith a speed characteristic of the medium.

acronical rising /-kron--kl/ (or setting)The rising (or setting) of a star at or justafter sunset.

Acrux /ay-kruks/ See Alpha Crucis.

actinometer /ak-t-nom--ter/ An in-strument for measuring the intensity of ra-diation.

3

actinometer

active galactic nucleus (AGN) See ac-tive galaxy.

active galaxy A galaxy that is emittingunusually large amounts of energy from avery compact central source hence the al-ternative name active galactic nucleus orAGN. (A separate category, STARBURSTGALAXY, is employed for a galaxy where ahigh infrared luminosity arises from in-tense star formation.) The central power-house may be observed directly, as inSEYFERT GALAXIES, BL LAC OBJECTS, orQUASARS; in RADIO GALAXIES it is the radio-emitting lobes created by BEAMS emanatingfrom the powerhouse that are observed. Ingeneral the host galaxies of powerful AGNare large, luminous elliptical galaxies,whereas the hosts of Seyferts are spiral. Al-though some radio-quiet QSO are found lo-cated in disklike hosts, most are found tolie in elliptical galaxies.

Observations of the motions of starsand gas in galaxies such as M87 (see VirgoA) and NGC 4151 (see Seyfert galaxy), inaddition to other arguments (see quasar;power-law spectrum), strongly suggest thatthe energy output is derived from the grav-itational potential of a supermassive BLACKHOLE: the energy arises from an accretiondisk of matter spiraling into the black hole.This material could come from the inter-stellar medium of a spiral galaxy (espe-cially when perturbed by gravitationaleffects in INTERACTING GALAXIES), from thetidal disruption of stars near the blackhole, or from flows of intergalactic gas onto the central galaxy of a CLUSTER OFGALAXIES, as the gas cools. The X-ray CON-TINUUM spectrum of many AGN featurescomponents of emission that are thoughtto be reflected off the accretion disk of theblack hole, producing a reflection spec-trum.

active optics The techniques by whichcorrections may be made to the shape of alarge mirror or radio dish to adjust forminute-long or hour-long drifts from itsdesigned shape. These variations in shapearise as a telescope is subjected to slowlychanging forces, including the effects ofgravity on different telescope orientations,

temperature changes within and exterior tothe structure, and wind; they result in animperfect image and are particularly severein large telescopes. With active optics, theshape of the reflector is adjusted to give ahighly accurate surface and so maintain thequality of the image; in a segmented PRI-MARY MIRROR, for example, the position,spacing, and tilt of individual segments canbe controlled, while in a thin monolithicmeniscus mirror, forces are applied at nu-merous positions on the mirror back tocounteract naturally occurring changes inshape. Active optics are used on the latestgeneration of giant telescopes, includingthe 3.5-m NTT of the European SouthernObservatory and the 10-m Keck Tele-scopes. An analogous system, in which thepanels making up the receiving surface of aradio telescope can be individually con-trolled by actuators in order to change theoverall shape of the dish, is in use on suchinstruments as the 100-m Green BankTelescope. See also adaptive optics.

active prominence See prominences.

active-region filament See prominences.

active regions Regions of intense local-ized magnetic field on the Sun that extendfrom the PHOTOSPHERE, through the CHRO-MOSPHERE, to the CORONA. They may en-compass a variety of phenomena, such asSUNSPOTS, FACULAEPLAGES, FILAMENTS (orPROMINENCES), and FLARES, and are charac-terized by enhanced emission of radiationat X-ray, extreme-ultraviolet, and radiowavelengths.

active Sun The term applied to the Sunaround the maximum of the SUNSPOTCYCLE, when the profusion of ACTIVE RE-GIONS ensures a high level of SOLAR ACTIV-ITY. Compare quiet Sun.

ADAF Short for advection-dominatedaccretion flow. See black hole.

adaptive optics The techniques bywhich corrections may be made veryrapidly (within hundredths of a second) tothe shape of a mirror in order to adjust for

active galactic nucleus

4

distortions in a telescope image arisingfrom turbulence in the Earths atmosphere.The effects of SEEING on the image are thusgreatly reduced or removed. Adaptive op-tics are being applied to new and modern-ized telescopes to increase the sensitivityand spatial RESOLUTION of the telescope,and should allow near-diffraction-limitedimaging over the full aperture of large op-tical and infrared telescopes (see Airy disk).Techniques have been developed to moni-tor the atmospheric disturbance on theimage of a bright reference star, or on anartificial reference star (or beacon), and tomake rapid compensating adjustments tothe shape of a small thin deformable mirrorin the light path of the telescope. See alsoactive optics.

Adhara /-day-rah, -dah-/ ( CMa) Avery luminous remote blue-white giant thatis the second brightest star in the constella-tion Canis Major. It is a visual binary starwith an 8th-magnitude companion at afixed separation of 8. mv: 1.5; Mv: 4.8;spectral type: B2 II; distance: 175 pc.

adiabatic process /ad-ee--bat-ik/ Aprocess that takes place in a system with noheat transfer to or from the system. In gen-eral, a temperature change usually occursin an adiabatic process.

Adonis /-don-is, -doh-nis/ ((2101)Adonis) A member of the APOLLO GROUPof ASTEROIDS. Discovered in 1936 by Eu-gne Joseph Delporte, when it passed0.015 AU from the Earth, it was not ob-served again until it was rediscovered in1977 by Chartles T. Kowal. Its PERIHELIONDISTANCE is 0.51 AU and it has a diameterof about 1 km. See Table 3, backmatter.

Adrastea /-dras-tee-/ A small irregu-larly shaped satellite of Jupiter, one of theplanets inner group of satellites, discov-ered in 1979. See Jupiters satellites; Table2, backmatter.

ADS 1. Abbrev. for Astrophysics DataSystem.2. Prefix used to designate an object aslisted in the New General Catalog of Dou-

ble Stars, 1932, a catalog of 17 180 doublestars in the northern and equatorial skies,between DECLINATIONS +90 and 30. Thecatalog was complied under the directionof the US astronomer Robert G. Aitken(the prefix is short for Aitkens DoubleStars).

Advanced Composition Explorer(ACE) A NASA satellite, launched in1997 to measure the isotope and elementABUNDANCES in the solar corona and COS-MIC RAYS over a wide range of energies. It isin orbit around L1, one of the Lagrangianpoints of the SunEarth system, 1.5 millionkm nearer to the Sun than Earth is.

Advanced X-ray Astrophysics FacilitySee AXAF.

advance of the perihelion The gradualmovement of the PERIHELION of a planetselliptical orbit in the same direction as thatof the planets orbital motion. This ad-vance results from the slow rotation of themajor axis of the planets orbit due to grav-itational disturbances by other planets andto the curvature of SPACETIME around theSun. The small contribution from the cur-vature of spacetime was predicted by Ein-steins general theory of RELATIVITY.

The value of this relativistic contribu-tion toward the advance of the perihelionof Mercury is about 43 arc seconds percentury. This agrees almost exactly withthe discrepancy between the experimen-tally determined value for the advance andthat predicted by classical Newtonian me-chanics. It was therefore an important con-firmation of general relativity. Recentmeasurements of the advance of the peri-helia of Venus and Earth have also beenvery close to Einsteins predicted values forthose planets.

aeon A period of one thousand millionyears: 109 years.

aerial See antenna.

aeronomy /air-on--mee/ The physicsand chemistry of the upper atmosphere ofthe Earth, i.e. its temperature, density, mo-

5

aeronomy

tions, composition, chemical processes, re-actions to solar and cosmic radiation, etc.The term has been extended to include thephysics and chemistry of the atmospheresof the other planets.

aerospace The Earths atmosphere andthe space beyond.

Ae stars /ay-ee/ Hot stars of SPECTRALTYPE A that in addition to the normal ab-sorption spectrum have bright emissionlines of hydrogen. These lines are thoughtto arise in an expanding atmospheric shellof matter lost from the star. Like some BE STARS they are probably similar to TTAURI STARS except that they have largermasses.

afocal system /ay-foh-kl/ An opticalsystem in which both the object and thesecondary image are infinitely distant. It isthe usual adjustment in a simple refractingtelescope and is produced when the objec-tive and eyepiece lenses are separated bythe sum of their focal lengths: the lenses arethen confocal.

AG Catalog See AGK.

