《航海学》课程参考文献（地文资料）CHAPTER 15 NAVIGATIONAL ASTRONOMY

CHAPTER 15NAVIGATIONALASTRONOMYPRELIMINARYCONSIDERATIONS1500.Definitioning principally with celestial coordinates, time, and theapparentmotions ofcelestial bodies,is thebranch of as-Astronomy predicts the futurepositions andmotionstronomy most important to the navigator.The symbolsof celestial bodies and seeks to understand and explaincommonly recognized in navigational astronomy aregiveninTable1500.their physical properties.Navigational astronomy,deal-Celestial BodiesOSunLower limbMoon0tCenter学Mercury7UpperlimbQVenusONewmoon@EarthCrescentmoonMarsOFirst quarter24JupiterOGibbous moonhSaturnOFullmoonUranusOGibbousmoonyNeptuneOLast quarterePlutoCrescentmoon★Star☆-P Star-planet altitude correction (alti-tude)Miscellaneous SymbolsYears米InterpolationimpracticalMonthsDegreesDays'Minutes of areHoursWSeconds of areMinutes of timed Conjunction· Seconds of time8OppositionRemsins belowhorizonQuadratureRemains above horizon88Ascending nodeUIll Twilight all nigheDescending nodeSigns of the ZodiaeT Aries (vernal equinox)Libra (autumnal equinox)8Taurusm Scorpius口Gemini1SagittariusCancer (summer solstice)Capricornus (wintersolstice)&Leo-AquariusVirgoXPisceeTable1500.Astronomicalsymbols.225

225 CHAPTER 15 NAVIGATIONAL ASTRONOMY PRELIMINARY CONSIDERATIONS 1500. Definition Astronomy predicts the future positions and motions of celestial bodies and seeks to understand and explain their physical properties. Navigational astronomy, deal￾ing principally with celestial coordinates, time, and the apparent motions of celestial bodies, is the branch of as￾tronomy most important to the navigator. The symbols commonly recognized in navigational astronomy are given in Table 1500. Table 1500. Astronomical symbols

226NAVIGATIONALASTRONOMY1501.TheCelestial Sphereserverat some distant point in space.When discussing therisingorsettingofabodvonalocalhorizon,wemustlocateLooking at the sky on a dark night, imagine that celes-the observer at a particularpoint on the earth because thetial bodies are located on the inner surface of a vast, earth-setting sun for one observer may be the rising sun foranother.centered sphere. This model is useful since we are only in-terested in the relativepositions and motions of celestialMotiononthecelestial sphereresultsfromthemotionsbodies on this imaginary surface.Understanding the con-in space of both the celestial body and the earth.Withoutcept of the celestial sphere is most important whenspecial instruments,motionstowardand awayfromthediscussingsightreduction inChapter20earthcannotbediscerned1502.RelativeAndApparentMotion1503.AstronomicalDistancesCelestial bodiesare in constantmotion.There isnoConsiderthecelestial sphere as having an infinite radi-fixed position in spacefrom whichonecan observeabso-usbecausedistancesbetweencelestial bodiesarelutemotion.Sinceall motionisrelative,thepositionoftheremarkablyvast.Thedifficultyofillustratingastronomicalobservermustbenotedwhendiscussingplanetarymotion.distances is indicated by thefact that ifthe earth were rep-Fromtheearthweseeapparentmotionsofcelestialbodiesresented by a circle one inch in diameter,themoon wouldon the celestial sphere.In considering how planetsfollowbeacircleone-fourthinchindiameteratadistanceof30their orbits around the sun, we assume a hypothetical ob-inches, the sun would be a circle ninefeet in diameter atFigure 1501.The celestial sphere

