230NAVIGATIONALASTRONOMYparts of the surface.awayfromthesuntowesternelongation,thenmovebacktoward the sun.After disappearing in the morningtwilight1510.Inferior Planetsit will move behind the sun to superior conjunction. Afterthis it willreappear in the evening sky,heading towardeast-Since Mercury and Venus are inside the earth's orbit,ernelongationthey always appear in the neighborhood of the sun.Over aMercury is never seen more than about 28°from theperiodofweeksormonths, they appearto oscillateback andsun.Forthisreason itisnot commonlyusedfor navigation.forth from one side of the sun to the other.They are seen ei-Near greatest elongation it appearsnear the western horizonther in the eastern sky before sunrise or in the western skyafter sunset, or the eastern horizon before sunrise.At theseaftersunset.Forbrief periodstheydisappearintothe sun'stimes it resembles afirstmagnitude starand is sometimesglare.At this time they arebetween the earth and sun (knownreported as a newor strange object in the sky.The intervalas inferior conjunction)or on the opposite side of the sunduringwhichitappearsasamorningoreveningstarcanfromtheearth(superiorconjunction).Onrareoccasionsatvary fromabout30to50 days.Around inferiorconjunction,inferior conjunction, the planet will cross the face ofthe sunMercury disappearsfor about 5days; near superior con-as seenfrom the earth.This isknown as a transit ofthe sun.junction, it disappears for about 35 days.Observed with aWhen Mercury or Venus appears most distant from thetelescope, Mercury is seen to go through phases similar tosun in the evening sky, it is at greatest eastern elongationthose of the moon.(Althoughtheplanetisinthewesternsky,itisatitseast-Venus can reacha distanceof47°fromthesun,allow-ernmost point from the sun.) From night to night the planeting ittodominatethemorningoreveningsky.Atmaximumwill approach the sun until it disappears into the glare ofbrilliance,aboutfive weeks before and after inferior con-twilight. At this time it is moving between the earth and sunjunction, it has a magnitude of about -4.4 and is brighterto inferior conjunction.Afewdays later,theplanetwill ap-than any other object in the sky except the sun and moon.pear in the morning sky at dawn. It will gradually moveConjunction-Orbinctio03.SUNGreatestGreotestElongationJinferiorCbnjunctioElongationEastWestEastWestQuadrafuruadratureEahhOppositionFigure1510.Planetaryconfigurations
230 NAVIGATIONAL ASTRONOMY parts of the surface. 1510. Inferior Planets Since Mercury and Venus are inside the earth’s orbit, they always appear in the neighborhood of the sun. Over a period of weeks or months, they appear to oscillate back and forth from one side of the sun to the other. They are seen either in the eastern sky before sunrise or in the western sky after sunset. For brief periods they disappear into the sun’s glare. At this time they are between the earth and sun (known as inferior conjunction) or on the opposite side of the sun from the earth (superior conjunction). On rare occasions at inferior conjunction, the planet will cross the face of the sun as seen from the earth. This is known as a transit of the sun. When Mercury or Venus appears most distant from the sun in the evening sky, it is at greatest eastern elongation. (Although the planet is in the western sky, it is at its easternmost point from the sun.) From night to night the planet will approach the sun until it disappears into the glare of twilight. At this time it is moving between the earth and sun to inferior conjunction. A few days later, the planet will appear in the morning sky at dawn. It will gradually move away from the sun to western elongation, then move back toward the sun. After disappearing in the morning twilight, it will move behind the sun to superior conjunction. After this it will reappear in the evening sky, heading toward eastern elongation. Mercury is never seen more than about 28° from the sun. For this reason it is not commonly used for navigation. Near greatest elongation it appears near the western horizon after sunset, or the eastern horizon before sunrise. At these times it resembles a first magnitude star and is sometimes reported as a new or strange object in the sky. The interval during which it appears as a morning or evening star can vary from about 30 to 50 days. Around inferior conjunction, Mercury disappears for about 5 days; near superior conjunction, it disappears for about 35 days. Observed with a telescope, Mercury is seen to go through phases similar to those of the moon. Venus can reach a distance of 47° from the sun, allowing it to dominate the morning or evening sky. At maximum brilliance, about five weeks before and after inferior conjunction, it has a magnitude of about –4.4 and is brighter than any other object in the sky except the sun and moon. Figure 1510. Planetary configurations