Agena /-jee-n/ See Centaurus.

age of the Earth The oldest rocks foundin the Earths crust have been assigned agesof 3.96 billion (109) years from radioiso-tope studies, but other properties of therocks support the belief that the planetshares a common origin with the rest of theSolar System, about 4.57 billion years ago.The cycling of the crustal material by platetectonics (see Earth) and the eroding effectsof ice, water, and wind mean that none ofthe earliest rocks survive intact and that ev-idence of an early meteoritic bombard-ment, so apparent on the Moon, Mercury,and Mars, is absent. See Earth; Solar Sys-tem, origin.

age of the Universe The observed ex-pansion and evolution of the Universe sug-gest that it has a finite age, considered asthe time since the BIG BANG. The inverseHUBBLE CONSTANT, 1/H0, gives a measure of

the age if the rate of expansion has alwaysbeen constant. Since gravitation tends todiminish the expansion rate, H0 can onlygive an upper limit. Using the value of H0of 75 km s1 Mpc1 gives an upper limit of13 billion (109) years.

In the standard COSMOLOGY (solutionsof Einsteins field equations without a cos-mological constant) with DECELERATION PA-RAMETER q0, the age is given as one of thethree alternatives:

H01q0(2q0 1)3/2[cos1(q01 1) q01(2q0 1)1/2]

2/3H01

H01q0(1 2q0)3/2[q01(1 2q0)1/2 cosh1(q01 1)]

The choice depends on whether q0 exceeds,equals, or is less than (and > 0), i.e. onwhether the Universe is closed, flat, oropen, respectively. Ages of 12 and 15 109years thus correspond to values of q0 of and 0.15, for H0 equal to 55 km s1 Mpc1.

Lower limits to the age of the Universe,other than through measurements of H0and q0, can be found from RADIOMETRICDATING of the Earth and Galaxy and fromstudies of GLOBULAR CLUSTERS. For exam-ple, the relative abundances of radioactiveelements, such as uranium, and their decayproducts yield an estimate of the time sinceformation of that body of material e.g., theEarth. The results give an age for the Uni-verse of 1416 109 years. The age of aglobular cluster may be estimated by com-parison of the observed main-sequenceTURNOFF POINT in the HertzsprungRusselldiagram for the cluster with theoreticalmodels. The oldest globular clusters in ourGalaxy then work out to be 1418 109years old. Both of these ages should be lessthan the age of the Universe and in partic-ular they must be less than H01. Resultsfrom WMAP give an age for the Universeof 13.7 109 years.

aggregation See accretion.

AGK (AG Catalog) Abbrev. for As-tronomische Gesellschaft Katalog. A gen-eral catalog of star positions originallyproposed by the Prussian astronomerFriedrich Argelander in 1867. Argelanderwas the founder (in 1863) of the As-

aerospace

6

tronomische Gesellschaft (AstronomicalSociety), which gave its name to the AGK.The first version, known as AGK1, cov-ered the northern sky and by 1912 DECLI-NATION zones between +90 and 18 hadbeen published. A revision, known asAGK2, was based on photographic meas-urements begun in the 1920s and was pub-lished 195158. A further set ofphotographic plates was produced in the1950s to determine PROPER MOTIONS,which appeared in the AGK3 version; thislists 183 145 stars and was distributed onmagnetic tape from 1969, finally appear-ing in print in 1975. AGK3R (197778)gave more accurate positions of 20 194northern-hemisphere reference stars, whilean updated version, AGK3U (1992), usingphotographic plates supplied for the HSTGUIDE STARS CATALOG in 1983, providedrefinements to the positions of 170 464stars.

AGN Abbrev. for active galactic nu-cleus. See active galaxy.

Ahnighito meteorite /ah-n-gee-toh/ The biggest (59 tonnes) of the three CapeYork iron METEORITES, now on display inNew York. It was brought back from thecoast of west Greenland in 1897 by RobertPeary.

AIPS Abbrev. for Astronomical Image-Processing System. A suite of computerprograms produced by the NATIONAL RADIOASTRONOMY OBSERVATORY (NRAO) cover-ing over 300 astronomical data processing

tasks. Designed primarily for radio astron-omy, AIPS can accept APERTURE SYNTHESISand VLBI data, process them to form imagesof the sky, and improve their quality usingDECONVOLUTION methods. It can also dis-play, analyze, and combine images. Dataare usually transferred to and from AIPSusing the FITS file format. The AIPS++ pro-ject is a more modern object-orientatedpackage, written in C++, intended to su-persede classic AIPS.

airglow /air-gloh/ (nightglow) The fainteverpresent glow arising in the Earths at-mosphere that is light emitted (along withinfrared radiation) during the recombi-nation of ionized atoms and moleculesfollowing collisions with high-energy parti-cles and radiation, mainly from the Sun.Airglow interferes with optical and in-frared observations of faint celestial bod-ies.

air shower See cosmic rays.

Airy disk /air-ee/ The bright disklikeimage of a point source of light, such as astar, as seen in an optical system with a cir-cular APERTURE. The disk is formed by DIF-FRACTION effects in the instrument and issurrounded by faint diffraction rings thatare only seen under perfect conditions (seeillustration). The disk diameter, first calcu-lated by George Airy in 1834, is the factorlimiting the angular RESOLUTION of the tele-scope.

Aitken /ayt-kin/ See table at craters.

7

Aitken

lightintensity

Light distribution in Airy disk image of single point source (left) and two just resolvable point sources

AI Velorum stars /vee-lor-m, -loh-rm/See dwarf Cepheids.

Alba Patera /al-b/ Probably the largestcentral-vent volcanic structure on Mars.Located north of the THARSIS RIDGE, it iscentered on the aerographic coordinates40 N latitude, 109 W longitude (see are-ography). It has a low dome containing acentral caldera and enclosed by a ring offractures 600 km in diameter. Associatedlava flows cover an area more than 1500km across. See Mars, volcanoes.

albedo /al-bee-doh/ The reflectingpower of a nonluminous object, such as aplanet or planetary surface feature. In gen-eral, it is the ratio of the total amount oflight reflected in all directions to theamount of incident light: an albedo of 1.0indicates a perfectly reflecting surfacewhereas a value of 0.0 indicates a totallyblack surface that absorbs all incidentlight.

Albedo can be expressed in severalways. The Bond albedo is the fraction ofthe total incident energy that a body re-flects in all directions. It is calculated overall wavelengths and its value therefore de-pends on the spectrum of the incident radi-ation. This parameter determines theenergy balance of a body such as a planet.The geometrical albedo is the ratio of thelight reflected in the backscattering direc-tion (zero phase angle or opposition) by anobject, to that which would be reflected bya perfectly diffusing disk of the same size.The wavelength or range of wavelengths atwhich the geometrical albedo applies mustbe defined. Geometrical albedo is com-monly used for Solar System objects. Thehemispherical albedo is the reflectingpower of a nonluminous body such as aplanet, assuming the body is a sphere witha diffuse surface reflecting incoming paral-lel light rays in all directions. The phase in-tegral is the ratio of the Bond albedo to thegeometrical albedo averaged over the inci-dent spectrum. See also Table 1, backmat-ter.

Albert /al-bert/ ((719) Albert) A smallasteroid (2.6 km across) discovered by J.

Palisa when it approached the Earthclosely in 1911, but since lost.

Albireo /al-beer-ee-oh/ ( Cyg) A beauti-ful double star, the second brightest star inthe constellation Cygnus. The primary isan orange giant with a deep blue compan-ion 35 away. mv: 3.1 (A), 5.1 (B); spectraltype: K5 II (A), B8 V (B).

Alcaid /al-kayd/ (Benatnasch; UMa)A blue-white star that is one of the sevenbrightest in the BIG DIPPER. mv: 1.86; spec-tral type: B3 V; distance: 34 pc.

Alcor /al-kor/ (80 UMa) A white starthat lies in the handle of the BIG DIPPERand forms an optical double with MIZAR.mv: 4.0; spectral type: A5 V.

Alcyone /al-see--nee, -s-/ ( Tau) Abluish-white giant star in the constellationTaurus that is the brightest star in thePLEIADES. mv: 2.86; Mv: 1.5; spectral type:B7 IIIe.

Aldebaran /al-deb--rn/ ( Tau) Aconspicuous red giant that is the brighteststar in the constellation Taurus and lies inthe line of sight of but much nearer thanthe Hyades. It is a slow irregular variable.It has two companions: one of 11th magni-tude at 122 separation, the other of 13thmagnitude at 30. mv: 0.85 (var.); Mv:0.3; spectral type: K5 III; radius (by inter-ferometer): 45 times solar radius; distance:18 pc.

