Astronomija

Orbita Merkurja

Orbita Merkurja


We are searching data for your request:

Forums and discussions:
Manuals and reference books:
Data from registers:
Wait the end of the search in all databases.
Upon completion, a link will appear to access the found materials.

Kot opazimo z zemlje, se zdi, da se sonce premika po približno krožni orbiti. Ali bi bilo to res za gibanje drugega planeta, kot je živo srebro, opaženo z zemlje? Če ne, zakaj? Kaj pa drugi planeti sončnega sistema?


Pojav krožnih orbit in razlog, zakaj so mnogi starodavni astronomi prišli do zaključka, da Sonce in zvezde krožijo okoli Zemlje, je posledica Zemljine rotacije. Vrtenje, nekoliko očitno, deluje na dnevni (blizu 24-urni) cikel.

Zdi se, da "fiksne" zvezde krožijo okoli Zemlje v najbližjem popolnem krogu, ker se Zemljina os ne spreminja glede na zvezde (spreminja se, vendar zelo postopoma) in v primerjavi z razdaljo zvezd, njihovim gibanjem in razmeroma majhnimi Zemljinimi Orbitalno gibanje 2 AU z ene strani sonca na drugo povzroči, da so zvezde videti fiksne.

Zdi se, da zvezde zaključijo eno orbito ali krožijo okoli Zemlje vsak zvezdni dan ali približno 23 ur 56 minut in 4 sekunde.

Zdi se, da sonce kroži okoli Zemlje v nepopolnem krogu, krog, tako kot pri zvezdah, poganja vrtenje Zemlje, vendar sprememba kroga, ki se kaže v tem, kako visoko je sonce na nebu in kako dolgo na dan je povezano z osnim nagibom Zemlje, kjer je Zemlja v letni orbiti in na vaši zemljepisni širini na Zemlji. Če bi živeli na severnem tečaju, bi se sonce poleti gibalo v krogu, pozimi pa bi izginilo.

Če dobro razumem vaše vprašanje, sprašujete, kako bi bili videti ti krogi, če bi živeli na površju Merkurja, Venere ali Neptuna. (Ne sprašujete se, kako bi bili videti ti planeti, če je to vaše vprašanje google "tavajoče zvezde", mislim pa, da je vaše vprašanje, kako bi bili videti orbitalni krogi z različnih planetov.

Kot pri Zemlji je tudi tu treba upoštevati tri glavne dejavnike: dolžino dneva, dolžino leta in osni nagib ter v Merkurjevem ali Plutonovem primeru ekscentričnost.

Merkur na primer v bistvu nima aksialnega nagiba, ima pa razmerje 3/2 spin / orbita, kar dejansko pomeni, da en dan na Merkurju traja 2 leti, saj se vrti le 50% hitreje, kot kroži. Merkur ima tudi znatno eliptično orbito, kjer se v periheliju sonce in zvezde na nebu ustavijo, kar bi trajalo nekaj zemeljskih dni.

tukaj vnesite opis povezave

Ker živo srebro dokaj hitro kroži okoli Sonca, traja 87 dni, da se sonce vrne na isto mesto na nebu glede na zvezde, ker pa ima Merkur dejansko dnevno svetlobo 87,6 dni in noč 87,6 dni, se očitno vrne tja, kjer začeli bi se pojavljali vsaki dve leti, vsakih 175,2 dni. Kljub temu bi na Merkurju imeli vtis, da vse kroži okoli sonca v krogih, čeprav si morda dovolj blizu in imaš pravi kot, da razbereš faze Venere, ki jih na Zemlji nismo mogli videti do Galilejevega teleskopa. Velika nenavadnost Merkurja bi bila ta, da bi se zvezde na nebu ustavile enkrat na 87 dni, vendar bi se premikale tudi zelo počasi, tako da bi to lahko ostalo neopaženo brez podrobnih meritev. Tudi na Merkurju bi opazovali, kako sonce narašča ali raste, odvisno od tega, kje je Merkur v svoji orbiti in kako blizu ali daleč od sonca.

Venera - edino kar je izjemno pri Veneri je, da je njen dan daljši kot leto, zato bi se krogi gibali v obratni smeri.

Uran je zanimiv, ker je nagnjen za skoraj 90 stopinj. To je sezonska sprememba sonca, ki bi bila veliko večja od Zemljine, toda leto na Uranu je 84 zemeljskih let, zato bi očitno gibanje sonca z naše perspektive trajalo dolgo.

Nazadnje bi lahko od Urana in Neptuna orbitalna razdalja zadostovala, da bi lahko opazili paralakso, čeprav bi bil učinek še vedno majhen, a mogoče opazen, iz pravega kota boste morda videli nekaj rahlih premikov bližnjih zvezd, kot je Alpha Centauri ali Sirius.

To je bistvo. Predvidevam, da Sončevemu navideznemu gibanju rečete "približno krožno", je stvar mnenja, vendar ga ne bi imenoval krožen. Kot najbližji krožniku bi želeli planet z aksialnim nagibom blizu 0 ali blizu 180 (Merkur, Venera, Jupiter) in mi ni posebej všeč primer Merkurja, ker se Sonce enkrat na leto ustavi na nebu . Predvidevam, da bi temu lahko rekli krožno gibanje s postankom na veliki dirkališču na nebu.

Včasih je bil super simuliran videoposnetek, kako bi izgledalo sonce, ki prehaja Merkur, zdaj pa ga ne najdem.


Orbita Merkurja - astronomija

Živo srebro je planet do Sonca. Merkur je v rimski mitologiji, rimskem dvojniku grškega boga Hermesa, dobil ime po bogu trgovine, potovanj in tatvine. Ime je planet najverjetneje prejel zaradi hitre hitrosti po nebu.

Orbita Merkurja ima največjo ekscentričnost med vsemi planeti Osončja (Pluton ima največjo ekscentričnost, vendar se ne šteje več za planet). Ekscentričnost Merkurja je 0,21, kar je vsekakor višja od ekscentričnosti Zemlje, ki je 0,02.

Živo srebro lahko vidimo na Zemlji blizu obzorja šele tik po sončnem zahodu (vzhodno od Sonca) ali tik pred sončnim vzhodom (zahodno od Sonca). To je razlog, da se Merkur pogosto imenuje jutranja ali večerna zvezda.