231NAVIGATIONALASTRONOMYAtthesetimes itcanbe seenduringthedayand is sometimesSaturn, the outermost of the navigational planetsobservedfora celestial lineofposition.Itappears as amorn-comestoopposition at intervals ofabout 380days.Itisvis-ingoreveningstarforapproximately263days insuccession.iblefor about175days beforeand afteropposition,anddisappears for about 25 days near conjunction. At opposi-Near inferior conjunction Venus disappears for 8 days,around superior conjunction it disappearsfor 50 days.Whention it becomes as brightas magnitude+0.8to -0.2it transits the sun, Venus can be seen to the naked eye as aThrough good,high powered binoculars, Saturn appears assmall dot about the size ofa group of sunspots.Through bin-elongated because of its system of rings.A telescope isoculars,Venus canbe seen togo througha full setofphases.needed to examine the rings in any detail. Saturn is nowknown to have atleast18 satellites,none of which arevisi-1511.SuperiorPlanetsble to the unaided eye.Uranus, Neptune and Pluto aretoo faint to be used forAs planets outside the earth's orbit, the superior plannavigation; Uranus, at about magnitude 5.5,is faintly visi-etsarenotconfined totheproximityofthesunasseenfrombleto the unaided eye.the earth. They can pass behind the sun (conjunction), but1512.TheMoontheycannotpassbetweenthesunandtheearth.Instead weseethemmoveawayfromthesununtiltheyareoppositeThemoon isthe only satellite of direct navigational in-the sun in the sky (opposition). When a superior planet isnear conjunction, it rises and sets approximatelywith theterest.It revolves around the earth once in about 27.3days,sun and is thus lost in the sun's glare.Gradually it becomesas measured with respect to the stars.This is called the si-visible in the earlymorning skybeforesunrise.Fromdaytoderealmonth.Becausethemoonrotatesonitsaxiswithday,it rises and sets earlier,becoming increasingly visiblethesameperiod with which itrevolves around theearth,thethrough the late night hours until dawn. Approaching oppo-same side of themoon is always turned toward the earth.sition, the planet will rise in the late evening,until atThecycleof phasesdependsonthemoon's revolution withopposition, it will rise when the sun sets, be visible through-respectto the sun.This synodic monthisapproximately29.53 days,but can vary from this average by up to a quar-out the night, and set when the sun rises.Observedagainstthebackground stars,theplanets norter of a day during any given month.mally move eastward in what is called direct motionWhen the moon is in conjunction with the sun (newApproachingopposition,howeveraplanetwillslowdownmoon), itrises and sets with the sun and is lost in the sun'sglare. The moon is always moving eastward at about 12.20pause (ata stationary point),and begin moving westward(retrograde motion), until it reaches the next stationaryper day,sothat sometimeafter conjunction (as little as16point and resumes its direct motion.This is not because thehours, or as long as two days),thethin lunar crescent canbeplanet is moving strangely in space.This relative,observedobserved after sunset, low in the west.For the next couplemotion resultsbecause thefaster moving earth is catchingofweeks,themoonwill wax,becoming morefully illumi-upwith and passingbythe slowermoving superior planet.nated.Fromdaytoday.themoonwillrise(andset)laterThe superior planets are brightest and closest tothebecoming increasingly visible in the evening sky,untilearth at opposition. The interval between oppositions is(about7daysafternewmoon)itreachesfirstquarter,whenknown as the synodic period.This period is longest for thethe moon rises about noon and sets about midnight.Overclosestplanet, Mars,andbecomes increasingly shorterforthenext week the moon will rise later and later in the after+the outer planets.noon until full