Alderamin /al-de-r-min/ See Cepheus.

Alfvns theory /al-venz/ A theory pro-posed by the Swedish physicist HannesAlfvn in 1942 for the formation of theplanets out of material captured by the Sunfrom an interstellar cloud of gas and dust.As atoms fall toward the Sun, they becomeionized and subject to the control of theSuns magnetic field. Ions are concentratedin the plane of the solar equator where theplanets coalesce. The theory, in its originalform, had difficulty in accounting for theinner planets, but was important in sug-gesting the role of MAGNETOHYDRODYNAM-

AI Velorum stars

8

ICS in the genesis of the Solar System. SeeSolar System, origin.

Alfvn waves /al-ven/ Disturbancestransmitted through a PLASMA in the pres-ence of a magnetic field. The direction ofpropagation is parallel to the mean mag-netic field, with the plasma particles vi-brating at right angles to this direction. Thespeed of propagation, the Alfvn speed, de-pends on the magnetic field strength andthe plasma density. The waves interactwith the plasma particles, for example byexerting radiation pressure on the plasma.As the waves dissipate, the plasma particlesare heated and accelerated. Alfvn wavesare a type of magnetohydrodynamic(MHD) wave. They have been directly ob-served in the SOLAR WIND, particularly inthe high-speed streams, and in planetarymagnetospheres.

Algenib /al-jee-nib/ ( Peg) A remotebluish-white subgiant that is one of thebrighter stars of the constellation Pegasusand lies on the Great Square of Pegasus. Itis a variable star with a very short period ofabout 4 hours, during which its visual mag-nitude ranges between 2.80 and 2.87. mv:2.83 (var.); Mv: 3.1; spectral type: B2 IV;distance: 153.5 pc. Algenib is also an oldname for the star MIRFAK.

Algieba /al-jee-b/ ( Leo) An orangegiant that is the second-brightest star in theconstellation Leo. It is a multiple star,forming a visual binary (separation 4.4,

period 619 years) with a 3rd-magnitudeyellow companion. mv: 2.2 (A), 1.84 (AB);Mv: 0.5; spectral type: K0p III (A), G7 III(B); distance: 32 pc.

Algol /al-gol/ (Demon Star; WinkingDemon; Per) A white star that is thesecond-brightest one in the constellationPerseus. It was the first ECLIPSING BINARY tobe discovered, being the prototype of theALGOL VARIABLES, although its variations inbrightness were known to early as-tronomers. The theory of a darker com-panion periodically cutting off the light ofthe brighter star (Goodricke, 1782; Picker-ing, 1880) was confirmed spectroscopi-cally in 1889. The brighter star (Algol A) isabout three times the Suns diameter; thefainter orange 3rd-magnitude companion(Algol B) is about 20% larger. The twostars revolve about one another in a periodof 68.8 hours, and the eclipses cause themagnitude to drop from 2.2 to 3.5 (see il-lustration). There is also a third more dis-tant star, Algol C, which orbits Algol Aand B in 1.86 years.

Algol A is about 3.7 times as massive asthe Sun, while Algol B has a mass of only0.8 solar masses. According to STELLAREVOLUTION theory, a more massive starevolves more rapidly; yet in the Algol sys-tem the more massive Algol A is still amain-sequence star while Algol B hasevolved to become a subgiant. This is theAlgol paradox, which is explained by slowand continuous MASS TRANSFER from AlgolB to Algol A (see Algol variables). This

9

Algol

0 10 20 30 40 50 60 70

2.0

2.4

2.8

3.2

3.6

_

_

_

_

__ _ _ _ _ _ _ _ _

hours

period 68.82 hoursperiod 68.82 hoursperiod 68.82 hours

abso

lute

mag

nit

ud

e

Light curve of Algol

mass transfer, along with APSIDAL MOTION,accounts for slight changes in the time ofAlgols eclipses. The streams of gas passingfrom Algol B to Algol A make Algol an er-ratic radio and X-ray source. mv: 2.1 (A),3.5 (B); Mv: 0.2 (A), 1.2 (B); spectral type:B8 V (A), K0 IV (B); distance: 29 pc.

Algol paradox See Algol variables;Algol.

Algol variables (Beta Persei stars) Asubclass of ECLIPSING BINARY stars, namedafter ALGOL, where the brighter and moremassive star is still on the MAIN SEQUENCEwhile the less massive companion hasevolved more and has become a SUBGIANT.This seemingly contradicts the theory ofstellar evolution, which predicts that moremassive stars evolve more rapidly, and isknown as the Algol paradox. It is ex-plained (Crawford 1955) as a result ofextensive MASS TRANSFER: the now less-massive star originally contained most ofthe systems mass, and it evolved rapidlybeyond the main sequence. As it expanded,this star lost up to 85% of its mass to thecompanion (see W Serpentis star) to end upas a faint low-mass subgiant, while thecompanion became a massive hot and bril-liant star, still on the main sequence. Masstransfer continues at a very slow rate inAlgol systems, causing variations in the or-bital period and feeble radio and X-rayemission.

As a result of the mass transfer, the twostars have the unusual property of beingroughly the same size (several times largerthan the Sun) but having very different lu-minosities. They thus have a LIGHT CURVEcharacterized by deep primary minimawhen the dim subgiant eclipses the brightmain-sequence star, alternating withscarcely detectable secondary minimawhen the subgiant is eclipsed.

Alhena /al-hee-n/ See Gemini.

alidade /al--dayd/ See astrolabe.

alignment effect See radio galaxy.

Alioth /al-ee-oth/ ( UMa) A white SPEC-

TROSCOPIC BINARY, period 4.15 years, thatis the brightest star in the constellationUrsa Major. It is a SPECTRUM VARIABLE star.mv: 1.78; spectral type: A0p; distance: 19pc. See also Big Dipper.

Alkaid /al-kayd/ See Alcaid.

Allegheny Observatory An observa-tory sited in Riverview Park, in the city ofPittsburgh, Pennsylvania, USA, and oper-ated by the Physics and Astronomy De-partment of the University of Pittsburgh.Founded in 1859, the observatory wastaken over by the university in 1867 andwas fully established on its present site by1912. The observatory has three telescopeshoused in three separate domes. The Fitz-Clark Refractor, a 33.02-centimeter instru-ment, was the observatorys originaltelescope, purchased in 1861; it has a focallength of 4.62 meters and a focal ratio off/14. The William Thaw Memorial Refrac-tor (aperture 76.2 cm, focal length 14.3 m,focal ratio f/18.8) was built in 1912 anddesigned for photographic work. In 1985its object lens was upgraded to focus redlight, a region of the spectrum in which thePittsburgh skies are still relatively clear.The James E. Keeler Memorial Reflector(aperture 73.7 cm, focal length 4.56 m,focal ratio f/6) is the observatorys main in-strument. Built in 1905 as a Cassegraintelescope, it was originally used for spec-troscopy. In 1992 its mirrors were replacedwith ones made from a Russian version ofCER-VIT and its optics were upgraded to anf/15 RitcheyChrtien system (see RitcheyChrtien optics). Among the observatorysother instruments is a nulti-channel astro-metric photometer (MAP), designed byGeorge A. Gatewood, the institutionsdirector from 1977. The Allegheny Obser-vatory is today a world leader in high-precision astrometry. Its measurements arebeing used most significantly in the searchfor extrasolar planetary systems.

Allende /ah-yen-day/ See meteorite.

ALMA /al-m/ See Atacama Large Mil-limeter Array.

Algol paradox

10

Almach /al-mak/ ( And) An orangegiant that is the third-brightest star in theconstellation Andromeda. It is a triple star,forming a very fine visual binary with itsapparently greenish companion (B), 10distant, with which it has a commonproper motion; B is a spectroscopic binary,period 61 years. mV: 2.28 (A), 5.1 (B), 2.16(AB); spectral type: K3 II (A), A0p (B).