Ker je Merkur mnogo bližje Soncu kot Zemlja in ima Sonce krajše obdobje kot mi, uporabite največjo ločitev med Merkurjem in Soncem, ko planet kroži, in uporabite ta opazovanja za risanje tangente na Merkurjevo orbito. Orbito Merkurja lahko načrtujemo tako, da opazujemo njegovo največjo kotno ločitev vzhodno ali zahodno od Sonca, kot jo vidimo z Zemlje na različne datume. Ta kot med vidno črto Zemlja-Sonce in vidno črto Zemlja-Živo srebro se imenuje kot raztezanja.

Ko je kot raztezanja največji, je vidna črta Zemlja-Merkur tangentna na Merkurjevo orbito.

Ker se tako Merkur kot Zemlja gibljeta po eliptičnih orbitah, se največji raztezalni kot spreminja od ene do druge rotacije. Spodnja tabela prikazuje največji raztezek živega srebra v letih 2000 do 2003.

Uporabite orbito Zemlje, ki ste jo začrtali v prejšnji preiskavi. Iz predhodno zgrajene slike narišite polmer, ki povezuje Zemljo s Soncem na vsakem od položajev.

Takole naj izgleda:

Zdaj bomo za podane kote raztezanja narisali črte od Zemlje do Merkurja. V našem koordinatnem sistemu gledamo s severnega pola navzdol. Zato je vzhodni raztezek levo od Sonca, zahodni pa desno. Da bi ugotovili, kje se nahaja Merkur, bomo z uporabo naše prejšnje predpostavke o raztezitvenem kotu pri njegovem maksimumu domnevali, da je Merkur v točki na tangentni omari na Sonce, to je točka tangente. Po iskanju teh točk bomo skicirali orbito Merkurja.

Pravokotne črte na polmer Zemlje do Sonca sem pobarval z rdečo, da bi se znebil zmede. Da najdem kot raztezanja, se bom zasukal po konstrukciji segmenta iz pravokotnih črt. Naučil sem se, da je koristno poskrbeti, da segmente sestavite v isti smeri. Na primer, če sem na vrhu pravokotno zgradil odsek za 7. marec, bom nadaljeval s sestavljanjem odsekov na ta način.

Po konstrukciji kota morate označiti središče, kjer se bo vrtenje zgodilo. Kot središče bom označil točke na orbiti Zemlje. Nato bom z uporabo kota raztezanja lahko narisal poti orbitu Merkurja.

Pot zemlje je zunaj, pot orbite Merkurja pa znotraj. Orbita Merkurja ni tako gladka kot zemeljska.


Mercury & # 8217s Spin-Orbit Resonance

Za vsaki 2 orbiti Merkurja okoli sonca se 3-krat zavrti na svoji osi. To je znano kot 3-2-orbitalno resonanco.

Opomba: Eliptična ekscentričnost Merkurjeve orbite okoli Sonca je v spodnji animaciji za poudarek pretirana.

Če pogledamo to animacijo, bi lahko opazovali tri vrtenja (vrtenja) Merkurja na vsaki dve orbiti in # 8211 fascinantno resonanco 3: 2 ali sinhronost. Živo srebro dokonča orbito v 87,97 zemeljskih dneh in zavrti enkrat na 58,65 zemeljskih dni.

Čeprav je Merkurjev zvezdični dan (glede na zvezde) dolg le 59 zemeljskih dni, sončni dan na Merkurju traja 176 zemeljskih dni. Z drugimi besedami, če bi lahko stali na površini Merkurja, bi trajalo 176 dni, da bi se Sonce vrnilo na isto mesto na nebu. Kombinacija zelo hitre orbite planeta in počasnega vrtenja povzroči zelo dolg Merkurjev dan & # 8211 dvakrat daljši od njegovega leta! Samo to # 8217 en merkurski dan na vsaki dve merkurski leti! Ali lahko to zaznate tako, da v zgornji animaciji spremljate gibanje vrtljive puščice, ko Merkur kroži okoli Sonca?

Ker je Merkur tako blizu Sonca (kroži med 28 in 43 milijoni milj od Sonca) in nima veliko ozračja, da bi ujel toploto, zelo dolgi dnevi povzročajo, da se temperatura močno spreminja. Temperature živega srebra se lahko gibljejo med -279 Fahrenheitov (-173 Celzija) ponoči do +801 Fahrenheitov (+427 Celzija) podnevi. To je dovolj vroče, da se stopi svinec!

Globalni mozaik Merkurja
iz misije NASA Messenger
(kliknite na sliko za povečavo)

"Zato pri tej urejeni ureditvi najdemo čudovito simetrijo v vesolju in določeno razmerje harmonije v gibanju in velikosti krogel, kakršne ni mogoče dobiti drugače."
- Johannes Kepler,
Svetovne harmonije, 1619

"Ah, kje bi bila hrana za duhovnost, brez noči in zvezd?" - Walt Whitman


Astronomska slika dneva Kazalo - Sončni sistem: živo srebro

APOD: 12. september 2004 - Živo srebro: kraterirani pekel
Pojasnilo: Merkurjeva površina je podobna površini naše Lune. Vsak je močno krateriran in izdelan iz kamenja. Premer Merkurja je približno 4800 km, medtem ko je Lunin nekoliko manjši na približno 3500 km (v primerjavi s približno 12.700 km pri Zemlji). Toda Merkur je v marsičem edinstven. Merkur je Soncu najbližji planet, ki kroži okoli 1/3 polmera zemeljske orbite. Ko se Merkur počasi vrti, se njegova površinska temperatura spreminja od neznosne mraz -180 stopinj Celzija do neznosno vročih 400 stopinj Celzija. Kraj najbližje Soncu v Merkurjevi orbiti nekoliko spremeni vsako orbito - dejstvo, ki ga je Albert Einstein uporabil za preverjanje pravilnosti svoje takrat na novo odkrite teorije gravitacije: Splošna relativnost. Zgornjo sliko je posnelo edino vesoljsko plovilo, ki je kdaj koli prečkalo Merkur: Mariner 10 leta 1974. Novo poslanstvo, Messenger, je za Merkur izstrelilo prejšnji mesec in naj bi leta 2011 vstopilo v orbito okoli najglobljega planeta Osončja.