moon, when it rises about sunset andUnlike Mercury and Venus, the superior planets do notdominates the skythroughout the night.During the nextgo through a full cycleof phases.They are always full orcoupleofweeksthemoonwillwane.risinglaterandlaterhighly gibbous.at night. By last quarter (a week after full moon), the moonMars can usually be identified by its orange color.Itrises about midnight and sets at noon.As it approaches newcan become asbright as magnitude-2.8but ismore oftenmoon,themoonbecomes an increasinglythin crescent,andbetween-1.0and-2.0atopposition.Oppositionsoccuratisseenonlyintheearlymorningsky.Sometimebeforeconintervals of about 780 days.Theplanet is visiblefor aboutjunction(16hoursto 2daysbefore conjunction)thethin330 dayson either sideofopposition.Near conjunction it iscrescent will disappear in the glare of morning twilight.lostfromviewforabout120days.ItstwosatellitescanonlyAtfullmoon,thesunandmoonareonoppositesidesofbeseeninalargetelescopetheecliptic.Therefore,in the winter thefull moon risesearlyJupiter, largest of the known planets, normally out-crosses the celestial meridian high in the sky,and sets late; asshines Mars,regularly reaching magnitude-2.0 or brighterthe sun does in the summer.In the summer thefull moon ris-at opposition.Oppositions occur at intervalsof about 400es in the southeasternpartofthesky (Northern Hemisphere)days, with the planet being visible for about 180 days be-remains relatively low in the sky,and sets along the south-fore and after opposition.The planet disappears for aboutwesternhorizonafterashorttimeabovethehorizon.32daysat conjunction.Four satellites (ofatotal 16current-Atthetimeoftheautumnal equinox,thepartofthelyknown)arebrightenoughtobeseeninbinoculars.Theirecliptic oppositethe sun is most nearlyparallel tothehori-motions around Jupitercanbeobservedoverthecourseofzon.Sincetheeastwardmotionofthemoon is approximatelyseveral hours.along the ecliptic,thedelay in thetimeofrisingofthefull
NAVIGATIONAL ASTRONOMY 231 At these times it can be seen during the day and is sometimes observed for a celestial line of position. It appears as a morning or evening star for approximately 263 days in succession. Near inferior conjunction Venus disappears for 8 days; around superior conjunction it disappears for 50 days. When it transits the sun, Venus can be seen to the naked eye as a small dot about the size of a group of sunspots. Through binoculars, Venus can be seen to go through a full set of phases. 1511. Superior Planets As planets outside the earth’s orbit, the superior planets are not confined to the proximity of the sun as seen from the earth. They can pass behind the sun (conjunction), but they cannot pass between the sun and the earth. Instead we see them move away from the sun until they are opposite the sun in the sky (opposition). When a superior planet is near conjunction, it rises and sets approximately with the sun and is thus lost in the sun’s glare. Gradually it becomes visible in the early morning sky before sunrise. From day to day, it rises and sets earlier, becoming increasingly visible through the late night hours until dawn. Approaching opposition, the planet will rise in the late evening, until at opposition, it will rise when the sun sets, be visible throughout the night, and set when the sun rises. Observed against the background stars, the planets normally move eastward in what is called direct motion. Approaching opposition, however, a planet will slow down, pause (at a stationary point), and begin moving westward (retrograde motion), until it reaches the next stationary point and resumes its direct motion. This is not because the planet is moving strangely in space. This relative, observed motion results because the faster moving earth is catching up with and passing by the slower moving superior planet. The superior planets are brightest and closest to the earth at opposition. The interval between oppositions is known as the synodic period. This period is longest for the closest planet, Mars, and becomes increasingly shorter for the outer planets. Unlike Mercury and Venus, the superior planets do not go through a full cycle of phases. They are always full or highly gibbous. Mars can usually be identified by its orange color. It can become as bright as magnitude –2.8 but is more often between –1.0 and –2.0 at