Almagest /al-m-jest/ (Arabic: theGreatest) An astronomical work compiledby Ptolemy of Alexandria in about AD 140.It was translated from the original Greekinto Arabic in the 9th century and becameknown in Europe when it was translatedfrom Arabic into Latin in the late 12th cen-tury. Its 13 volumes cover the whole of as-tronomy as conceived in ancient times,with a detailed description of the PTOLE-MAIC SYSTEM of the Solar System. It also in-cluded a star catalog giving positions andMAGNITUDES (from 1 to 6) of 1022 stars.This catalog was based mainly on the oneproduced in the 2nd century BC by Hip-parchus of Nicaea.

almucantar /al-my-kan-ter/ A SMALLCIRCLE on the CELESTIAL SPHERE parallel tothe horizon.

Alnath /al-nath/ See Elnath.

Alnilam /al-n-lm/ See Orion.

Alnitak /al-n-tak/ See Orion.

alpha /al-f/ () 1. The first letter of theGreek alphabet, used in STELLAR NOMEN-CLATURE usually to designate the brighteststar in a constellation or sometimes to in-dicate a stars position in a group.2. Symbol for right ascension.

Alpha Capricornids See Capricornids.

Alpha Centauri /sen-tor-, -ee/ (RigilKentaurus; Cen) A binary star that isthe brightest star in the constellation Cen-taurus, one of the brightest in the sky, andthe second nearest star to the Sun. The twocomponents, A and B, form a yellow-or-ange VISUAL BINARY (separation 17.7, pe-

riod 80.1 years) and are similar in massand size to the Sun. PROXIMA CENTAURI ap-pears to be physically associated with Cen; it is the nearest star to the Sun. mv:0.01 (A), 1.33 (B), 0.27 (AB); Mv: 4.4(A), 5.7 (B), 4.1 (AB); spectral type: G2 V(A), K1 V (B); mass: 1.09 (A), 0.89 (B)times solar mass; distance: 1.33 pc.

Alpha Crucis /kroo-sis/ (Acrux; Cru) A conspicuous white star that is thebrightest in the constellation Crux. It is avisual binary (separation 4), both compo-nents being spectroscopic binaries. Alpha() and Gamma () Crucis point towardsthe south celestial pole. mv: 1.3 (A), 1.7 (B),0.76 (AB); Mv: 4.2 (A), 3.2 (B); spectraltype: B1 IV (A), B1 V (B); distance: 160 pc.

alpha particle ( particle) The nucleusof a helium atom, i.e. a positively chargedparticle consisting of two protons and twoneutrons. It is thus a fully ionized heliumatom. Alpha particles are very stable. Theyare often ejected in nuclear reactions, in-cluding alpha decay in which a parent nu-cleus disintegrates or breaks up into analpha particle and a lighter daughter nu-cleus.

Alphard /al-fard/ ( Hya) An orangegiant that is the brightest star in the con-stellation Hydra and lies in a part of thesky where there are few other stars of com-parable brightness. mv: 2.05; spectral type:K4 III; distance: 35 pc.

Alpha Regio The first feature identifiedon Venus using Earth-based radar (it wasdiscovered in 1963). It is a highland pla-teau about 1300 km across situated in thesouthern hemisphere of the planet (cen-tered on the Venusian coordinates 25 Slatitude, 4 E longitude). It exhibits multi-ple sets of intersecting ridges, troughs, andflat-floored fault valleys that together forma polygonal outline. Directly south of thecomplex ridged terrain is a large oval-shaped feature named Eve. A radar brightspot within Eve marks the location of theprime meridian of Venus.

Alphecca /al-fek-/ (Alphekka; Gemma;

11

Alphecca

CrB) A blue-white star that is thebrightest one in the constellation CoronaBorealis. It is an ECLIPSING BINARY (separa-tion 42, period 17.36 days). mv: 2.23;spectral type: A0 V (A), G5 V (B); distance:22 pc.

Alpheratz /al-feer-ats/ (Sirrah; And) Abluish-white giant that is the brightest starin the constellation Andromeda. It is botha spectroscopic binary (period 96.7 days)and a SPECTRUM VARIABLE. It originally layin the constellation Pegasus, as Delta ()Pegasi, and is still considered part of theGreat Square of Pegasus. mv: 2.07; spectraltype: B9p III; distance: 31 pc.

Alphonsus /al-fon-ss, -zs/ See table atcraters.

Alps (Montes Alpes) See table at moun-tains, lunar.

ALSEP Abbrev. for Apollo Lunar Sur-

face Experiments Package. Any of the ex-perimental packages carried to the Moonby APOLLOS 1117, set up by the astro-nauts, and left there to transmit informa-tion back to Earth. The content of eachpackage differed, becoming more sophisti-cated as the program progressed. In theApollo 17 ALSEP, a central station andthermal generator provided the mainpower; the experiments included an analy-sis of any residual atmosphere, detectionand measurement of any lunar ejecta andmeteorite impacts, measurement of anygravitational anomalies, lunar surfacestudies, and seismic measurements.

Altair /al-tair/ ( Aql) A nearby conspic-uous white star that is the brightest one inthe constellation Aquila. mv: 0.77; Mv: 2.3;spectral type: A7 Vn; distance: 5.0 pc.

altazimuth mounting /al-taz--mth/(azimuthal mounting) A MOUNTING inwhich the telescope swings in azimuth

Alpheratz

12

altitude axis

azimuthaxis

Altazimuth mounting

about a vertical axis and in ALTITUDE abouta horizontal axis (see illustration). It is easyto make and use and needs no counterpoiseweights to balance the telescope. It pro-vides a very firm support and is welladapted for terrestrial observations and forfollowing rapidly moving objects such asartificial satellites. Its great disadvantage isthe need to adjust both altitude and az-imuth simultaneously and at different ratesto follow the diurnal motion of a heavenlybody. The application of very precise com-puter-controlled drive mechanisms to al-tazimuth mountings has led to their use inlarge optical and radio telescopes. The al-tazimuth design allows a smaller and lesscostly observatory DOME to be used on anoptical telescope. Compare equatorialmounting.

altitude The angular distance of a pointor celestial object above or below the HORI-ZON, or of an object, such as an artificialsatellite, above mean sea level. Altitudeand azimuth are coordinates in the HORI-ZONTAL COORDINATE SYSTEM.

aluminizing /-loo-m-nz-ing/ A processwhereby a very thin but perfectly uniformcoating of aluminum is deposited by evap-oration on a suitable base. It is used in as-tronomy to produce the reflective layer ofa mirror. The aluminizing is done in anevacuated chamber. The aluminum layer isusually protected by a transparent coatingof silica or magnesium fluoride. The sur-face is harder and more stable than silverand is also able to reflect shorter wave-lengths than silver. The process was intro-duced into telescope manufacture in 1931by the US instrument-maker John Strong.See silvering.

Amalthea /-mal-thee-/ One of theinner group of JUPITERS SATELLITES that wasdiscovered in 1892 by E.E. Barnard anduntil 1979 was thought to be the innermostsatellite. It has been greatly distorted by the gravitational pull of Jupiter, becomingmarkedly elongated in shape (270 166 150 km); its long axis points towardJupiter. Photographs from the VOYAGERPROBES show the surface is very red, due

probably to sulfur contamination from IO.The ALBEDO is generally low, but the brightrather greenish patches have a reflectivityof up to 20% (albedo 0.2) and may be dueto recently exposed material that containsless sulfur-rich glass. Its two main cratersare bowl-shaped: Pan is 90 km in diameterand Gaea is 75 km. The Galileo orbiterflew past Amalthea in November 2002 butdid not image it. See Table 2, backmatter.

AMANDA Telescope An interna-tional facility operational since 1997 andsited in Antarctica for the purpose of de-tecting neutrinos. An improved version,AMANDA-II, came into service in 2000.AMANDA the Antarctic Muon and Neu-trino Detector Array consists of an ar-rangement of 677 optical modules, eachone as big as a glass basketball, strung on19 electrical cables and buried deep in thepure clear ice beneath the South Pole. TheAMANDA Telescope points downward todetect neutrinos passing right through theEarth from the skies above the northernhemisphere. Interference from other parti-cles and from light radiation is filtered outby the body of the Earth and by the situa-tion of the modules, located more than twokilometers below the surface of the ice.

A small number of the countless neutri-nos that continually bombard and tearthrough our planet interact with subatomicparticles in the polar ice and rock to pro-duce negatively charged muons. Eachmuon speeding through the ice emitsCerenkov radiation, visible as a short-livedstreak of faint blue light that illuminatesphototubes within the modules. The pho-totubes convert the radiation to an electri-cal signal and transmit it to computersabove ground. After an interaction, themuon continues to travel in almost exactlythe same direction as the neutrino that pro-duces it. Thus analysis of the muons sig-nals allows scientists to extrapolateinformation about their originating neutri-nos, such as their paths, velocity, and fre-quency. Thus AMANDA makes it possibleto track neutrinos back to their cosmic ori-gins, such as gamma-ray bursts.