APOD: 2003, 12. april - Živo srebro na obzorju
Pojasnilo: Ste že kdaj videli planet Merkur? Ker Merkur kroži tako blizu Sonca, nikoli ne tava daleč od Sonca na zemeljskem nebu. Če gre za Soncem, bo Merkur viden nizko na obzorju le kratek čas po sončnem zahodu. Če vodi Sonce, bo Merkur viden le malo pred sončnim vzhodom. Tako lahko v določenih letnih časih informirani skygazer z malo odločnosti običajno izbere Merkur s kraja z nezaščitenim obzorjem. Zgoraj je bilo veliko odločnosti kombinirano z malo digitalnimi triki, ki so prikazovali zaporedne položaje Merkurja marca 2000. Vsaka slika je bila posneta z iste lokacije v Španiji, ko je bilo Sonce samo 10 stopinj pod obzorjem in postavljena na najbolj fotogenični sončni zahod. Do srede tega meseca bo Merkur ob sončnem zahodu spet primeren za ogled nad zahodnim obzorjem, do konca aprila pa bo zbledel in padel v mrak. 7. maja bo Merkur prečkal Sončev disk.

APOD: 2003, 16. februarja - jugozahodno živo srebro
Pojasnilo: Planet Merkur je podoben luni. Stara površina Merkurja je močno kraterirana kot številne lune. Merkur je večji od večine lun, vendar manjši od Jupitrove lune Ganimed in Saturnove lune Titan. Živo srebro pa je veliko gostejše in bolj masivno kot katera koli luna, ker je večinoma iz železa. Pravzaprav je Zemlja edini planet, ki je bolj gost. Obiskovalec Merkurjevega površja bi videl nekaj čudnih znamenitosti. Ker se Merkur vrti natanko trikrat na dve orbiti okoli Sonca in ker je Merkurjeva orbita tako eliptična, lahko obiskovalec Merkurja vidi sonce, kako se ustavi, ustavi na nebu, se vrne proti vzhajajočemu obzorju, ponovno ustavi in ​​nato hitro nastavi čez drugo obzorje. > Z Zemlje je bližina Merkurja do Sonca videti, da je viden le kratek čas tik po sončnem zahodu ali tik pred sončnim vzhodom.


Orbita Merkurja - astronomija

KAKO JE BILO ODKRITO Merkur

(Bill Yenne, "Atlas sončnega sistema," Brompton Books Corp., Greenwich, 1987, str. 23.)

* Johann Hieronymus Schroeter je prvi opazoval planet Merkur in posnel podrobne risbe površinskih značilnosti Merkurja. Schroeter je živel od 1745 do 1816. Žal njegove skice niso bile zelo natančne.

* Proge, podobne tako imenovanim "marsovskim kanalom", sta na Merkurju videla tudi Schiaparellit in Percival Lowell (1855-1916).

* Astronom z imenom Eugenios Antoniadi (1870-1944) je zelo podrobno začrtal površino Merkurja. Njegove zemljevide so uporabljali skoraj 50 let. Uporabil je enega močnejših teleskopov svojega časa in ugotovil, da so kanali optične iluzije.

* Mariner 10 je natančno pogledal Merkur, ki je preoblikoval prejšnje karte in zemljevide teleskopov.

(Bevan M. French in Stephen P. Maran, ur., "Srečanje z vesoljem", NASA EP-177, tiskarna vlade ZDA, 1981.)

* Merilne radarske meritve na zemeljskem srebru so ugotovile (1965), da je obdobje vrtenja 59 dni, ne pa 88 dni, kot so dolgo verjeli.

* Mariner 10 je leta 1974 prvi preletel Merkur z vesoljskim plovilom (v resnici je trikrat preletel Merkur) in pridobil nekaj tisoč fotografij.

* Med rezultati preiskav Mariner 10 so bili naslednji:

* Masa živega srebra je bila natančno določena.

* Vsaka preostala atmosfera ima pritisk zemeljske atmosfere na morski gladini manj kot milijon milijard. Vendar pa je bila najdena sled helija, ki je bila morda pridobljena z izpuščanjem iz Merkurjeve notranjosti.

* Ugotovljeno je bilo, da ima Merkur notranje magnetno polje, podobno, vendar šibkejše od zemeljskega.

* Merkurjeva površina je močno kraterirana in je podobna površini Lune. * Odkrit je bil velik krožni bazen (Mare Caloris) s premerom približno 1300 kilometrov.

* Ugotovljena je bila planetarna značilnost, edinstvena za Merkur, sestavljena iz dolgih škarp ali pečin, ki so očitno nastale s stiskanjem v velikem krčenju planeta.

* Najdene so ravne ravnice, morda tokovi lave.

* Ugotovljeno je bilo, da je živo srebro bližje popolni krogli kot Zemlja.

(Sledi iz "Osončja" NASA / ASEP, 1989, str. 2.)

* Merkur je glasnik rimskih bogov.

* Merkur je najgloblji planet.

* Živo srebro spominja na Zemljino luno.

* Merkurjev dan je enak 3 zemeljskim mesecem.

* Teža živega srebra je približno ena tretjina gravitacije Zemlje.

* Premer Merkurja je 3.025 milj.

* Živo srebro potuje okoli Sonca vsakih 88 zemeljskih dni.

* Živo srebro je skoraj brez ozračja.

* Živo srebro ima šibko magnetno polje in sled ozračja (eno bilijonto gostoto Zemlje, sestavljeno predvsem iz argona, neona in helija).

* Merkurjeva orbita je bolj eliptična kot kateri koli drug planet razen Plutona.

* Temperatura živega srebra je 950 F, sončna stran 210 F, temna stran.

* Živo srebro ima skorjo lahke silikatne kamnine.

* Železovo jedro Merkurja je približno velikosti Zemljine lune.

* Marca 1974 nam je Mariner 10 dal prve fotografije Merkurjeve površine od blizu.

(NASA, Laboratorij za reaktivni pogon, "Kratek pregled našega sončnega sistema", Povzetki informacij NASA, PMS 010-A (JPL), junij 1991.)