226 NAVIGATIONAL ASTRONOMY 1501. The Celestial Sphere Looking at the sky on a dark night, imagine that celes￾tial bodies are located on the inner surface of a vast, earth￾centered sphere. This model is useful since we are only in￾terested in the relative positions and motions of celestial bodies on this imaginary surface. Understanding the con￾cept of the celestial sphere is most important when discussing sight reduction in Chapter 20. 1502. Relative And Apparent Motion Celestial bodies are in constant motion. There is no fixed position in space from which one can observe abso￾lute motion. Since all motion is relative, the position of the observer must be noted when discussing planetary motion. From the earth we see apparent motions of celestial bodies on the celestial sphere. In considering how planets follow their orbits around the sun, we assume a hypothetical ob￾server at some distant point in space. When discussing the rising or setting of a body on a local horizon, we must locate the observer at a particular point on the earth because the setting sun for one observer may be the rising sun for another. Motion on the celestial sphere results from the motions in space of both the celestial body and the earth. Without special instruments, motions toward and away from the earth cannot be discerned. 1503. Astronomical Distances Consider the celestial sphere as having an infinite radi￾us because distances between celestial bodies are remarkably vast. The difficulty of illustrating astronomical distances is indicated by the fact that if the earth were rep￾resented by a circle one inch in diameter, the moon would be a circle one-fourth inch in diameter at a distance of 30 inches, the sun would be a circle nine feet in diameter at Figure 1501. The celestial sphere

227NAVIGATIONALASTRONOMYadistanceof nearlyafifth ofa mile,and Plutowould bea1504.Magnitudecircle half an inch in diameter at a distance ofabout sevenThe relativebrightness of celestial bodies is indicatedmiles.Theneareststarwouldbeone-fifththeactual dis-by a scale of stellarmagnitudes.Initially,astronomers di-tancetothemoonvided the stars into 6groups accordingto brightness.TheBecause ofthe size ofcelestialdistances,itis inconve-20 brightest wereclassifiedas of thefirstmagnitude, andnientto measurethem in common units such as themile orthedimmestwereofthesixthmagnitude.Inmoderntimeskilometer.Themeandistanceto our nearest neighbor, thewhenitbecamedesirabletodefinemorepreciselythe limitsmoon, is 238,900miles.For conveniencethisdistance isof magnitude,a first magnitude star was considered 100sometimes expressed in units oftheequatorial radiusofthetimes brighter than one of the sixth magnitude.Since theearth:60.27earthradii.fifthrootof100is2.512.thisnumberisconsideredtheDistances between theplanets are usuallyexpressed inmagnitude ratio.A first magnitude star is 2.512 times asterms of the astronomical unit (AU),the mean distancebright as a second magnitude star, which is 2.512 times asbetween the earth and the sun.This is approximatelybright as a third magnitude star,.A second magnitude is92,960,000miles.Thusthemeandistanceoftheearthfrom2.512x2.512=6.310timesasbrightasafourthmagnitudestar. A first magnitude star is 2.51220 times as bright as athe sun is 1 A.U.The mean distance of Pluto, the outermoststar ofthe 2ist magnitude,the dimmestthat can be seenknownplanetinoursolarsystem,is39.5A.U.Expressed inthrough a 200-inchtelescope.astronomical units,the mean distancefrom theearthto theBrightness is normally tabulated to the nearest 0.1moonis0.00257A.U.magnitude,aboutthesmallestchangethatcanbedetectedDistances to the stars require another leap in units.Aby the unaided eye ofa trained observer.All stars of magcommonly-used unit is the light-year, the distance lightnitude 1.50 or brighter are popularly called firsttravels in one year.Since the speed of light is about 1.86xmagnitude"stars.Thosebetween 1.51and 2.50 are called105milespersecondandthereareabout3.16×107seconds"second magnitude"stars,thosebetween2.51and3.50areper year, the length of one light-year is about 5.88 × 1012called "third magnitude"stars,etc.Sirius, thebrightest star.miles.Theneareststars,AlphaCentaurianditsneighborhasamagnitudeof-1.6.TheonlyotherstarwithanegativeProxima,are 4.3 light-years away.Relativelyfew stars aremagnitude is Canopus, -0.9. At greatest brilliance Venusless than 100 light-years away.The nearest galaxies, thehasamagnitudeofabout-4.4.Mars,Jupiter,andSaturnareClouds of Magellan, are 150,000 to 200,000 light yearssometimes of negative magnitude.The full moon has aaway.Themost distantgalaxies observed byastronomersmagnitudeofabout-12.6,but varies somewhat.The magare severalbillion light years away.nitude of the sun is about -26.7.THEUNIVERSE1505.TheSolarSystemThehierarchies of motions in the universe are caused bytheforce of gravity.As a result ofgravity,bodies attract eachotherinproportiontotheirmassesandtotheinversesquareThe sun, the most conspicuous celestial object in the sky,of thedistancesbetweenthem.This forcecauses theplanetsis the central body ofthe solar system.Associated with it are atto go around the sun in nearly circular,elliptical orbitsleast nine principal planets and thousands of asteroids, com-In eachplanet'sorbit,thepointnearestthesun iscalledets,andmeteors.Someplanets likeearthhave satellitesthe perihelion.The point farthest from the sun is called theaphelion. The line joining perihelion and aphelion is called1506.MotionsOfBodiesOfTheSolarSystemthe line of apsides.In the orbit ofthemoon, the point near-est the earth is called the perigee, and that point farthestAstronomers distinguish between two principal mo-from the earth is called the apogee. Figure 1506 shows thetions of celestial bodies.Rotation is a spinning motionorbit of the earth (with exaggerated eccentricity), and theabout an axis within the body, whereas revolution is theorbit ofthemoon around the earth.motionofabodyinitsorbitaroundanotherbody.Thebodyaroundwhichacelestialobjectrevolvesisknownasthat1507.The Sunbody's primary.For the satellites,the primary is a planet.For the planets and other bodies of the solar system,thepri-Thesundominates our solar system.Itsmass is nearlyamary is the sun.The entire solar system is held together bythousand timesthatofall otherbodiesofthe solarsystemcom-thegravitational force of the sun.The whole system re-bined. Its diameter is about 866,000 miles. Since it is a star, itvolvesaround thecenteroftheMilky Waygalaxy(sectiongenerates its own energy throughthermonuclear reactions,1515), and the Milky Way is in motion relative to its neigh-thereby providing heat and light for the entire solar systemboring galaxies.Pluto