The AMANDA Telescope serves as asmall-scale forerunner to a much larger

13

AMANDA Telescope

array of neutrino detectors currentlynamed Ice Cube. This device, covering a re-gion of ice one cubic kilometer in volumeand consisting of a planned 4800 opticalmodules on 80 cables, will include and ex-pand upon the existing AMANDA detec-tors. Astronomers hope that it will be inoperation by 2008.

ambipolar diffusion /am-bee-poh-ler/ A process by which magnetic fluxand angular momentum are removed fromDENSE CORES in molecular clouds, ulti-mately leading to the collapse of the coreand the formation of a low-mass star. Seestar formation.

American Ephemeris and Nautical Al-manac See Astronomical Almanac.

Ames Research Center /aymz/ A NASAresearch establishment near San Francisco,California.

AM Herculis /her-ky-liss/ The arche-typal magnetic white dwarf binary system,or polar, identified in 1976 from an UHURUX-ray source. Optically it is a faint variablebinary star (mag. 1215), exhibiting strongoptical polarization and a photometric pe-riod of 185 minutes. X-ray emission isstrongly modulated at the same period, dueto rotation of the white dwarf. The charac-teristic high XUV-to-optical luminosityratio has led to several other similar sys-tems being found from X-ray and XUVsurveys. See magnetic cataclysmic vari-ables.

AM Herculis stars See magnetic cata-clysmic variables.

Amor group /ay-mor, am-or/ A groupof near-Earth ASTEROIDS that come withinthe PERIHELION DISTANCE of Mars but notwithin the orbit of the Earth. They haveSEMIMAJOR AXES greater than 1 AU but per-ihelion distances between 1.017 and 1.3AU. The group is named after the asteroid(1221) Amor, discovered in 1932 by Eu-gne Joseph Delporte. Compare Apollogroup. See Table 3, backmatter.

amplitude 1. Symbol: m. The differ-ence between the maximum and minimumMAGNITUDES of a VARIABLE STAR, i.e. therange in magnitude of the star. The ampli-tude of pulsating variables is related to thelogarithm of the period.2. The maximum instantaneous deviationof an oscillating quantity from its averagevalue.

AMPTE Abbrev. for active magnetos-pheric particle tracer explorer. Any ofthree satellites built by West Germany, theUK, and the USA that were launched froma Thor Delta rocket into different orbits on9 Aug. 1984 for a series of interactive ex-periments to study the Earths MAGNETOS-PHERE. The German and UK satellites wereput in highly eccentric orbits into the SOLARWIND while the US craft was in a lowerorbit. Lithium and later barium atoms re-leased into the magnetosphere from theGerman satellite formed positive ions.These were used as tracers in a coordinatedeffort to study how solar energy, carried bythe solar wind, is intercepted and stored inthe magnetosphere as charged particles.

Am stars PECULIAR main-sequence starsof spectral types from A0 to F0 in whichthere is an over-abundance of heavier el-ements and rare earths and (less so) of iron,and an apparent under-abundance of cal-cium. They rotate slower than normal Astars and are almost all short-period SPEC-TROSCOPIC binaries. Current theory sug-gests that tidal effects slow the A starsrotation, leading to an unusually stable at-mosphere where heavy elements can dif-fuse up from the interior. Compare Apstars.

analemma /an--lem-/ The shape re-sembling a figure of eight obtained by plot-ting the position of the Sun relative to theintersection of a meridian and the celestialequator at a fixed (mean) time every daythroughout the year. The analemmas ver-tical extent is a reflection of the changes inthe Suns declination arising from the incli-nation of the Earths axis to the perpendic-ular to its orbit. The horizontal extentarises from the fact that the Earths orbit is

ambipolar diffusion

14

elliptical, which produces a difference be-tween the length of the apparent solar day(the actual time between successive merid-ian transits of the Sun) and the mean solarday.

Ananke /-nank-ee/ A small satellite ofJupiter, one of the planets many outersatellites, discovered in 1951 by the US as-tronomer Seth Nicholson (18911963).See Jupiters satellites; Table 2, backmat-ter.

anastigmatic system /an--stig-mat-ik/ An optical system that is able to pro-duce an image essentially free of SPHERICALABERRATION, COMA, and ASTIGMATISM.

Andromeda /an-drom--d/ A constella-tion in the northern hemisphere close to PE-GASUS, the brightest stars being ALPHERATZ(), the red giant Mirach (), and the finevisual binary ALMACH (). It contains theMIRA STARS R and W Andromedae, the spi-ral ANDROMEDA GALAXY and its companiongalaxies M32 (NGC 221) and M110(NGC 205), and the bright planetary neb-ula NGC 7662. Other objects include theopen cluster NGC 752 and the edge-onspiral galaxy NGC 891. The 4th-magni-tude Upsilon () Andromedae, a yellowdwarf star of spectral class F8 V located ata distance of 15.9 pc, is believed to havethree giant planets in orbit around it. Ab-brev.: And; genitive form: Andromedae;approx. position: RA 1h, dec +40; area:722 sq deg.

Andromeda galaxy (M31; NGC 224)The largest of the nearby galaxies, visibleto the unaided eye as a faint oval patch oflight in the constellation Andromeda. It isan intermediate (Sb) spiral (see Hubbleclassification), orientated at an angle ofabout 15 from the edge-on position, andhas a bright elliptical-shaped nucleus. Itsdistance is currently estimated as 725 kilo-parsecs (2.36 million light years). With atotal luminosity roughly double that of ourown Galaxy and an overall diameter of ap-proximately 60 kiloparsecs, M31 is thelargest of the established members of theLOCAL GROUP. It has at least four elliptical

satellites, including NGC 205 and NGC221 (M32).

Andromedids /an-drom--didz/ SeeBielids.

anemic galaxies Disk galaxies that areintermediate in type between lenticular andnormal spirals, possessing diffuse weakspiral arms; the BLACKEYE GALAXY is an ex-ample. See galaxies.

Anglo-Australian Observatory (AAO)An observatory at the Siding Spring Obser-vatory site in New South Wales, Australia.The chief instruments are the 3.9-meterANGLO-AUSTRALIAN TELESCOPE and the 1.2-meter UK SCHMIDT TELESCOPE.

Anglo-Australian Telescope (AAT)The 3.9-meter reflecting telescope of theANGLO-AUSTRALIAN OBSERVATORY, sited atan altitude of 1150 meters. It was fundedjointly by the governments of Australiaand the UK, each country having a halfshare in observing time. It began regularoperation in 1975 and is now equipped forboth optical and infrared observations. Ithas RITCHEYCHRTIEN OPTICS and a CER-VIT mirror and works at a FOCAL RATIO off/3.3 at the prime focus, f/8 at theCassegrain foci, f/15 for infrared work,and f/35 at the coud focus. It has a limit-ing magnitude of 2325 (depending onwavelength and technique used), the maxi-mum field of view covering about onesquare degree of sky. The telescope has anEQUATORIAL MOUNTING, and is computercontrolled so that the pointing and track-ing are exceptionally accurate.

angstrom /ang-strm/ Symbol: . A unitof length equal to 1010 meters. It was for-merly used to specify interatomic dis-tances.

angular acceleration See angular ve-locity.

angular diameter See apparent diame-ter.

angular distance See apparent distance.

15

angular distance

angular measure Units of angle orlength given in terms of degrees (), arcminutes (), and ARC SECONDS ():

60 = 160 = 1360 = 2 radians (a full circle).

angular momentum Symbol: L. Aproperty of any rotating or revolving sys-tem whose value depends on the distribu-tion of mass and velocity about the axis ofrotation or revolution. It is a vector di-rected along the axis and for a body orbit-ing about a point it is given by the vectorproduct of the bodys linear momentumand position vector r, i.e. it is the productm(v r). It is also expressed as the productof the MOMENT OF INERTIA (I) and ANGULARVELOCITY () of the body.