opposition. Oppositions occur at intervals of about 780 days. The planet is visible for about 330 days on either side of opposition. Near conjunction it is lost from view for about 120 days. Its two satellites can only be seen in a large telescope. Jupiter, largest of the known planets, normally outshines Mars, regularly reaching magnitude –2.0 or brighter at opposition. Oppositions occur at intervals of about 400 days, with the planet being visible for about 180 days before and after opposition. The planet disappears for about 32 days at conjunction. Four satellites (of a total 16 currently known) are bright enough to be seen in binoculars. Their motions around Jupiter can be observed over the course of several hours. Saturn, the outermost of the navigational planets, comes to opposition at intervals of about 380 days. It is visible for about 175 days before and after opposition, and disappears for about 25 days near conjunction. At opposition it becomes as bright as magnitude +0.8 to –0.2. Through good, high powered binoculars, Saturn appears as elongated because of its system of rings. A telescope is needed to examine the rings in any detail. Saturn is now known to have at least 18 satellites, none of which are visible to the unaided eye. Uranus, Neptune and Pluto are too faint to be used for navigation; Uranus, at about magnitude 5.5, is faintly visible to the unaided eye. 1512. The Moon The moon is the only satellite of direct navigational interest. It revolves around the earth once in about 27.3 days, as measured with respect to the stars. This is called the sidereal month. Because the moon rotates on its axis with the same period with which it revolves around the earth, the same side of the moon is always turned toward the earth. The cycle of phases depends on the moon’s revolution with respect to the sun. This synodic month is approximately 29.53 days, but can vary from this average by up to a quarter of a day during any given month. When the moon is in conjunction with the sun (new moon), it rises and sets with the sun and is lost in the sun’s glare. The moon is always moving eastward at about 12.2° per day, so that sometime after conjunction (as little as 16 hours, or as long as two days), the thin lunar crescent can be observed after sunset, low in the west. For the next couple of weeks, the moon will wax, becoming more fully illuminated. From day to day, the moon will rise (and set) later, becoming increasingly visible in the evening sky, until (about 7 days after new moon) it reaches first quarter, when the moon rises about noon and sets about midnight. Over the next week the moon will rise later and later in the afternoon until full moon, when it rises about sunset and dominates the sky throughout the night. During the next couple of weeks the moon will wane, rising later and later at night. By last quarter (a week after full moon), the moon rises about midnight and sets at noon. As it approaches new moon, the moon becomes an increasingly thin crescent, and is seen only in the early morning sky. Sometime before conjunction (16 hours to 2 days before conjunction) the thin crescent will disappear in the glare of morning twilight. At full moon, the sun and moon are on opposite sides of the ecliptic. Therefore, in the winter the full moon rises early, crosses the celestial meridian high in the sky, and sets late; as the sun does in the summer. In the summer the full moon rises in the southeastern part of the sky (Northern Hemisphere), remains relatively low in the sky, and sets along the southwestern horizon after a short time above the horizon. At the time of the autumnal equinox, the part of the ecliptic opposite the sun is most nearly parallel to the horizon. Since the eastward motion of the moon is approximately along the ecliptic, the delay in the time of rising of the full