NAVIGATIONAL ASTRONOMY 227 a distance of nearly a fifth of a mile, and Pluto would be a circle half an inch in diameter at a distance of about seven miles. The nearest star would be one-fifth the actual dis￾tance to the moon. Because of the size of celestial distances, it is inconve￾nient to measure them in common units such as the mile or kilometer. The mean distance to our nearest neighbor, the moon, is 238,900 miles. For convenience this distance is sometimes expressed in units of the equatorial radius of the earth: 60.27 earth radii. Distances between the planets are usually expressed in terms of the astronomical unit (AU), the mean distance between the earth and the sun. This is approximately 92,960,000 miles. Thus the mean distance of the earth from the sun is 1 A.U. The mean distance of Pluto, the outermost known planet in our solar system, is 39.5 A.U. Expressed in astronomical units, the mean distance from the earth to the moon is 0.00257 A.U. Distances to the stars require another leap in units. A commonly-used unit is the light-year, the distance light travels in one year. Since the speed of light is about 1.86 × 105 miles per second and there are about 3.16 × 107 seconds per year, the length of one light-year is about 5.88 × 1012 miles. The nearest stars, Alpha Centauri and its neighbor Proxima, are 4.3 light-years away. Relatively few stars are less than 100 light-years away. The nearest galaxies, the Clouds of Magellan, are 150,000 to 200,000 light years away. The most distant galaxies observed by astronomers are several billion light years away. 1504. Magnitude The relative brightness of celestial bodies is indicated by a scale of stellar magnitudes. Initially, astronomers di￾vided the stars into 6 groups according to brightness. The 20 brightest were classified as of the first magnitude, and the dimmest were of the sixth magnitude. In modern times, when it became desirable to define more precisely the limits of magnitude, a first magnitude star was considered 100 times brighter than one of the sixth magnitude. Since the fifth root of 100 is 2.512, this number is considered the magnitude ratio. A first magnitude star is 2.512 times as bright as a second magnitude star, which is 2.512 times as bright as a third magnitude star,. A second magnitude is 2.512 × 2.512 = 6.310 times as bright as a fourth magnitude star. A first magnitude star is 2.51220 times as bright as a star of the 21st magnitude, the dimmest that can be seen through a 200-inch telescope. Brightness is normally tabulated to the nearest 0.1 magnitude, about the smallest change that can be detected by the unaided eye of a trained observer. All stars of mag￾nitude 1.50 or brighter are popularly called “first magnitude” stars. Those between 1.51 and 2.50 are called “second magnitude” stars, those between 2.51 and 3.50 are called “third magnitude” stars, etc. Sirius, the brightest star, has a magnitude of –1.6. The only other star with a negative magnitude is Canopus, –0.9. At greatest brilliance Venus has a magnitude of about –4.4. Mars, Jupiter, and Saturn are sometimes of negative magnitude. The full moon has a magnitude of about –12.6, but varies somewhat. The mag￾nitude of the sun is about –26.7. THE UNIVERSE 1505. The Solar System The sun, the most conspicuous celestial object in the sky, is the central body of the solar system. Associated with it are at least nine principal planets and thousands of asteroids, com￾ets, and meteors. Some planets like earth have satellites. 1506. Motions Of Bodies Of The Solar System Astronomers distinguish between two principal mo￾tions of celestial bodies. Rotation is a spinning motion about an axis within the body, whereas revolution is the motion of a body in its orbit around another body. The body around which a celestial object revolves is known as that body’s primary. For the satellites, the primary is a planet. For the planets and other bodies of the solar system, the pri￾mary is the sun. The entire solar system is held together by the gravitational force of the sun. The whole system re￾volves around the center of the Milky Way galaxy (section 1515), and the Milky Way is in motion relative to its neigh￾boring galaxies. The hierarchies of motions in the universe are caused by the force of gravity. As a result of gravity, bodies attract each other in proportion to their masses and to the inverse square of the distances between them. This force causes the planets to go around the sun in nearly circular, elliptical orbits. In each planet’s orbit, the point nearest the sun is called the perihelion. The point farthest from the sun is called the aphelion. The line joining perihelion and aphelion is called the line of apsides. In the orbit of the moon, the point near￾est the earth is called the perigee, and that point farthest from the earth is called the apogee. Figure 1506 shows the orbit of the earth (with exaggerated eccentricity), and the orbit of the moon around the earth. 1507. The Sun The sun dominates our solar system. Its mass is nearly a thousand times that of all other bodies of the solar system com￾bined. Its diameter is about 866,000 miles. Since it is a star, it generates its own energy through thermonuclear reactions, thereby providing heat and light for the entire solar system