In a closed system, such as the SolarSystem or an isolated star, there is alwaysCONSERVATION OF MOMENTUM.

angular resolution See resolution.

angular separation See apparent dis-tance.

angular velocity Symbol: . The rate atwhich a body or particle moves about afixed axis, i.e. the rate of change of angulardisplacement. It is expressed in radians persecond. Angular acceleration is the rate ofchange of angular velocity.

anisotropy /an--sot--pee/ A depend-ence of physical properties upon direction;a lack of ISOTROPY. A lack of HOMOGENEITYimplies anisotropy but the reverse is notnecessarily true.

Ankaa /ank-ay-/ See Phoenix.

annihilation A reaction between an EL-EMENTARY PARTICLE and its ANTIPARTICLE inwhich the two particles disappear and pho-tons or other particles and antiparticles arecreated; energy and momentum are con-served. An electron and a positron, for ex-ample, interact and produce twogamma-ray photons. Hadrons, such as theproton and the antiproton, also undergoannihilation, as do QUARKS and antiquarks.

annual aberration See aberration.

annual equation (annual inequality)The periodic INEQUALITY in the Moonsmotion that arises from variations in solarattraction due to the ECCENTRICITY of theEarths orbit. Its period is the ANOMALISTICYEAR and the maximum displacement inlongitude is 118.9. See also evection; par-allactic inequality; variation.

annual inequality See annual equation.

annual parallax (heliocentric parallax)The PARALLAX of a celestial body that re-sults from the change in the position of anobservational point during the Earths an-nual revolution around the Sun: nearbystars are seen to be displaced in positionrelative to the more remote backgroundstars. The instantaneous parallax of agiven star is the angle formed by the radiusof the Earths orbit at the star; it variesthrough the year as the orbital radiusvaries. If the positions of a star are deter-mined during one year they are found todescribe an ellipse a parallactic ellipse on the celestial sphere. The annual paral-lax, , of the star is equal to and can bemeasured from the semimajor axis of theellipse.

Annual parallax is the maximum paral-lactic displacement of the star and occurswhen the angle Earth-Sun-star is 90 (seeillustration). It is thus expressed by the re-lation sin = a/d, where a is the semimajoraxis of the Earths orbit and d is the starsdistance from Earth. Since is extremelysmall ( 0.03). Space-craft such as HIPPARCOS can measure an-

angular measure

16

nual parallax with a very much greater ac-curacy, to 0.002, allowing distances up to500 parsecs to be determined. Microarc-second measurements are planned for fu-ture spacecraft. Only about 3000 stars areknown to have annual parallaxes exceed-ing 0.04, the nearest star Proxima Cen-tauri having the greatest value (0.772).

annular eclipse See eclipse.

anomalistic month The time intervalof 27.554 55 days, on average, betweentwo successive passages of the Moonthrough the PERIGEE of its orbit.

anomalistic year The time interval of365.259 64 days between two successivepassages of the Earth through the PERIHE-LION of its orbit. The anomalistic year islonger than the SIDEREAL and TROPICALYEARS because of the ADVANCE OF THE PERI-HELION caused (mainly) by planetary per-turbations.

anomaly Any of three related angles bymeans of which the position, at a particu-lar time, of a body moving in an ellipticalorbit can be calculated. For a body S mov-ing around the focus, F, of an orbit (see il-lustration), the true anomaly is the angle vmade by the body, the focus, and the point,P, of nearest approach. For a body orbitingthe Sun, P is the perihelion. The angle ismeasured in the direction of motion of S. Ifan auxiliary circle is drawn centered on themidpoint, C, of the major axis of the ellip-tical orbit, then the eccentric anomaly isthe angle E between CS and CP, where Slies on the circle and is vertically above S.The mean anomaly is the angle M betweenP, F, and a hypothetical body moving at aconstant angular speed equal to the MEANMOTION of S. It is thus the product of themean motion and the time interval since Spassed P.

The eccentric and mean anomalies arerelated by Keplers equation:

E e sinE = M

17

anomaly

2 x annual parallax

parallactic ellipse

star

instantaneous parallax

Sun

Earths orbit

Annual parallax

where e is the ECCENTRICITY of the orbit.The coordinates (x, y) of the body S can befound from the equations

x = a(cosE e) y = a sinE (1 e2)

where a is the semimajor axis of the orbit.See also equation of center; orbital el-ements.

ANS Abbrev. for Astronomical Nether-lands Satellite.

ansae /an-see/ The parts of SATURNSRINGS that are visible on each side of theplanet as viewed from the Earth. They ap-pear rather like handles on a double-han-dled cup (the Latin word ansa meanshandle).

antapex /ant-ay-peks/ (solar antapex) Seeapex.

Antarctic astronomy The high alti-tude, extreme cold, and low atmosphericwater vapor content found on the Antarc-tic Plateau make it possibly the best site onEarth for astronomy at millimeter, submil-limeter, and infrared wavelengths. TheCenter for Astronomical Research inAntarctica (CARA) coordinates year-round observations at the South Pole.

These contribute to cosmic microwavebackground anisotropy studies and to in-vestigations of young stars and molecularclouds.

Antares /an-tair-eez/ ( Sco) A huge re-mote but conspicuous red supergiant thatis the brightest star in the constellationScorpius. It is in an advanced stage of evo-lution. It is a visual binary (separation 3,period 900 years), its 5th-magnitude B-type companion appearing green by con-trast. The companion orbits inside the coolstellar wind and is a peculiar hot radiosource. mv: 0.96 (var.); Mv: 5.2; spectraltype: M1.5 Iab; diameter: about 285 timessolar diameter; distance: 160 pc.

antenna /an-ten-/ (aerial) A device usedfor the transmission or reception of radiowaves. When connected to a transmitter,the oscillating electric currents induced inthe antenna launch radio waves into space.When connected to a RECEIVER, incomingradio waves from a distant source generateoscillating electric currents in the antenna,which are detected by the receiver. A prac-tical antenna neither radiates nor receivesequally in all directions, which may bedemonstrated by its antenna pattern aplot of relative GAIN as a function of direc-

ANS

18

y

S

S

P xFC

a

E v

Anomaly

tion. The plot may be made in terms of theantennas voltage response or power re-sponse, giving a voltage pattern or a powerpattern. By the principle of reciprocity theantenna pattern for transmission is identi-cal with that for reception.

A number of distinct lobes may often beidentified in the antenna pattern. The lobecorresponding to the direction of besttransmission or reception is the main lobeor main beam; all the others are called sidelobes and are usually unwanted (see alsobeam). The ratio of the power received inthe main beam to the total power receivedby all the lobes is called the beam effi-ciency. The angle between the two direc-tions in the main beam at which the powerresponse has fallen to half its maximumvalue is called the beamwidth; it is a meas-ure of the antennas DIRECTIVITY. If the an-gular separation of two cosmic radiosources is less than the beamwidth, thesources will not be resolved but will be ob-served as a single source. Some highly di-rectional antennas have pencil beams,which are narrow main beams of circularcross section. Others have fan beamswhere the cross section of the main beam isgreatly extended in one direction. See alsoarray; dish; radiation resistance.

Antennae /an-ten-ee/ (NGC 4038/9) Apair of INTERACTING GALAXIES that are inthe process of merging. They lie at a dis-tance of only 22 megaparsecs and are thusthe best-studied example of this phenome-non. They were normal galaxies beforetheir interaction, but during their point ofclosest approach about 200 million yearsago tides have drawn out two long curvedtails, hence their popular name. The galax-ies are currently turning around towardtheir final MERGER.

antenna pattern (field pattern; polar dia-gram) See antenna.

antenna temperature A measure of thestrength of signal received by a RADIO TELE-SCOPE from a RADIO SOURCE. It is defined asthe noise power received per unit BAND-WIDTH, divided by the BOLTZMANN CON-STANT, k. Antenna temperature depends on

the surface brightness of the sky weightedby the telescopes BEAM rather than physi-cal temperature of the antenna. A radiosource of flux density S with an angular di-ameter that is much smaller than the an-tennas beam will give an antennatemperature of SAe/(2k), where Ae is the ef-fective area (see array) of the antenna. Ifthe beam is filled by a uniform source ofBRIGHTNESS TEMPERATURE TB, the antennatemperature will also be TB.

