pale blue dot -carl sagan-第22章

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—SAMUEL　TAYLOR　COLERIDGE；　NOTEBOOKS　（1805）

The　blue　of　a　cloudless　May　morning；　or　the　reds　and　oranges　of　a　sunset　at　sea；　have　roused　humans　to　wonder；　to　poetry；　and　to　science。　No　matter　where　on　Earth　we　live；　no　matter　what　our　language；　customs；　or　politics；　we　share　a　sky　in　mon。　Most　of　us　expect　that　azure　blue　and　would；　for　good　reason；　be　stunned　to　wake　up　one　sunrise　to　find　a　cloudless　sky　that　was　black　or　yellow　or　green。　（Inhabitants　of　Los　Angeles　and　Mexico　City　have　grown　accustomed　to　brown　skies；　and　those　of　London　and　Seattle　to　gray　ones—but　even　they　still　consider　blue　the　planetary　norm。）

And　yet　there　are　worlds　with　black　or　yellow　skies；　and　maybe　even　green。　The　color　of　the　sky　characterizes　the　world。　Plop　me　down　on　any　planet　in　the　Solar　System；　wsithout　sensing　the　gravity；　without　glimpsing　the　ground；　let　me　take　a　quick　look　at　the　Sun　and　sky；　and　I　can；　I　think；　pretty　well　tell　you　where　I　am。　That　familiar　shade　of　blue；　interrupted　here　and　there　by　fleecy　white　clouds；　is　a　signature　of　our　world。　The　French　have　an　expression；　sacre…bleu！；　which　translates　roughly　as　〃Good　heavens！〃*　Literally；　it　means　〃sacred　blue！〃　Indeed。　If　there　ever　is　a　true　flag　of　Earth；　this　should　be　its　color。

*　Like　〃gosh…darned〃　and　〃geez；〃　this　phrase　was　originally　a　euphemism　for　those　who　considered　Sacre…Dieu！；　〃Sacred　God！；〃　too　strong　an　oath；　the　Second　mandment　duly　considered；　to　be　uttered　aloud。

Birds　fly　through　it；　clouds　are　suspended　in　it；　humans　admire　and　routinely　traverse　it；　light　from　the　Sun　and　stars　flutters　through　it。　But　what　is　it？　What　is　it　made　of？　Where　does　it　end？　How　much　of　it　is　there？　Where　does　all　that　blue　e　from？　If　it's　a　monplace　for　all　humans；　if　it　typifies　our　world；　surely　we　should　know　something　about　it。　What　is　the　sky？

In　August　1957；　for　the　first　time；　a　human　being　rose　above　the　blue　and　looked　around—when　David　Simons；　a　retired　Air　Force　officer　and　a　physician；　became　the　highest　human　in　history。　Alone；　he　piloted　a　balloon　to　an　altitude　of　over　100；000　feet　（30　kilometers）　and　through　his　thick　windows　glimpsed　a　different　sky。　Now　a　professor　at　the　University　of　California　Medical　School　in　Irvine；　Dr。　Simons　recalls　it　was　a　dark；　deep　purple　overhead。　He　had　reached　the　transition　region　where　the　blue　of　ground　level　is　being　overtaken　by　the　perfect　black　of　space。

Since　Simons'　almost　forgotten　flight；　people　of　many　nations　have　flown　above　the　atmosphere。　It　is　now　clear　from　repeated　and　direct　human　（and　robotic）　experience　that　in　space　the　daytime　sky　is　black。　The　Sun　shines　brightly　on　your　ship。　The　Earth　below　you　is　brilliantly　illuminated。　But　the　sky　above　is　black　as　night。

Here　is　the　memorable　description　by　Yuri　Gagarin　of　what　he　saw　on　the　first　spaceflight　of　the　human　species；　aboard　Vostok　1；　on　April　12；　1961：

The　sky　is　pletely　black；　and　against　the　background　of　this　black　sky　the　stars　appear　somewhat　brighter　and　more　distinct。　The　Earth　has　a　very　characteristic；　very　beautiful　blue　halo；　which　is　seen　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
　　　　　
　　　　　　Anank