pale blue dot -carl sagan-第27节

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n　well　when　you　observe　the　horizon。　There　is　a　smooth　color　transition　from　tender　blue；　to　blue；　to　dark　blue　and　purple；　and　then　to　the　pletely　black　color　of　the　sky。　It　is　a　very　beautiful　transition。

Clearly；　the　daylit　sky—all　that　blue—　is　somehow　connected　with　the　air。　But　as　you　look　across　the　breakfast　table；　your　panion　is　not　（usually）　blue；　the　color　of　the　sky　must　be　a　property　not　of　a　little　air；　but　of　a　great　deal。　If　you　look　closely　at　the　Earth　from　space；　you　see　it　surrounded　by　a　thin　band　of　blue；　as　thick　as　the　lower　atmosphere；　indeed；　it　is　the　lower　atmosphere。　At　the　top　of　that　band　you　can　make　out　the　blue　sky　fading　into　the　blackness　of　space。　This　is　the　transition　zone　that　Simons　was　the　first　to　enter　and　Gagarin　the　first　to　observe　from　above。　In　routine　spaceflight；　you　start　at　the　bottom　of　the　blue；　penetrate　entirely　through　it　a　few　minutes　after　liftoff；　and　then　enter　that　boundless　realm　where　a　simple

breath　of　air　is　impossible　without　elaborate　life…support　systems。　Human　life　depends　for　its　very　existence　on　that　blue　sky。　We　are　right　to　consider　it　tender　and　sacred。

We　see　the　blue　in　daytime　because　sunlight　is　bouncing　off　the　air　around　and　above　us。　On　a　cloudless　night；　the　sky　is　black　because　there　is　no　sufficiently　intense　source　of　light　to　be　reflected　off　the　air。　Somehow；　the　air　preferentially　bounces　blue　light　down　to　us。　How？

The　visible　light　from　the　Sun　es　in　many　colors—violet；　blue；　green；　yellow；　orange；　red—corresponding　to　light　of　different　wavelengths。　（A　wavelength　is　the　distance　front　crest　to　crest　as　the　wave　travels　through　air　or　space。）　Violet　and　blue　light　waves　have　the　shortest　wavelengths；　orange　and　red　the　longest。　What　we　perceive　as　color　is　how　our　eyes　and　brains　read　the　wavelengths　of　light。　（We　Might　just　as　reasonably　translate　wavelengths　of　light　into；　say；　heard　tones　rather　than　seen　colors—but　that's　not　how　our　senses　evolved。）

When　all　those　rainbow　colors　of　the　spectrum　are　mixed　together；　as　in　sunlight；　they　seem　almost　white。　These　waves　travel　together　in　eight　minutes　across　the　intervening　93　million　miles　（150　million　kilometers）　of　space　from　the　Sun　to　the　Earth。　They　strike　the　atmosphere；　which　is　made　mostly　of　nitrogen　and　oxygen　molecules。　Some　waves　are　reflected　by　the　air　back　into　space。　Some　are　bounced　around　before　the　light　reaches　the　ground　and　they　can　be　detected　by　a　passing　eyeball。　（Also；　some　bounce　off　clouds　or　the　ground　back　into　space。）　This　bouncing　around　of　light　waves　in　the　atmosphere　is　called　〃scattering。〃

But　not　all　waves　are　equally　well　scattered　by　the　molecules　of　air。　Wavelengths　that　are　much　longer　than　the　size　of　the　molecules　are　scattered　less；　they　spill　over　the　molecules；　hardly　influenced　by　their　presence。　Wavelengths　that　are　closer　to　the　size　of　the　molecules　are　scattered　snore。　And　waves　have　trouble　ignoring　obstacles　as　big　as　they　are。　（You　can　see　this　in　water　waves　scattered　by　the　pilings　of　piers；　or　bathtub　waves　from　a　dripping　faucet　encountering　a　rubber　duck。）　The　shorter　wavelengths；　those　that　we　sense　as　violet　and　blue　light；　are　more　efficiently　scattered　than　the　longer　wavelengths；　those　that　we　sense　as　orange　and　red　light。　When　we　look　up　on　a　cloudless　day　and　admire　the　blue　sky；　we　are　witnessing　the　preferential　scattering　of　the　short　waves　in　sunlight。　This　is　called　Rayleigh　scattering；　after　the　English　physicist　who　offered　the　first　coherent　explanation　for　it。　Cigarette　smoke　is　blue　for　just　the　same　reason：　The　particles　that　make　it　up　are　about　as　small　as　the　wavelength　of　blue　light。