Pridobivanje prvih pogledov na Merkur od blizu je bil glavni cilj vesoljskega plovila Mariner 10, ki je bilo 3. novembra 1973 iz vesoljskega centra Kennedy na Floridi izstreljeno. Po skoraj petmesečnem potovanju, vključno s preletom Venere, je vesoljsko plovilo 29. marca 1974 prešlo 703 kilometre (437 milj) od najglobljega planeta sončnega sistema.

Do Marinerja 10 se je o Merkurju malo vedelo. Tudi najboljši teleskopski pogledi z Zemlje so Merkur prikazovali kot nerazločen predmet, ki mu manjka nobena površinska podrobnost. Planet je tako blizu Sonca, da se običajno izgubi v sončnem bleščanju. Ko je planet viden na obzorju Zemlje tik po sončnem zahodu ali pred svitanjem, ga zakrivata meglica in prah v našem ozračju. Le radarski teleskopi so namignili na površinske razmere Merkurja pred potovanjem Marinerja 10.

Fotografije, ki jih je Mariner 10 posnel nazaj na Zemljo, so razkrile starodavno močno kraterizirano površino, ki je bila zelo podobna naši Luni. Slike so prikazovale tudi ogromne pečine, ki prečkajo planet. Očitno so nastali, ko se je notranjost Merkurja ohladila in skrčila ter upognila zemeljsko skorjo. Pečine so visoke do 3 kilometre (2 milje) in do 500 kilometrov (310 milj).

Instrumenti na Marinerju 10 so odkrili, da ima Merkur šibko magnetno polje in sled ozračja - triljonto gostote zemeljske atmosfere, sestavljeno predvsem iz argona, neona in helija. Ko ga orbita planeta približa Soncu, se površinske temperature gibljejo od 467 stopinj Celzija (872 stopinj Fahrenheita) na Merkurjevi sončni strani do -183 stopinj Celzija (-298 stopinj Celzija) na temni strani. To območje površinske temperature - 650 stopinj Celzija (1170 stopinj Fahrenheita) - je največje za posamezno telo v sončnem sistemu. Živo srebro dobesedno peče in zamrzne hkrati.

Dnevi in ​​noči so na Merkurju dolgi. Kombinacija počasnega vrtenja glede na zvezde (59 zemeljskih dni) in hitre revolucije okoli Sonca (88 zemeljskih dni) pomeni, da en Merkurjev sončni dan traja 176 zemeljskih dni ali dve Merkurjevi leti - čas, ki ga potrebuje najgloblji planet dokončati dve orbiti okoli Sonca!

Zdi se, da ima živo srebro skorjo lahke silikatne kamnine, kot je zemeljska. Znanstveniki verjamejo, da ima Merkur težko jedro, bogato z železom, in predstavlja nekaj manj kot polovico njegove prostornine. Zaradi tega bi bilo Merkurjevo jedro sorazmerno večje od Luninega jedra ali jedra katerega koli planeta.

Po začetnem srečanju z Merkurjem je Mariner 10 izvedel dva dodatna leta - 21. septembra 1974 in 16. marca 1975 - preden je bil izčrpan krmilni plin za orientacijo vesoljskega plovila in misija zaključena. Vsak prelet se je zgodil ob istem lokalnem času Merkurja, ko je bila osvetljena enaka polovica planeta, še vedno nismo videli polovice površine planeta.


Merkur: Planet skrajnosti

Živo srebro ima eksosfero namesto ozračja. (Slika: David Lugasi / Shutterstock)

Živo srebro je planet skrajnosti. Je najmanjši planet v sončnem sistemu in najbližji Soncu. Okrog Sonca potuje veliko hitreje kot kateri koli drug planet, 60% hitreje kot Zemlja, s povprečno hitrostjo več kot 100.000 milj na uro. Tudi vstop v orbito okoli Merkurja je nenavadno zahteven. To je zato, ker se vesoljska plovila, pa tudi kometi ali karkoli drugega, pospešijo, ko se približujejo Soncu.

Sila gravitacije povzroči, da se vse pospeši, ko se približuje Soncu in se pomakne globlje v tisto, čemur dobro rečemo sončna gravitacija. Predmeti, ki padejo na površje Zemlje, se pospešijo, ko se globlje v zemeljski gravitaciji dobro poglobijo.

Vesoljska plovila, kot je MESSENGER, nam lahko pomagajo odkriti nove podrobnosti o atmosferi Mercury & # 8217s in eliptični orbiti. (Slika: NASA / javna domena)

Če želite torej vesoljsko plovilo poslati v Merkur, se velika težava upočasnjuje. Na primer, pot, ki jo je sledilo vesoljsko plovilo MESSENGER, ko se je leta 2004 izstrelilo od Zemlje do Merkurja, je vključevala en prelet Zemlje, 2 preleta Venere in 3 prelete samega Merkurja. Zdaj vsak prelet uporablja gravitacijo planeta, da upočasni vesoljsko plovilo ali spremeni njegovo smer. Vse to zmanjšanje hitrosti s pomočjo gravitacije je trajalo dolgo in veliko dodatne razdalje.

MESSENGER je začel krožiti okoli Merkurja 7 let po izstrelitvi. Misija Bepi Colombo, ki se je začela leta 2018, sledi isti sedemletni časovni liniji in prispe v Merkur leta 2025.

To je prepis iz video serije Terenski vodnik po planetih. Pazi zdaj, Wondrium.

Merkur & # 8217s Nenavadne eliptične orbite

Ko je tam, je še ena skrajnost, da ima Merkur neverjetno dolge dni. Pravzaprav je en sam & # 8216day & # 8217 na Merkurju daljši kot celo leto. Orbita Merkurja okoli Sonca je tudi manj krožna - bolj eliptična - kot kateri koli drug planet. Torej eno leto vključuje veliko več pospeševanja in upočasnjevanja v eni orbiti.

Prav tako si ne moremo kaj, da ne bi opazili Sonca. Sonce je v povprečju 2/2 krat večje, merjeno na nebu od Merkurja kot od Zemlje. To je zato, ker je Merkur v povprečju 2 1⁄2 krat bližje Soncu kot Zemlji. Zdaj pravimo & # 8216v povprečju & # 8217, ker se Merkurjeva oddaljenost od Sonca med njegovo orbito zelo spremeni.