228NAVIGATIONALASTRONOMYThe distance from the earth to the sun varies fromphotosphere.91,300,000 atperihelion to94,500,000milesat aphelion.The sun is continuously emitting charged particles,When the earth is at perihelion, which always occurs earlywhich form the solar wind. As the solar wind sweeps pastin January,the sun appears largest, 32.6'in diameter.Sixthe earth, these particles interact with the earth's magneticfield. If the solar wind is particularly strong, the interactionmonths later at aphelion, the sun's apparent diameter is aminimumof31.5'.can producemagnetic storms which adverselyaffectradioObservations of the sun's surface (called the photo-signals on the earth. At such times the auroras are particu-sphere) reveal small dark areas called sunspots. These arelarlybrilliantandwidespread.areas of intensemagneticfields inwhichrelativelycool gas (atThe sun is moving approximately in the direction of7000°F.)appears dark in contrast to the surrounding hotter gasVega at about 12 miles per second, or about two-thirds as(10,000°F.).Sunspots vary in sizefrom perhaps 50,000 milesfast as the earth moves in its orbit around the sun.This is inindiameter to the smallest spots thatcan bedetected (afewadditionto thegeneral motion ofthe sunaround the centerhundred miles in diameter).They generally appear in groups.of ourgalaxy.Large sunspots canbe seen withoutatelescope if the eyes areprotected, as by the shade glasses ofa sextant1508.PlanetsSurrounding the photosphere is an outer corona ofThe principal bodies orbiting the sun are called planets.very hot but tenuous gas. This can only be seen during aneclipse of the sun, when the moon blocks the light of theNineprincipal planets areknown:Mercury,Venus,Earth,(April)PERIHELION(Januory)LINEOF APSIDIAPHELUION(uuly)(Odober)Figure1506.Orbits of the earth and mo0n