232NAVIGATIONALASTRONOMYFirstQuarterLGHTGibbousCrescen4FROM+FulMoorMoor44-4SUNGibbousCrescent4LastQuarter4Figure1512.Phases of themoon.The inner figures of the moon represent its appearance fromthe earthmoonfromnighttonightis lessthan at othertimes ofthespeedawayfromthesolarsystemaftergainingvelocityasyear. The full moon nearest the autumnal equinox is calledthey pass by Jupiter or Saturn.theharvestmoon,thefullmoonamonthlateriscalledtheThe short-period comets long ago lost the gasses need-hunter'smoon.SeeFigure1512.ed to form a tail. Long period comets, such as Halley'scomet, are more likely to develop tails.The visibility of a1513.Comets And Meteorscometdependsverymuchonhowcloseitapproachestheearth. In 1910, Halley's comet spread across the sky.YetAlthough comets are noted as great spectacles of na-when it returned in 1986, the earth was not well situated toture, very few arevisible without a telescope.Those thatgeta good view,and itwas barely visibletotheunaidedeyebecome widely visible do so because they develop long,Meeors,popularlycalled shooing stars,are tinyoglowingtails.Cometsareswarmsof relativelysmall solididbodiestoosmalltobeseenuntil heatedtoincandescencebodies held together by gravity. Around the nucleus, a gas-by air friction while passing through the earth's atmo-eous head or coma and tail may form as the cometsphere.A particularly bright meteor is called a fireballapproaches the sun. The tail is directed away from the sun,One thatexplodes is calledabolide.Ameteor that survivesso that itfollows theheadwhile thecomet is approaching theits tripthroughtheatmosphere and lands as a solidparticlesun,and precedesthehead whilethecometis receding.Theiscalled ameteoritetotal mass ofa comet is very small, and the tail is so thin thatVastnumbersofmeteors exist.Ithasbeenestimatedstars can easilybe seen through it. In 1910, the earth passedthat anaverageofabout 1,000,000 brightenoughto be seenthrough thetail of Halley's comet without noticeable effect.entertheearth'satmosphereeachhour,andmanytimesthisCompared to the well-ordered orbits of the planets,number undoubtedly enter, but are too small to attractcomets are erratic and inconsistent. Some travel eastto westattention.andsomewesttoeast.inhighlyeccentricorbitsinclinedatMeteor showers occur at certain times of the year whenany angle to theecliptic.Periods ofrevolution rangefromabout 3 years to thousands of years. Some comets maytheearthpassesthroughmeteorswarms,thescatteredre
232 NAVIGATIONAL ASTRONOMY moon from night to night is less than at other times of the year. The full moon nearest the autumnal equinox is called the harvest moon; the full moon a month later is called the hunter’s moon. See Figure 1512. 1513. Comets And Meteors Although comets are noted as great spectacles of nature, very few are visible without a telescope. Those that become widely visible do so because they develop long, glowing tails. Comets are swarms of relatively small solid bodies held together by gravity. Around the nucleus, a gaseous head or coma and tail may form as the comet approaches the sun. The tail is directed away from the sun, so that it follows the head while the comet is approaching the sun, and precedes the head while the comet is receding. The total mass of a comet is very small, and the tail is so thin that stars can easily be seen through it. In 1910, the earth passed through the tail of Halley’s comet without noticeable effect. Compared to the well-ordered orbits of the planets, comets are erratic and inconsistent. Some travel east to west and some west to east, in highly eccentric orbits inclined at any angle to the ecliptic. Periods of revolution range from about 3 years to thousands of years. Some comets may speed away from the solar system after gaining velocity as they pass by Jupiter or Saturn. The short-period comets long ago lost the gasses needed to form a tail. Long period comets, such as Halley’s comet, are more likely to develop tails. The visibility of a comet depends very much on how close it approaches the earth. In 1910, Halley’s comet spread across the sky. Yet when it returned in 1986, the earth was not well situated to get a good view, and it was barely visible to the unaided eye. Meteors, popularly called shooting stars, are tiny, solid bodies too small to be seen until heated to incandescence by air friction while passing through the earth’s atmosphere. A particularly bright meteor is called a fireball. One that explodes is called a bolide. A meteor that survives its trip through the atmosphere and lands as a solid particle is called a meteorite. Vast numbers of meteors exist. It has been estimated that an average of about 1,000,000 bright enough to be seen enter the earth’s atmosphere each hour, and many times this number undoubtedly enter, but are too small to attract attention. Meteor showers occur at certain times of the year when the earth passes through meteor swarms, the scattered reFigure 1512. Phases of the moon. The inner figures of the moon represent its appearance from the earth