Na najbolj oddaljeni točki je Merkur oddaljen 70 milijonov kilometrov od Sonca, medtem ko je na najbližji točki oddaljen le 46 milijonov kilometrov. To pomeni, da bi se izmerjena velikost Sonca na nebu spreminjala v različnih točkah Merkurjeve orbite. To ni samo optična iluzija. Ko je Merkur na najbližji točki Sonca, je Sonce res približno 50% večje kot takrat, ko je Merkur na najbolj oddaljeni točki od Sonca.

Ta eliptična orbita daje tudi Merkurjeve letne čase, ki so zelo različni. Na Zemlji so letni časi posledica 23-stopinjskega nagiba Zemljine osi, kjer severna zima sovpada z južnim poletjem in obratno. Nasprotno pa Merkurjeva os vrtenja skoraj nima nagiba.

Tako Merkurjeva severna in južna polobla doživljata skoraj isto stvar. Pa vendar ima Merkur kot celota sezone neenakomerne velikosti, ker Merkurjeva eliptična orbita približuje cel planet Soncu za kratek čas in dlje za dlje. Ko je Merkur v periheliju, je Sonce dvakrat svetlejše kot takrat, ko je Merkur na najbolj oddaljeni točki, imenovani afelij. Torej obstajajo letni časi, ki so bolj vroči in hladnejši.

V našem osončju ima Merkur daleč najbolj eliptično orbito od osmih planetov. Morali bomo iti do Plutona in divjih orbit nekaterih eksoplanetov, da bomo videli kaj primerljivega.

Živo srebro & # 8217s Atmosfera

Merkur nima atmosfere v smislu, da tam ne bodo delovali baloni, krila, padala ali druge letalske naprave. A to še ne pomeni, da nad površino ne plavajo delci, gravitacijsko vezani na Merkur. Vsi planeti in celo nekatere lune imajo delce, ki obkrožajo planet. Na Zemlji so ti delci večinoma dušik, skupaj s kisikom in veliko manjše količine ogljikovega dioksida.

Delci so gravitacijsko vezani na Zemljo in tukaj je pomemben del: Gostota delcev je dovolj velika, da ti delci trčijo med seboj in jih učinkovito zadržujejo v zraku. Takšni delci se ob trku obnašajo kot plin. Ta trčenja omogočajo tisto, na kar običajno pomislimo, ko rečemo & # 8216atmosfera & # 8217.

Vesoljsko plovilo MESSENGER je to fotografijo posnelo med kroženjem in preučevanjem Merkurja (2011-2015). (Slika: NASA / Laboratorij za uporabno fiziko Univerze Johns Hopkins / Inštitut Carnegie v Washingtonu / javna domena)

Merkur v tem smislu nima ozračja. Toda včasih še vedno obstajajo delci, ki obkrožajo planet in ga gravitacijsko vežejo. Preprosto, da skoraj nikoli ne komunicirajo z drugimi delci. Na primer, včasih lahko visokoenergijski delci iz sončnega vetra vplivajo na površino Merkurja in odbijejo atom s površine. Toda ta atom ima premalo drugih delcev, da bi ga podprl. Torej lahko nekje pristane. Ali pa ga odpihne in začne krožiti okoli planeta. To območje delcev, ki krožijo, je običajno znano kot eksosfera.

Znanstveniki so odkrili vse vrste elementov v eksosferi Merkurja, vključno z vodikom, helijem, kisikom, natrijem, kalcijem in magnezijem. Tudi Zemlja in drugi planeti imajo eksosfere. Zemeljska eksosfera pa se začne daleč nad tem, kar mislimo kot ozračje, na višinah, kjer gostota postane dovolj nizka, da delci ne trdijo več.

Razlika je v tem, da ima Merkur samo eksosfero. To je še en način, da je planet Merkur bolj podoben luni.

Pogosta vprašanja o živem srebru

Orbite večine planetov so ekscentrične. Zemljina orbita je rahlo ekscentrična, medtem ko je orbita Merkurja najbolj ekscentrična, ker je najbližji Soncu planet.

Orbite povzročajo interakcije planeta s soncem, ko se sonce premika okoli njega. Pospeši ali upočasni vrtenje Merkurja, odvisno od tega, kje je planet na svoji eliptični orbiti.

Orbite so rezultat popolnega ravnovesja med gibanjem telesa v vesolju, kot je planet ali luna, in delovanjem gravitacije nanj iz drugega telesa v vesolju, kot je zvezda.

Namesto običajne atmosfere ima Merkur tanko eksosfero, sestavljeno iz atomov, ki so jih s površine odstrelile sile, kot so sončni veter in udarni meteoroidi.


Ep. 49: Živo srebro

Še vedno kopljemo po tisočih komentarjih in predlogih iz ankete med poslušalci, vendar poslušamo vaše zahteve in predloge, zdaj pa lahko začnete izkoriščati prednosti. Danes z Merkurjem začenjamo raziskavo sončnega sistema. Katere skrivnosti nam skriva? Kako podoben je Merkur ostalim skalnatim planetom? Koliko v resnici vemo o tej prvi sončni skali?

Razstavne opombe

  • NASA & # 8217s Mercury Fact Sheet & # 8211 samo številke Pregled Ključ do magnetnega polja in razvoja magnetnega polja Zemeljskega planeta & # 8211 Živo srebro in splošna relativnost, splošna relativnost in sončna izboklina

Astronomija Cast Archived Episodes
Naš arhiv je poln osnovnih informacij. Ne pozabite si ogledati teh oddaj iz preteklosti!

Brezsramna samopromocija
Preverite, kaj počnemo, ko ne podcastujemo!

  • Pamela blogira o pošastih, prehranjevanju blaznosti in medijih, potem ko je več govorila o iskanju temnih galaksij. Vse to in še več si lahko preberete na njenem blogu Star Stryder.
  • Fraser rad govori o odličnih amaterskih astrofotografih, revija Wired pa mu je dala napisati članek za njihovo spletno stran, ki poudarja nekatere astrofotografe in njihovo impresivno opremo. Ko končate, pojdite na Universe Today po najnovejše astronomske novice.

Prepis: Živo srebro

Fraser: Hvala vsem, ki ste izpolnili anketo med poslušalci: zdaj boste izkoristili prednosti. Imeli smo na tisoče komentarjev, predlogov, povratnih informacij - še vedno se kopljemo po njem, a nekaj, kar smo slišali velikokrat, je bilo, da bi ljudje radi slišali raziskavo vseh planetov v sončnem sistemu, samo vzemite jih enega za drugim in vas vse seznanite z najnovejšo znanostjo.