So　why　is　the　sunset　red？　The　red　of　the　sunset　is　what's　left　of　sunlight　after　the　air　scatters　the　blue　away。　Since　the　atmosphere　is　a　thin　shell　of　gravitationally　bound　gas　surrounding　the　solid　Earth；　sunlight　must　pass　through　a　longer　slant　path　of　air　at　sunset　（or　sunrise）　than　at　noon。　Since　the　violet　and　blue　waves　are　scattered　even　more　during　their　now…longer　path　through　the　air　than　when　the　Sun　is　overhead；　what　we　see　when　we　look　toward　the　Sun　is　the　residue—the　waves　of　sunlight　that　are　hardly　scattered　away　at　all；　especially　the　oranges　and　reds。　A　blue　sky　makes　a　red　sunset。　（The　noontime　Sun　seems　yellowish　partly　because　it　emits　slightly　more　yellow　light　than　other　colors；　and　partly　because；　even　with　the　Sun　overhead；　some　blue　light　is　scattered　out　of　the　sunbeams　by　the　Earth's　atmosphere。）

It　is　sometimes　said　that　scientists　are　unromantic；　that　their　passion　to　figure　out　robs　the　world　of　beauty　and　mystery。　But　is　it　not　stirring　to　understand　how　the　world　actually　works—that　white　light　is　made　of　colors；　that　color　is　the　way　we　perceive　the　wavelengths　of　light；　that　transparent　air　reflects　light；　that　in　so　doing　it　discriminates　among　the　waves；　and　that　the　sky　is　blue　for　the　same　reason　that　the　sunset　is　red？　It　does　no　harm　to　the　romance　of　the　sunset　to　know　a　little　bit　about　it。

Since　most　simple　molecules　are　about　the　same　size　（roughly　a　hundred　millionth　of　a　centimeter）；　the　blue　of　the　Earth's　sky　doesn't　much　depend　on　what　the　air　is　made　of—as　long　as　the　air　doesn't　absorb　the　light。　Oxygen　and　nitrogen　molecules　don't　absorb　visible　light；　they　only　bounce　it　away　in　some　other　direction。　Other　molecules；　though；　can　gobble　up　the　light。　Oxides　of　nitrogen—produced　in　automotive　engines　and　in　the　fires　of　industry—are　a　source　of　the　murky　brown　coloration　of　smog。　Oxides　of　nitrogen　（made　from　oxygen　and　nitrogen）　do　absorb　light。　Absorption；　as　well　as　scattering；　can　color　a　sky。



OTHER　WORLDS；　OTHER　SKIES：　Mercury；　the　Earth's　Moon；　and　most　satellites　of　the　other　planets　are　small　worlds；　because　of　their　feeble　gravities；　they　are　unable　to　retain　their　atmospheres—which　instead　trickle　of　into　space。　The　near…vacuum　of　space　then　reaches　the　ground。　Sunlight　strikes　their　surfaces　unimpeded；　neither　scattered　nor　absorbed　along　the　way。　The　skies　of　these　worlds　are　black；　even　at　noon。　This　has　been　witnessed　firsthand　so　far　by　only　12　humans；　the　lunar　landing　crews　of　Apollos　11；　12；　and　14…17。

A　full　list　of　the　satellites　in　the　Solar　System；　known　as　of　this　writing；　is　given　in　the　acpanying　table。　（Nearly　half　of　them　were　discovered　by　Voyager。）　All　have　black　skies—except　Titan　of　Saturn　and　perhaps　Triton　of　Neptune；　which　are　big　enough　to　have　atmospheres。　And　all　asteroids　as　well。