233NAVIGATIONALASTRONOMYmainsofcometsthathavebrokenup.AtthesetimestheApril26April27Aprll30May2May3May4May6Halley'sCometin1910May15May23May28June3June6June9June1fFigure1513.Halley'sComet:fourteenviews,madebetweenApril26and June11,1910Courtesyof Mt.Wilson and PalomarObservatories.numberofmeteorsobservedismanytimestheusualnumberStrikingconfigurations,knownasconstellations,werenot-A faint glow sometimes observed extending upwarded byancientpeoples, who supplied themwith names andmyths.Todayastronomers use constellations-88 in all-approximatelyalongtheeclipticbefore sunriseandafterto identifyareasofthe sky.sunsethas been attributed tothereflection of sunlightfromquantities of this material.This glow is called zodiacalUnder ideal viewing conditions,the dimmest star thatlight.Afaintglow at that point ofthe ecliptic 180°from thecanbeseenwiththeunaidedeveisofthesixthmagnitudesun is called the gegenschein or counterglowIn the entire skythere are about 6000 stars of this magni-tude or brighter.Half of these are below the horizon at any1514.Starstime.Becauseofthegreaterabsorptionoflightnearthehorizon, where the path of a ray travels for a greater distancethroughtheatmosphere,notmorethanperhaps2,5o0 starsStars aredistant suns, in many ways resembling theare visible to the unaided eye at any time.However,the av-body which provides the earth with most of its light anderagenavigator seldomusesmorethanperhaps20or30ofheat.Likethesun,starsaremassiveballsofgasthatcreatethebrighter stars.theirown energy through thermonuclear reactionsStars which exhibit a noticeablechange of magnitudeAlthoughstarsdifferinsizeandtemperature,thesedifare called variable stars.A star which suddenlybecomesferences areapparent onlythroughanalysis byastronomers.several magnitudes brighter and then gradually fades isSomedifferences incolor arenoticeabletotheunaidedeye.called a nova.A particularly bright nova is called aWhilemoststarsappearwhite,some(thoseoflowertemper-ature)havea reddish hue.In Orion, blueRigel and redsupernovaBetelgeuse, located on opposite sides ofthe belt, constituteTwo starswhich appear tobevery closetogether areanoticeablecontrast.calledadoublestar.IfmorethantwostarsareincludedinThe stars are not distributed uniformlyaround the skythe group, it is called a multiple star.Agroup ofa few doz-
NAVIGATIONAL ASTRONOMY 233 mains of comets that have broken up. At these times the number of meteors observed is many times the usual number. A faint glow sometimes observed extending upward approximately along the ecliptic before sunrise and after sunset has been attributed to the reflection of sunlight from quantities of this material. This glow is called zodiacal light. A faint glow at that point of the ecliptic 180° from the sun is called the gegenschein or counterglow. 1514. Stars Stars are distant suns, in many ways resembling the body which provides the earth with most of its light and heat. Like the sun, stars are massive balls of gas that create their own energy through thermonuclear reactions. Although stars differ in size and temperature, these differences are apparent only through analysis by astronomers. Some differences in color are noticeable to the unaided eye. While most stars appear white, some (those of lower temperature) have a reddish hue. In Orion, blue Rigel and red Betelgeuse, located on opposite sides of the belt, constitute a noticeable contrast. The stars are not distributed uniformly around the sky. Striking configurations, known as constellations, were noted by ancient peoples, who supplied them with names and myths. Today astronomers use constellations—88 in all— to identify areas of the sky. Under ideal viewing conditions, the dimmest star that can be seen with the unaided eye is of the sixth magnitude. In the entire sky there are about 6,000 stars of this magnitude or brighter. Half of these are below the horizon at any time. Because of the greater absorption of light near the horizon, where the path of a ray travels for a greater distance through the atmosphere, not more than perhaps 2,500 stars are visible to the unaided eye at any time. However, the average navigator seldom uses more than perhaps 20 or 30 of the brighter stars. Stars which exhibit a noticeable change of magnitude are called variable stars. A star which suddenly becomes several magnitudes brighter and then gradually fades is called a nova. A particularly bright nova is called a supernova. Two stars which appear to be very close together are called a double star. If more than two stars are included in the group, it is called a multiple star. A group of a few dozFigure 1513. Halley’s Comet; fourteen views, made between April 26 and June 11, 1910. Courtesy of Mt. Wilson and Palomar Observatories