SYNCHROTRON EMISSION from COSMICRAYS in the Galaxy produces a diffuse radioemission centered broadly on the galacticplane. Any practical observation of a radiosource has to be made against this back-ground emission, which at low frequencieslimits the SENSITIVITY of the RADIO TELE-SCOPE. The antenna temperature of the dif-fuse emission is called the backgroundtemperature; the electrical NOISE it pro-duces in radio receivers is often called cos-mic noise.

anthropic principle /an-throp-ik/ Aprinciple that was put forward in the 1960sby R. Dicke and maintains that the pres-ence of life in the Universe places con-straints on the ways in which the very earlyUniverse evolved: the possible initial condi-tions are limited to those that give rise toan inhabited Universe, i.e. what we observemust be restricted by the conditions neces-sary for our presence as observers.

antimatter Matter composed entirely ofANTIPARTICLES. Ordinary matter and anti-matter would annihilate on contact. Al-though individual antiparticles areproduced in cosmic-ray showers and inhigh-energy particle accelerators, thesearch for antimatter in the Universe has sofar proved unsuccessful. It is thought there-fore that the Universe is not now symmet-ric between matter and antimatter,although initially equal amounts were cre-ated. An excess of matter over antimattermay have resulted from processes occur-ring very early in the evolution of the Uni-verse while it was out of equilibrium.

antiparticles A pair of ELEMENTARY PAR-TICLES, such as the electron plus positron or

19

antiparticles

the proton plus antiproton, that have anidentical rest mass but a charge, strange-ness, and other fundamental properties ofequal magnitude but opposite sign (posi-tive rather than negative and vice versa).Thus, the electron has a negative chargeand the positron an equal positive charge.A reaction of a particle with its antiparticleis called ANNIHILATION. See also pair pro-duction.

antitail A small spiky taillike structureon a comet that, unlike most COMET TAILS,seems to point toward the Sun. It is usuallyonly seen when the observer is in the planeof the cometary orbit. The particles re-sponsible for scattering the sunlight aremuch larger than those that produce thenormal dust tail.

Antlia /ant-lee-/ (Air Pump, Pump) Asmall inconspicuous constellation in thesouthern hemisphere near Centaurus, thebrightest stars being of 4th magnitude. Ab-brev.: Ant; genitive form: Antliae; approx.position: RA 10h, dec 35; area: 239 sqdeg.

Antoniadi scale /an-toh-nee-ah-dee/ Seeseeing.

Apache Point Observatory An obser-vatory near Sunspot, New Mexico, thatopened in 1990 and is owned and operatedby the Astrophysical Research Consortium(ARC), a consortium of US universities.The observatory is sited at an altitude of2787.9 meters. The main instrument is a3.5-meter telescope with a lightweighthoneycomb PRIMARY MIRROR, for opticaland infrared observations. The SLOAN DIGI-TAL SKY SURVEYs 2.5-meter telescope wascommissioned there in 1997. There is alsoa 1-meter telescope operated by New Mex-ico State University.

apastron /ap-ass-tron/ See periastron.

Apennines /ap--nnz/ (Montes Apenni-nus) See table at mountains, lunar.

aperture The diameter of the unob-scured portion of the objective lens in a re-

fracting telescope or of the primary mirrorin a reflector. In a radio telescope it is thephysical size of the antenna. As the aper-ture is increased, the telescope gathersmore light, radio waves, etc., and thus willdiscern fainter objects: the radiation-gath-ering power depends on area, i.e. on thesquare of the aperture. A larger aperturealso produces a smaller AIRY DISK and sohas greater spatial RESOLUTION.

aperture ratio (relative aperture) Theratio d/f of the effective diameter (i.e. APER-TURE), d, of a lens or mirror to its FOCALLENGTH, f. The ratio f/d is the FOCAL RATIO.

aperture synthesis A technique origi-nally of radio astronomy devised by SirMartin Ryle in the late 1950s. It is used toobtain high RESOLUTION images by combin-ing signals from a number of relativelysmall ANTENNAS spaced over an area equiv-alent to a large aperture. Although origi-nally used only for synthesized aperturesthat were fully filled by smaller antennas,the term is also applied to instruments forwhich no clearly definable equivalent largeaperture exists. The antennas are con-nected in interferometric pairs (see radiotelescope) and the AMPLITUDES and PHASESof the signals from a radio source arerecorded on all antenna spacings necessaryto cover the required area. The informa-tion is then processed in a computer wherea FOURIER TRANSFORM is carried out to gen-erate a radio map of the source.

In practice the rotation of the Earth isoften used to move the antennas in space a technique known as Earth-rotation syn-thesis. In such an arrangement, just 12hours of observation generate all the base-lines available from a particular configura-tion; the same baselines are repeated withthe opposite sense during the next 12hours. For a radio interferometer in whichall the baselines are oriented eastwest (e.g.the RYLE TELESCOPE and the WSRT), eachbaseline produces an elliptical track of syn-thesized aperture. The major axis of the el-lipse is determined by the baseline length,whereas the minor axis depends on the sineof the DECLINATION of the source. Low dec-lination sources cannot therefore be

antitail

20

mapped with a uniformly high resolutionby an eastwest interferometer. The VLAlargely overcomes this difficulty by arrang-ing its dishes in a Y-shaped configuration.

The computer processing involved inmap-making can become very sophisti-cated, and special software (such as AIPS) isusually used. The Fourier transform of thesynthesized aperture defines the synthe-sized beam (or dirty beam) of the telescope,with which the final image will appear tobe convolved (see convolution). A synthe-sized aperture that is not fully filled willproduce false features or artifacts in themap, and these must be suppressed usingdeconvolution algorithms such as Clean orMEM. An aperture consisting of only a fewequally spaced strips will show bright,widely spaced concentric rings or gratingresponses (see array) around features in themap. Even a fully filled synthesized aper-ture may need to be graded, by graduallyreducing the weight of contributions fromthe longer baselines. This reduces the side-lobe response of the telescope, which is theresidual structure of the synthesized beamlying outside its central region. Other tech-niques such as self-calibration are used toremove artifacts generated by instrumentaldrifts and the effects of the atmosphere.

apex 1. (solar apex) The point on the ce-lestial sphere toward which the Sun and theSolar System are moving relative to thestars in our vicinity. The apex lies in theconstellation Hercules at a position RA18h, dec +30, close to the star Vega. Thepoint diametrically opposite to the direc-tion of this relative solar motion is the an-tapex (or solar antapex); it lies in theconstellation Columba. The position of theapex can be determined from an analysis ofthe proper motions, radial velocities, andparallaxes of stars, based on the assump-tion that solar motion is equal and oppo-site to the group motion of stars. Thevelocity of this motion has been calculatedas about 19.5 km s1 with respect to theLOCAL STANDARD OF REST, when all avail-able information on proper motion and ra-dial velocity is used. The value is lowerwhen only the stars in the Suns immediateneighborhood are considered. Solar mo-

tion produces the SECULAR PARALLAX ofstars.2. The point on the celestial sphere towardwhich the Earth appears to be moving, at agiven time, as a result of its orbital motionaround the Sun.

aphelion /ap-hee-lee-n/ The point inthe orbit of a planet, comet, or artificialsatellite in solar orbit that is farthest fromthe Sun. The Earth is at aphelion on orabout July 3.

Aphrodite Terra /af-r-d-tee/ A veryextensive highland area on VENUS, situatedmainly in the southern hemisphere butpartly across the equator between about70 and 210 Venusian longitude. It is ascorpion-shaped feature, 9700 3200 kmin extent, with eastern and western moun-tains separated by a lower area. Severalvolcanoes are at its eastern limit, includingthe giant Maat Mons, which is 5 km high,Ozza Mons, and Sapas Mons. See alsoIshtar Terra; Beta Regio.

aplanatic Gregorian /ap-l-nat-ik/ SeeGregorian telescope.

aplanatic system An optical systemthat is able to produce an image essentiallyfree from SPHERICAL ABERRATION andCOMA.

APM Abbrev. for automatic plate-meas-uring machine. A facility at the RoyalGreenwich Observatory at Cambridge,used to digitize optical sky-survey photo-graphic plates.

apocenter /ap--sen-ter/ See pericenter.

apochromatic lens /ap--kr-mat-ik/See achromatic lens.

apodization /ap--di-zay-shn/ Anyprocess suppressing the secondary maximaof a diffraction pattern, such as the faintrings around the AIRY DISK of an opticalimage; this allows finer detail to be re-solved. It may be achieved, for example, byprogressively reducing the transmission of

21

apodization

the telescope from the center of the aper-ture to its periphery.

apogee /ap--jee/ 1. The point in theorbit of the Moon or an artificial Earthsatellite that is farthest from the Earth andat which the bodys velocity is at a mini-mum. Strictly the distance to the apogee istaken from the Earths center. The distanceto the Moons apogee varies; on average itis about 405 500 km. When a spacecraft orsatellite reaches apogee, its apogee rocketcan be fired to boost it into a more circularorbit or to allow it to escape from Earthorbit. See also perigee.2. The highest point above the Earths sur-face attained by a rocket, missile, etc.