Venus　has　about　90　times　more　air　than　Earth。　It　isn't　mainly　oxygen　and　nitrogen　as　here—it's　carbon　dioxide。　But　carbon　dioxide　doesn't　absorb　visible　light　either。　What　would　the　sky　look　like　from　the　surface　of　Venus　if　Venus　had　no　clouds。　With　so　much　atmosphere　in　the　way；　not　only　are　violet　and　blue　waves　scattered；　but　all　the　other　colors　as　well…green　yellow；　orange；　red。　The　air　is　so　thick；　though；　that　hardly　any　blue　light　makes　it　to　the　ground；　it's　scattered　back　to　space　by　successive　bounces　higher　up。　Thus；　the　light　that　does　reach　the　ground　should　be　strongly　reddened…like　an　Earth　sunset　all　over　the　sky。　Further；　sulfur　in　the　high　clouds　will　stain　the　sky　yellow。　Pictures　taken　by　the　Soviet　Venera　landers　confirm　that　the　skies　of　Venus　are　a　kind　of　yellow…orange。

　

SIXTY…TWO　WORLDS　FOR　THE　THIRD　MILLENNIUM：　KNOWN　MOONS　OF　THE　PLANETS　（AND　ONE　ASTEROID）—

LISTED　IN　ORDER　OF　DISTANCE　FROM　THEIR　PLANET

EARTH；　1
　MARS；　2
　IDA；　1
　JUPITER；　16
　SATURN；　18
　URANUS；　15
　NEPTUNE；　8
　PLUT0；　1
　
Moon
　Phobos
　Dactyl
　Metis
　Pan
　Cordelia
　Naiad
　Charon
　
　　Deimos
　　　Adrastea
　Atlas
　Ophelia
　Thalassa
　　　
　　　　　　Amalthea
　Prometheus
　Bianca
　Despina
　　　
　　　　　　Thebe
　Pandora
　Cressida
　Galatea
　　　
　　　　　　to
　Epimetheus
　Desdemona
　Larissa
　　　
　　　　　　Europa
　Janus
　Juliet
　Proteus
　　　
　　　　　　Ganymede
　Mimas
　Portia
　Triton
　　　
　　　　　　Callisto
　Enceladus
　Rosalind
　Nereid
　　　
　　　　　　Leda
　Tethys
　Belinda
　　　　　
　　　　　　Himalia
　Telesto
　Puck
　　　　　
　　　　　　Lysithea
　Calypso
　Miranda
　　　　　
　　　　　　Elara
　Diane
　Ariel
　　　　　
　　　　　　Ananke
　Helene
　Umbriel
　　　　　
　　　　　　Carme
　Rhea
　Titania
　　　　　
　　　　　　Pasiphae
　Titan
　Oberon
　　　　　
　　　　　　Sinope
　Hyperion
　　　　　　　
　　　　　　　　Iapetus
　　　　　　　
　　　　　　　　Phoebe
　　　　　　　

Mars　is　a　different　story。　It　is　a　smaller　world　than　Earth；　with　a　much　thinner　atmosphere。　The　pressure　at　the　surface　of　Mars　is；　in　fact；　about　the　same　as　the　altitude　in　the　Earth's　stratosphere　to　which　Simons　rose。　So　we　might　expect　the　Martian　sky　to　be　black　or　purple…black。　The　first　color　picture　from　the　surface　of　Mars　was　obtained　in　July　1976　by　the　American　Viking　1　lander—the　first　spacecraft　to　touch　down　successfully　on　the　surface　of　the　Red　Planet。　The　digital　data　were　dutifully　radioed　from　Mars　back　to　Earth；　and　the　color　picture　assembled　by　puter。　To　the　surprise　of　all　the　scientists　and　nobody　else；　that　first　image；　released　to　the　press；　showed　the　Martian　sky　to　be　a　fortable；　homey　blue—impossible　for　a　planet　with　so　insubstantial　an　atmosphere。　Something　had　gone　wrong。

The　picture　on　your