234NAVIGATIONALASTRONOMYen to several hundred stars moving through spacetogetherLarge and Small Magellanic Clouds (named afterFerdi-is called an open cluster.ThePleiades is an example of annand Magellan)are the nearestknown neighbors oftheopen cluster.Thereare also spherically symmetric clustersMilkyWay.Theyare approximately1,700,000lightyearsof hundreds of thousands of starsknown asglobularclus-distant. The Magellanic Clouds can be seen as sizableters.Theglobularclustersarealltoodistanttobeseenwithglowingpatches inthesouthern sky.the naked eye.Acloudypatch ofmatter in the heavens is calleda neb-ula.If it is within the galaxy of which the sun is a part, it iscalled a galactic nebula, ifoutside, it is called an extraga-lactic nebula.Motion ofa star through spacecan be classified by itsvectorcomponents.That component inthelineof sight iscalled radialmotion,while that component across the lineof sight, causinga star to change its apparent positionrela-tive to the background of more distant stars,is calledpropermotion.1515.GalaxiesAgalaxy is a vastcollection ofclusters ofstarsandcloudsof gas.The earth islocated in theMilkyWaygalaxy,a slowlyspinning disk more than 100,000 light years in diameter. Allthe bright stars in the sky are in the Milky Way.However, themost dense portions ofthe galaxy are seen as thegreat, broadband thatglows in the summer nighttime sky.When we looktowardtheconstellation Sagittarius,wearelookingtoward thecenteroftheMilkyWay,30,000lightyears awayDespite their size and luminance, almost all other gal-Figure 1515. Spiral nebula Messier 51, In Canes Venetici.axies are too far away to be seenwith the unaided eye.AnSatellitenebulaisNGC5195exception in the northern hemisphere is the Great GalaxyCourtesy of Mt.Wilson and Palomar Observatories.(sometimes called the Great Nebula) in Andromeda, whichappearsasafaintglow.Inthe southernhemisphere,theAPPARENTMOTION1516.ApparentMotionDueToRotationOfTheEarthshowninFigure1516aForan observer atoneof thepoles,bodieshaving con-Apparentmotion caused by theearth'srotation isstant declination neither risenorset(neglectingprecessionmuchgreaterthan any other observed motion of celestialof the equinoxes and changes in refraction),but circle thebodies.It is this motion that causes celestial bodies to ap-sky,alwaysatthesamealtitude,makingonecompletetrippeartorise alongthe eastern halfof thehorizon,climbtoaround thehorizoneachday.AttheNorthPolethemotionmaximumaltitudeas theycross themeridian,and setalongis clockwise,and attheSouthPoleitis counterclockwise.the western horizon,ataboutthesamepointrelativetodueApproximatelyhalf the stars are always abovethehorizonwest as the rising point was to due east.This apparentmo-and the otherhalf never are.The parallel sphere at the polestion along the daily path, or diurnal circle, of the body isisillustratedinFigure1516b.approximatelyparallel totheplaneof theequator.ItwouldBetween these two extremes, the apparent motion is abeexactlyso ifrotationof theearthweretheonlymotioncombinationofthetwo.Onthisobliquesphere,illustrated inandtheaxisofrotationoftheearthwerestationaryinspaceFigure1516c,circumpolarcelestial bodiesremain abovetheThe apparent effect due to rotation of the earthvarieshorizon during the entire24hours,circling the elevated ce-with the latitude of theobserver. At the equator,where thelestial pole each day.The stars of Ursa Major (theBigequatorial plane is vertical (sincethe axis ofrotation oftheDipper)andCassiopeiaarecircumpolarformanyobserversearth is parallel to theplane of thehorizon),bodies appearin the United States.An approximately equal part ofthe ce-to rise and set vertically.Every celestial body is above thehorizon approximately half the time.The celestial sphere aslestial sphere remains belowthe horizon during the entireseen by an observer at the equator is called theright sphere,day.Crux is not visibleto most observers in the United