228 NAVIGATIONAL ASTRONOMY The distance from the earth to the sun varies from 91,300,000 at perihelion to 94,500,000 miles at aphelion. When the earth is at perihelion, which always occurs early in January, the sun appears largest, 32.6' in diameter. Six months later at aphelion, the sun’s apparent diameter is a minimum of 31.5'. Observations of the sun’s surface (called the photo￾sphere) reveal small dark areas called sunspots. These are areas of intense magnetic fields in which relatively cool gas (at 7000°F.) appears dark in contrast to the surrounding hotter gas (10,000°F.). Sunspots vary in size from perhaps 50,000 miles in diameter to the smallest spots that can be detected (a few hundred miles in diameter). They generally appear in groups. Large sunspots can be seen without a telescope if the eyes are protected, as by the shade glasses of a sextant. Surrounding the photosphere is an outer corona of very hot but tenuous gas. This can only be seen during an eclipse of the sun, when the moon blocks the light of the photosphere. The sun is continuously emitting charged particles, which form the solar wind. As the solar wind sweeps past the earth, these particles interact with the earth’s magnetic field. If the solar wind is particularly strong, the interaction can produce magnetic storms which adversely affect radio signals on the earth. At such times the auroras are particu￾larly brilliant and widespread. The sun is moving approximately in the direction of Vega at about 12 miles per second, or about two-thirds as fast as the earth moves in its orbit around the sun. This is in addition to the general motion of the sun around the center of our galaxy. 1508. Planets The principal bodies orbiting the sun are called planets. Nine principal planets are known: Mercury, Venus, Earth, Figure 1506. Orbits of the earth and moon

229NAVIGATIONALASTRONOMYFigure1507.Wholesolardiskand anenlargementoftheFigure1509.Oblatespheroidorellipsoidof revolutiongreatspotgroupofApril7,1947.Courtesy of Mt. Wilson and Palomar Observatories.Mars,Jupiter,Saturn,Uranus,Neptune,andPluto.fThe orbits of manythousands of tinyminorplanets orasteroids lie chieflybetween the orbits of Mars and Jupiter.these,onlyfourarecommonlyusedforcelestial navigationThese are all too faint to be seen with thenaked eyeVenus,Mars, Jupiter,and SaturnExceptfor Pluto,the orbits of the planets lie in nearly1509.TheEarththe same plane as the earth's orbit.Therefore, as seen fromthe earth, theplanets areconfined toa stripof the celestialIn common with other planets, theearth rotates on itssphere called the ecliptic.axis and revolves in its orbit around the sun.These motionsThetwo planets with orbits smaller than that of theeartharetheprincipal sourceofthedailyapparentmotions ofare called inferiorplanets, and those with orbits largerthanothercelestialbodies.Theearth'srotationalsocausesadethat of the earth are called superior planets. The four planetsflection of waterand air currents totheright in the Northernnearestthesunaresometimescalledtheinnerplanets,andtheHemisphereandtotheleft inthe SouthernHemisphere.Be-others the outer planets.Jupiter, Saturn, Uranus, and Neptunecause of the earth's rotation, high tides on the open sea lagare so much larger than the others that they are sometimesbehind the meridian transit of themoon.classed as major planets. Uranus is barely visible to the unaid-Formostnavigationalpurposes,theearthcanbeconedeye,NeptuneandPlutoarenotvisiblewithoutatelescopesidered a sphere.However, like the other planets, the earthPlanets can be identified in the sky because, unlike theis approximatelyan oblate spheroid,or ellipsoid of revo-stars.thevdonottwinkle.Thestarsaresodistantthattheylution, flattened at thepoles and bulged at the equator. Seearevirtuallypointsources of light.Therefore thetiny streamFigure1509.Therefore, the polar diameter is less than theoflightfromastariseasilyscatteredbynormalmotionsofequatorial diameter, and the meridians are slightly ellipti-air in the atmospherecausing theaffectoftwinkling.Thena-cal, rather than circular.The dimensions of the earth areked-eyeplanets.however,areclose enoughto presentrecomputed from timetotime,asadditional andmorepre-perceptible disks. The broader stream of light from a planetcisemeasurementsbecomeavailable.Sincetheearthis notis noteasily disrupted unless theplanet is low on the horizonexactly an ellipsoid, results differ slightly when equallyor the air is especially turbulent.precise and extensivemeasurements are made on different