Apollo /-pol-oh/ The American spaceprogram for landing astronauts on theMOON and returning them safely to Earth.A manned lunar landing, to be achieved be-fore 1970, was proposed to Congress in1961 by President J.F. Kennedy followingthe first manned flight by what was thenthe USSR. Prior to Apollo 7, the missionswere designed to test the Saturn 1B andSaturn V launch vehicles, the commandand service module (CSM), and the lunarmodule (LM). After three astronauts werekilled in a flash fire inside the commandmodule (CM) during ground tests in Jan.1967, the program was delayed for 18months while the CM was redesigned.NASA commemorated the three astronautsby redesignating the mission that theywould have flown in as Apollo 1. The sub-sequent Apollos 26 were crewless testflights.

The method selected for the flight pro-gram was lunar orbit rendezvous. The as-tronauts were to travel to and from theMoon in the command module, whichcontained the controls and instruments.Rocket engines and fuel supplies werehoused in a separate service module (SM).On entering lunar orbit the command-module pilot would remain in the CMwhile the commander and lunar-modulepilot made the landing in the LM. On com-pletion of the mission on the Moons sur-face, the descent stage was to remain on theMoon: the ascent stage of the LM was to

carry the astronauts into lunar orbit andrendezvous with the command and servicemodules. The craft would then embark onits return journey, the LM being jettisoned.The SM was to be jettisoned just prior toreentering the Earths atmosphere.

Details of the Apollo flights are given inthe table. Apollo 7 was the first piloted testflight of the CSM. Apollo 8 was the first tobe launched by Saturn V and to enter lunarorbit. The LM was tested in Earth orbit byApollo 9, in lunar orbit by Apollo 10, andlanded on the Moon for the first time byApollo 11 on July 20 1969. Apollo 11slanding site, in Mare Tranquillitatis, wasdubbed Tranquility Base. Apollo 12landed with higher accuracy than its prede-cessor, touching down in Oceanus Procel-larum close to SURVEYOR 3, parts of whichwere returned to Earth. Both missions re-turned basalts from the MARIA. Apollo 13was aborted safely after an explosion in theservice module and its target was takenover by Apollo 14. This was to collectrocks from Cone crater (25 million yearsold) in the FRA MAURO FORMATION. Mobil-ity was increased by the use of a ModularEquipment Transporter but the terrain wasunexpectedly rugged. Complex non-marebasalt breccias were returned.

The last three Apollos visited sites withmultiple objectives, collected deep (twometer) drill cores (in addition to widecores, soils, pebbles, and larger rocks), andused a LUNAR ROVING VEHICLE. Apollo 15returned samples of anorthosite and greenglass from the Apennine MOUNTAINS andmare basalts from HADLEY RILLE. Apollo 16landed in the vicinity of South Ray andNorth Ray craters (2 and 50 million yearsold respectively) in the HIGHLANDS to sam-ple the CAYLEY and DESCARTES FORMATIONS,but returned only anorthositic brecciasrather than volcanic rocks. Apollo 17 in-cluded a scientist-astronaut for the firsttime. It landed in a dark and LIGHT MAN-TLED mare-filled valley between high mas-sifs on the borders of Mare SERENITATIS.Several boulders were sampled and un-usual orange glass was found near a 30-million-year-old DARK HALO crater, calledShorty.

apogee

22

Experiments performed on the Moonby Apollo included SOLAR-WIND, lunar-atmosphere, COSMIC-RAY and neutron de-tection, heat-flow and magnetic-fieldmeasurements, active and passive seis-mometry, and laser ranging. Apollos 15,

16, and 17 were equipped with experi-ments in the service module, including met-ric and panoramic photography, laseraltimetry, radar sounding, magnetometry,and gamma-ray, ultraviolet, and alpha-particle spectrometry. Subsatellites were

23

Apollo

APOLLO MISSIONS

Apollo Astronauts Date Landing site EVA Dist- SampleMission (Commander) of coordinates time tance weight

(LM pilot) landing (accomplishment) (hr) traversed (kg)(CM pilot) (km)

7 Schirra Oct. (first test of Eisele 1968 CSM in earthCunningham orbit)

8 Borman Dec. (first men in Lovell 1968 lunar orbit)Anders

9 McDivitt Mar. (test of LM in Scott 1969 earth orbit)Schweickart

10 Stafford May (full dress Young 1969 rehearsal forCernan lunar landing)

11 Armstrong* (20) July M. Tranquillitatis 2.2 0.5 21.7Aldrin* 1969 067N2349ECollins (first landing)

12 Conrad* (19) Nov. Oc. Procellarum 7.7 1.3 34.4Bean* 1969 312S2323WGordon (Surveyor 3)

13 Lovell Apr. (Mission aborted: Swigert 1970 explosion inHaise service module)

14 Shepard* (31) Jan. Fra Mauro 9.2 3.4 42.9Mitchell* 1971 formationRoosa 340S1728E

(Cone crater)15 Scott* (30) July Hadley Rille 18.3 27.9 76.8

Irwin* 1971 2606N339EWorden (first test of LRV)

16 Young* (21) Apr. Cayley-Descartes 20.1 27.0 94.7Duke* 1972 859S1531EMattingley (lunar highlands)

17 Cernan* (11) Dec. Taurus-Littrow 22.0 30.0 110.5Schmitt* 1972 2010N3046EEvans (first scientist)

*Astronauts marked with an asterisk landed on the moon.

ejected from the SM for particle and fieldmeasurements. Spent boosters werecrashed as seismic sources. After Apollo17, the remaining hardware of the pro-gram was used in SKYLAB and the APOLLO-SOYUZ TEST PROJECT.

See also Gemini project; Lunar Orbiterprobes; Luna probes; Mercury project;Moon rocks; Ranger; Zond probes.

Apollo-Amor objects See Apollogroup.

Apollo group A group of near-Earth AS-TEROIDS, including (1566) ICARUS and(2201) OLJATO, that have PERIHELIA insidethe orbit of the Earth (i.e. less than 1.017AU). Like members of the AMOR GROUP,most are very small bodies and can be ob-served only when close to the Earth. Thegroup takes its name from the 1.4-km-di-ameter asteroid (1862) Apollo, which wasdiscovered when it approached within 0.07AU of the Earth in 1932 by Karl Reinmuthbut was lost because of uncertainties in itsorbit; it was rediscovered in 1973. Radarobservations of Apollo in 2005 by theARECIBO RADIO TELESCOPE revealed that asatellite is in close orbit around it. Thespectra of several of the Apollo group as-teroids closely resemble those of CHON-DRITE meteorites. Members of the Apolloand Amor groups are often classed collec-tively as ApolloAmor objects, the perihe-lia of such bodies usually being taken assmaller than 1.3 AU. See Table 3, back-matter.

Apollo-Soyuz test project (ASTP) Thefirst international manned space flight, fi-nally agreed to in 1972 and achieved by theUSA and the USSR in 1975. An AmericanAPOLLO spacecraft Apollo 18 and a So-viet SOYUZ CRAFT Soyuz 19 werelaunched into Earth orbit on July 15, ren-dezvoused on July 17 at an altitude of 225km, and successfully docked. The crewsvisited each others craft and conductedjoint experiments and surveys. The missioninvolved major design modifications inboth spacecraft.

Apollo Telescope Mount (ATM) The

complex telescope mount on NASAs or-biting space laboratory SKYLAB. The sixprincipal instruments carried on the mountoperated in the X-ray and/or ultravioletspectral regions, not accessible to ground-based instruments. Two X-ray telescopes(0.36, 0.63.3 nm) and a white-lightcoronagraph (350700 nm) were used instudying the solar CORONA. An XUV spec-troheliograph (1562 nm), a UV spectro-heliometer (30140 nm), and a UVspectrograph (97394 nm) provided dataon the Suns CHROMOSPHERE. The observa-tions were made between May 1973 andFeb. 1974.

apparent Denoting a property of a staror other celestial body, such as altitude or