Vaša želja je naš ukaz, začeli bomo na sredini in se potrudili. Torej, Pamela, pogovorimo se o Merkurju.

Pamela: No, na sredini je # 8217. To je prva skala s Sonca. to je majhen svet, izredno gost, veliko, veliko kovin, tanka skorja in se ne razlikuje toliko od Lune. Pravzaprav je manjša od dveh lun v sončnem sistemu. Manjši je od Titana in manjši od Ganimeda.

Nekaj ​​časa nazaj, pred 3000 leti pred našim štetjem, so ljudje mislili, da gre dejansko za dva različna predmeta na nebu, ker se ta pojavi le v mraku zgodaj zvečer in zgodaj zjutraj. Ljudje so rabili nekaj časa, da so ugotovili, da gre za samo en objekt, ki je tako blizu Sonca, da ga nikoli ne vidimo visoko na nebu.

Fraser: Začnimo z oblikovanjem. Kako je nastal Merkur?

Pamela: Kot vsi planeti v sončnem sistemu je tudi on nastal iz sončne meglice. Nastala je iz diska plina in prahu, ki se je vrtinčil okoli Sonca, ko se je Sonce gravitacijsko sesuvalo v zvezdo, ki je danes.

Nismo povsem prepričani, ali je nastalo točno tako, kot je danes. Obstaja veliko različnih idej, s katerimi lahko poskusite razložiti, kako dobite ta majhen majhen planet, ki je & # 8217s tako neverjetno gost (je & # 8217s eden najgostejših objektov v sončnem sistemu).

Nekateri ljudje mislijo, da je nastal veliko večji kot danes, vendar se je nekaj zgodilo, udarilo in odvrnilo del površine. Ponovno se je strdila na manjši planet in odpadle snovi so padle v Sonce ali pa se preselile v druge dele sončnega sistema.

Fraser: Torej, če ga primerjamo z Zemljo, ima jedro, kot je Zemlja, vendar nič umazanije zunaj.

Pamela:Točno tako. V bistvu je jedro obkroženo s samo 600 km debelim plaščem, narejenim iz običajnih plaščnih materialov (kot so stvari, iz katerih so sestavljene celine). Povrh je še 100–200 km debela skorja, to je kamnina, umazanija in vse te vrste stvari, ki so močno poškodovane zaradi stvari, ki so prizadele Merkur.

To je osrednji del, ki je večinoma kovinski, debeline 800 km.

Fraser: Torej gre za samo kroglo železa, ki kroži okoli Sonca.

Pamela: Točno tako. Menimo, da bi lahko prvotno nastal z debelejšim plaščem, debelejšo skorjo in ga kaj prizadelo.

Druga možnost je, da je Sonce dejansko zgodaj razstrelilo skorjo. Mlado sonce je bilo jezna, bliskovita, visokoenergijska, nasilna zvezda. To je faza, skozi katero prehajajo vse zvezde, tako kot strašni dvojki. Zvezde imajo svojo grozno malčkovsko fazo. V tej visokoenergijski fazi je bilo mogoče, da je Sonce dejansko razstrelilo skorjo Merkurja.

Nismo prepričani. Obe teoriji sta nekako urejeni in delujeta tako, da razložita to majhno, nenavadno, prvo sončno skalo.

Fraser: Pravite, da se je razstrelil s sončnim vetrom. Kam bi šel material?

Pamela: Pravkar bi se prerazporedil po sončnem sistemu. Ves čas Mercury & # 8217s razbijajo sonci. sevalni in visokoenergijski delci, ki prihajajo iz Sonca, zadenejo njegovo površino in počasi odbijajo delce z Merkurja.

Živo srebro dejansko ima to resnično tanko atmosfero, ki jo v celoti tvorijo delci, ki nenehno izvirajo iz materiala, ki poskuša sestaviti skorjo Merkurja. Torej kamnina sedi tam, udari jo sončna energija, nekaj atomov v skali pa se odlepi, plava okoli, oblikuje ozračje in sčasoma odnese v globok vesolj (ali vsaj v notranji del sončnega sistema, čeprav nekateri verjetno pobegnejo v globok vesolje).

Fraser: Pred kratkim sem napisal članek o tem. Nekako napačno ga imenujemo ozračje, saj imamo pri Zemlji ozračje oblake delcev kisika in dušika, ki trčijo drug v drugega. The atmosphere on Mercury doesn’t actually collide in that way, it’s like the particles zip past each other and only occasionally actually bump into each other.

Pamela: We know the atmosphere is there: we can see it using spectrographs, sunlight passes through it, but here atmosphere simply means there is gas near the surface of the planet. It doesn’t mean the gas is there to stay: it’s not. It’s a transitory phenomenon. It certainly isn’t an atmosphere with winds or rain or anything else that we associate with an atmosphere.

Fraser: All right, let’s move down to the surface and land. What would we see?

Pamela: Mercury is this really neat looking planet. When it formed it was incredibly hot, and it’s cooled over time. One of the things that happens when some things cool, is they contract. Water is anomalous: if you freeze water, it gets bigger. Pretty much everything else gets smaller when you freeze it (this is what happens with the pipes in your house).

With this planet, as its iron core contracted, it eventually shrank to the point that the entire surface cracked. It started off with this big surface, or at least a 0.01% bigger surface, that was on top of a bigger (by a very small percentage) core. The core got cooler, contracted, and the surface cracked because there wasn’t as much volume of stuff supporting it any longer.

So the planet is riddled with these really cool cracks that come from the planet cooling.

Fraser: Is it fairly cratered like the Moon?

Pamela: It’s cratered just like the Moon. That’s one of the neat things about this world. You look at it, and it’s like looking at a red version of the Moon. They’re not even that different in size, just in density.

It has no way to resurface itself, so any rock, asteroid or anything that hits it, makes a crater and that crater stays there until something else comes along and hits it and re-craters the surface. So its entire surface is covered in all sorts of different craters that trace back the history of collisions that have happened throughout the entire time our solar system has been around.