NAVIGATIONAL ASTRONOMY 229 Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. Of these, only four are commonly used for celestial navigation: Venus, Mars, Jupiter, and Saturn. Except for Pluto, the orbits of the planets lie in nearly the same plane as the earth’s orbit. Therefore, as seen from the earth, the planets are confined to a strip of the celestial sphere called the ecliptic. The two planets with orbits smaller than that of the earth are called inferior planets, and those with orbits larger than that of the earth are called superior planets. The four planets nearest the sun are sometimes called the inner planets, and the others the outer planets. Jupiter, Saturn, Uranus, and Neptune are so much larger than the others that they are sometimes classed as major planets. Uranus is barely visible to the unaid￾ed eye; Neptune and Pluto are not visible without a telescope. Planets can be identified in the sky because, unlike the stars, they do not twinkle. The stars are so distant that they are virtually point sources of light. Therefore the tiny stream of light from a star is easily scattered by normal motions of air in the atmosphere causing the affect of twinkling. The na￾ked-eye planets, however, are close enough to present perceptible disks. The broader stream of light from a planet is not easily disrupted unless the planet is low on the horizon or the air is especially turbulent. The orbits of many thousands of tiny minor planets or asteroids lie chiefly between the orbits of Mars and Jupiter. These are all too faint to be seen with the naked eye. 1509. The Earth In common with other planets, the earth rotates on its axis and revolves in its orbit around the sun. These motions are the principal source of the daily apparent motions of other celestial bodies. The earth’s rotation also causes a de￾flection of water and air currents to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. Be￾cause of the earth’s rotation, high tides on the open sea lag behind the meridian transit of the moon. For most navigational purposes, the earth can be con￾sidered a sphere. However, like the other planets, the earth is approximately an oblate spheroid, or ellipsoid of revo￾lution, flattened at the poles and bulged at the equator. See Figure 1509. Therefore, the polar diameter is less than the equatorial diameter, and the meridians are slightly ellipti￾cal, rather than circular. The dimensions of the earth are recomputed from time to time, as additional and more pre￾cise measurements become available. Since the earth is not exactly an ellipsoid, results differ slightly when equally precise and extensive measurements are made on different Figure 1507. Whole solar disk and an enlargement of the great spot group of April 7, 1947. Courtesy of Mt. Wilson and Palomar Observatories. Figure 1509. Oblate spheroid or ellipsoid of revolution

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 ei￾ther 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 east￾ernmost 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 ap￾pear 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 east￾ern 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 con￾junction, 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, allow￾ing it to dominate the morning or evening sky. At maximum brilliance, about five weeks before and after inferior con￾junction, 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 morn￾ing 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 bin￾oculars, Venus can be seen to go through a full set of phases. 1511. Superior Planets As planets outside the earth’s orbit, the superior plan￾ets 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 oppo￾sition, the planet will rise in the late evening, until at opposition, it will rise when the sun sets, be visible through￾out the night, and set when the sun rises. Observed against the background stars, the planets nor￾mally 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 out￾shines 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 be￾fore and after opposition. The planet disappears for about 32 days at conjunction. Four satellites (of a total 16 current￾ly 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 vis￾ible for about 175 days before and after opposition, and disappears for about 25 days near conjunction. At opposi￾tion 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 visi￾ble to the unaided eye. Uranus, Neptune and Pluto are too faint to be used for navigation; Uranus, at about magnitude 5.5, is faintly visi￾ble to the unaided eye. 1512. The Moon The moon is the only satellite of direct navigational in￾terest. It revolves around the earth once in about 27.3 days, as measured with respect to the stars. This is called the si￾dereal 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 quar￾ter 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 illumi￾nated. 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 after￾noon 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 con￾junction (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 ris￾es in the southeastern part of the sky (Northern Hemisphere), remains relatively low in the sky, and sets along the south￾western 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 hori￾zon. 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 na￾ture, 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 gas￾eous 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 need￾ed 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, sol￾id bodies too small to be seen until heated to incandescence by air friction while passing through the earth’s atmo￾sphere. 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 re￾Figure 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 dif￾ferences 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 temper￾ature) 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 not￾ed 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 magni￾tude or brighter. Half of these are below the horizon at any time. Because of the greater absorption of light near the ho￾rizon, 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 av￾erage 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 doz￾Figure 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 clus￾ters. 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 neb￾ula. 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 extraga￾lactic 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 rela￾tive 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 gal￾axies 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 Ferdi￾nand 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 ap￾pear 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 mo￾tion 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 con￾stant 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 ce￾lestial 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 ce￾lestial 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