234 NAVIGATIONAL ASTRONOMY en to several hundred stars moving through space together is called an open cluster. The Pleiades is an example of an open cluster. There are also spherically symmetric clusters of hundreds of thousands of stars known as globular clusters. The globular clusters are all too distant to be seen with the naked eye. A cloudy patch of matter in the heavens is called a nebula. If it is within the galaxy of which the sun is a part, it is called a galactic nebula; if outside, it is called an extragalactic nebula. Motion of a star through space can be classified by its vector components. That component in the line of sight is called radial motion, while that component across the line of sight, causing a star to change its apparent position relative to the background of more distant stars, is called proper motion. 1515. Galaxies A galaxy is a vast collection of clusters of stars and clouds of gas. The earth is located in the Milky Way galaxy, a slowly spinning disk more than 100,000 light years in diameter. All the bright stars in the sky are in the Milky Way. However, the most dense portions of the galaxy are seen as the great, broad band that glows in the summer nighttime sky. When we look toward the constellation Sagittarius, we are looking toward the center of the Milky Way, 30,000 light years away. Despite their size and luminance, almost all other galaxies are too far away to be seen with the unaided eye. An exception in the northern hemisphere is the Great Galaxy (sometimes called the Great Nebula) in Andromeda, which appears as a faint glow. In the southern hemisphere, the Large and Small Magellanic Clouds (named after Ferdinand Magellan) are the nearest known neighbors of the Milky Way. They are approximately 1,700,000 light years distant. The Magellanic Clouds can be seen as sizable glowing patches in the southern sky. APPARENT MOTION 1516. Apparent Motion Due To Rotation Of The Earth Apparent motion caused by the earth’s rotation is much greater than any other observed motion of celestial bodies. It is this motion that causes celestial bodies to appear to rise along the eastern half of the horizon, climb to maximum altitude as they cross the meridian, and set along the western horizon, at about the same point relative to due west as the rising point was to due east. This apparent motion along the daily path, or diurnal circle, of the body is approximately parallel to the plane of the equator. It would be exactly so if rotation of the earth were the only motion and the axis of rotation of the earth were stationary in space. The apparent effect due to rotation of the earth varies with the latitude of the observer. At the equator, where the equatorial plane is vertical (since the axis of rotation of the earth is parallel to the plane of the horizon), bodies appear to rise and set vertically. Every celestial body is above the horizon approximately half the time. The celestial sphere as seen by an observer at the equator is called the right sphere, shown in Figure 1516a. For an observer at one of the poles, bodies having constant declination neither rise nor set (neglecting precession of the equinoxes and changes in refraction), but circle the sky, always at the same altitude, making one complete trip around the horizon each day. At the North Pole the motion is clockwise, and at the South Pole it is counterclockwise. Approximately half the stars are always above the horizon and the other half never are. The parallel sphere at the poles is illustrated in Figure 1516b. Between these two extremes, the apparent motion is a combination of the two. On this oblique sphere, illustrated in Figure 1516c, circumpolar celestial bodies remain above the horizon during the entire 24 hours, circling the elevated celestial pole each day. The stars of Ursa Major (the Big Dipper) and Cassiopeia are circumpolar for many observers in the United States. An approximately equal part of the celestial sphere remains below the horizon during the entire day. Crux is not visible to most observers in the United Figure 1515. Spiral nebula Messier 51, In Canes Venetici. Satellite nebula is NGC 5195. Courtesy of Mt. Wilson and Palomar Observatories