Fraser: We talked in the past about extrasolar planets. If you get a planet close enough to its parent star, it can get tidally locked, where the planet is only facing one side. Mercury isn’t tidally locked though, is it?

Pamela: It’s sort of/kind of tidally locked. It’s an object that’s confused astronomers for a long time, and it’s only within the past 50 years that we’ve started to fully understand its motion.

It’s really hard to observe Mercury. If you only try and look at it when the Sun is below the horizon, it’s never more than 28 degrees above the horizon. To get a sense of how low on the sky that is, try holding one arm all the way out straight in front of you, parallel with the ground. Walk up the sky with one fist at a time. When you have three fists above the horizon, on top of your fist is higher than Mercury can ever get in the sky.

There are certain times it’s easier to observe Mercury than others. It just happens to work out that the times it’s easiest to observe Mercury, the exact same face of Mercury is always facing us. Up until the 1960s, astronomers thought Mercury was tidally locked: the same side of Mercury always faced the Sun, and the same side of Mercury always faced toward the outer parts of the solar system.

In the 1960s, we started to use radio telescopes. With radio telescopes, we can measure how warm things are. When astronomers measured how warm the side of Mercury facing away from the Sun was, they expected to find it really cold. There’s not an atmosphere to transfer heat, we thought the backside of Mercury never faced the Sun… there was nothing to warm it up. Instead we found it really hot. The only way to explain this is if the planet is rotating, or if you invoked crazy physics.

The first thing they did was they tried invoking crazy physics. It happens. But in the mid to late 1960s, we started using radar imaging. By bouncing radar light off of Mercury, we could actually watch it slowly rotate. It was realized that for every two times that Mercury goes around the Sun, it rotates three times around its axis.

So one year on Mercury is one and a half days long. This is a weird form of tidal locking that happens because Mercury’s orbit isn’t circular. It is in fact, the least circular orbit of any of the eight planets in the solar system.

Fraser: Don’t send us mail!!

Pamela: Okay, we’re going to just not touch on the Pluto issue today. Ignoring Pluto, ignoring all the other Kuiper Belt Objects, Mercury has the most elliptical, the most oval-looking in the solar system.

As Mercury goes from being about 43 million miles away from the Sun, to about 29 million miles away from the Sun, this change in distance does some really weird pushes and pulls on Mercury. It’s this change in distance that causes it to get locked into a 2:3 resonance, where for every two years, it rotates three times.

Fraser: So if it were circular, then it probably would be tidally locked, but because it has this elliptical orbit, it has this other weird thing.

Pamela: It’s still called tidal locked, it’s just not a 1:1 tidal locking like with the Moon, which orbits once and rotates once. Here, it orbits twice, orbits three times. It’s just a different kind of tidal locking.

Fraser: I know we’ve sent some missions to Mercury in the past. Can we talk a bit about that?

Pamela: Trying to observe Mercury even with a spacecraft is hard. You have to fling things to the inner part of the solar system and the Sun likes to try and catch them. To date, we’ve only sent one mission to image Mercury that was Mariner 10. It did two high-speed flybys, taking images as it went by. In fact, it only saw one side of Mercury. So we only have images of half a planet.

The other problem with going to Mercury is it’s kind of hot. Anyone who’s had a computer fan stop working knows that hot electronics are non-functioning electronics. So when we send things to Mercury, we have to try and figure out how to protect them from the heat and sunlight.

So we have two big problems to solve: temperature and getting there without landing in the Sun or missing the planet. Mariner 10 did the job, got some really good images of half the planet, but we want more. Scientists always want more data (it’s a problem we have).

Currently, NASA has a mission called Messenger. It just finished flying past Venus. It’s on its way into Mercury. It’s going to take it a few tries to settle into orbit around Mercury – it’s actually going to fly past it a couple of times and use different gravitational effects to try and slow itself down.

Fraser: I guess it’s very different from the spacecraft they send to mars. When they go to Mars, they can use the atmosphere to aerodynamically break their orbit. I know the mars spacecraft come through the atmosphere several times, skimming the top of the atmosphere slowing themselves down a little bit more until they’re in whatever orbit they want to be in.

With Mercury, that non-atmosphere isn’t going to participate, so they’ve got to be doing it entirely with rockets.

Pamela: They actually do it almost entirely with gravity, that’s one of the cool things.

A better way to think of it is when we send things out to Jupiter and Saturn, we often use some of the inner planets to give gravity boosts. We’ll send things into an orbit where they go once around the Sun and then they start to catch up on Earth. As they catch up on Earth, its gravity pulls them in and they eventually fly past Earth. As they fly past, the Earth tries to slow them down, but Earth and this object are moving in the same direction, so the amount of push we can give an object heading out toward the outer solar system, that’s going in the same direction of orbit we’re going in, is a lot more than the pull we give it as it goes past us.

This is called gravity-assist. It’s away to speed things up by allowing the Earth’s gravity to pull in the direction we’re all orbiting.

If you try going around the Sun in the opposite direction, such that it comes around the Sun and is headed into a head-on collision with the Earth, the planet’s gravity will still pull it toward the Earth, but as it starts to go past the Earth, because we’re now moving in the other direction, we slow the object down more than we speed it up.

So you can use gravity-assist to slow things down if you try and go against the flow of orbits, or to speed things up if you go in the flow of the orbit.

Fraser: So that’s what Messenger’s going to do: orbit in the “wrong” direction and use that gravity to actually slow it down until it can put itself into orbit.

Pamela: Exactly, so they’re gradually breaking themselves (in this case using Venus and Earth to break themselves) to get to Mercury. They’re going to have to go past Mercury a couple of times before they settle into a nice orbit and then just image, image, image that entire planet.

Mercury has a lot of neat stuff it’s hiding, and Mariner had image resolutions on the order of kilometres. You couldn’t make out anything small. With Messenger, we’ll be able to make out smaller features on the surface of the planet. One of the cool mysteries about Mercury is it might actually be hiding ice.

Fraser: Where would there be ice?

Pamela: So, with the planet Earth, we’re kind of tilted. The entire planet gets to see sunlight now and then, depending which side of the planet is facing the Sun. Mercury hangs out perfectly straight. Its rotational axis is absolutely perpendicular to its orbit. This means the poles of Mercury never see sunlight at all, if you’re in a crater. So the crater shadows are always, always, always in shadow.

Fraser: I see, so you have a crater on top of the planet, and as the planet is turning, that crater is like a bowl on the very top of the planet and is always in shadow. Different rim/edges of the crater would be brightened, but at the very bottom it would always be in shadow.

It could actually have ice remain in there? As you said, the whole planet itself is so hot (as we image from Earth) how could ice remain in there? It’s not water sloshing around inside the crater?

Pamela: No, we think it’s actual water ice. The reason we think this is when we do radar imaging of the surface, we find these areas that are extremely smooth and reflective in the exact same way we generally associate with ice. The surface temperature, while it’s hot on both sides, it actually at the poles has really cold areas, areas that are significantly below zero. We’ve actually measured temperatures that are only 80 Kelvin in the extreme north polar regions.

Fraser: Okay, so the only reason the planet is so heated up is because it’s bathed in sunlight for a good chunk of time. It’s not like it’s convective, where the heat moves around and warms up the whole planet, but in this case just because they’re in the darkness they can stay frigid cold.

Pamela: Its complete lack of a reasonable atmosphere doesn’t provide anything that holds onto the heat, so in this lack of atmosphere area, you only stay warm when there’s air to trap in the temperatures.

Fraser: I wonder though, with the Moon they’re talking about a similar situation. There could be ice trapped in craters at the poles of the Moon, and that would be a wonderful resource for future astronauts who land on the Moon and want to use that ice to breathe, make water and fuel and so on. What would ice tell us about Mercury?

Pamela: It actually raises more questions than it answers, because Mercury really couldn’t have formed with water. it’s in a part of the solar system that the Sun baked quite nicely. If you want to get the water out of something, you back it. This is part of how pottery’s made: you stick it in a kiln and get all the water out of the clay. Mercury has been in that kiln, so how is there water?

The only way there could be water on Mercury is from it getting hit by comets. Something brought water in from the outer parts of the solar system. So if we find ice, it means at some point Mercury was getting hit with not just asteroids, but also with comets, and the comets left their icy remains locked on the poles. The water could just be burial ground for comets.

Fraser: What else is Messenger going to be doing while it’s at Mercury?

Pamela: It’s also going to be trying to understand Mercury’s magnetic field.

Fraser: Mercury has a magnetic field?

Pamela: Right! It’s one of those curious, “how did that happen?” kind of things. It’s not a strong magnetic field. Most compasses probably wouldn’t respond that strongly to its magnetic field… but it has one. That means it has a liquid iron core.

Now, when we look at Mars, it doesn’t have a magnetic field. It cooled off. Mercury is a lot smaller, so this raises the question of why didn’t it cool off enough that its magnetic field froze out?

Fraser: Isn’t it right in front of the Sun? People have said the temperatures on Venus are hot enough to melt lead. Mercury’s even closer, could the heat from the Sun be keeping it warm?

Pamela: Venus is actually hotter than Mercury. It’s the greenhouse effect on Venus that’s trapping all the heat inside. Mercury at its hottest is only about 700K, which is really hot, but not hot enough to explain the heat necessary to have a magnetic dynamo in the core.

Here what we actually think is happening is as the planet moves closer and further away from the Sun, this is squeezing the planet, just like Io (one of the moons of Jupiter) is getting squeezed, and this squeezing is creating the magnetic field.

That’s just kind of neat. It’s another characteristic that some of the rocky moons share with some of the rocky worlds.

Fraser: All right. Any other mysteries that Messenger will uncover?

Pamela: Well, there’s always the “what’s on the other side? ” question.

Fraser: Right, I guess we have no photographs whatsoever of that other side.

Pamela: There are some tantalizing hints at fascinating structure in some of the images people have tried to take.

If you go out and use a really good telescope, and you’re really careful, you can image Mercury during the day, but you’re imaging through an atmosphere. When you’re looking at something during the day, you can’t use artificial stars to correct your telescope for atmospheric issues (you can’t use adaptive optics). Folks who are expert imagers, working as hard as they can, using really good telescopes and really good telescope techniques, have put together low resolution images of Mercury.

There are hints that there is a giant crater on Mercury that has a mountain on the other side. So it’s possible that something came along at some point, nailed Mercury, created a crater on one side, and the shocks went through the planet and actually affected the other side of the planet.

Fraser: Could that maybe be one of the things that knocked off some of its material?

Pamela: Since the surface was already there to get hit and form ripples and craters, whatever knocked the surface off of Mercury probably happened long beforehand.

If you hit something hard enough that the whole planet falls apart, it’s going to reform as a nice, smooth ball. You get to start over from ground zero, re-cratering the planet however you will.

So this is something that came along later and threatened Mercury’s life a second time. It’s not good to be something that close to the Sun, where you’re constantly in harm’s way.

Fraser: Right, but we have other moons in the solar system, like Mimus going around Saturn, who looks like the Death Star. It has a crater on it that’s so large it completely dwarfs everything on the planet. Not quite big enough to make everything have to reform, but still a pretty devastating impact.

Well, that’d be great. As soon as we start to see those pictures, I think everyone’s going to be really, really impressed. I know that Messenger isn’t the only mission that’s probably going to be headed to Mercury. What does the future hold?

Pamela: The Europeans are also looking to launch their own mission, Bebicolumbo. This is a follow-up mission to Messenger that’s going to go in and take additional images, be there for another year, and whatever questions Messenger opens, Bebicolumbo will be there to answer them (we hope).

Currently, we’re looking at getting Messenger to Mercury in March 2011. it’s currently leaving Venus and Bebicolumbo won’t even be launching until Messenger’s already gotten to Mercury, so they have time to change their mind and update their equipment to fill whatever needs are open.

Fraser: We talked about Mercury a couple of times in the past, in reference to relativity. It’s got a pretty neat story to it, do you want to go into that?

Pamela: Mercury’s orbit is this weird ellipse. It gets close, gets far away, gets close, and gets far away. This ellipse is slowly rotating. This means that if you’re looking down on the solar system and you pretend the whole solar system isn’t moving, as you watch, this ellipse that Mercury’s rotating on, itself rotates. We see this rotation in terms of when Mercury is furthest from the Sun, and highest in our sky.

This is something people have been able to measure since before Newton. It confused us. When Newton first went through and figured out his orbits, he was left scratching his head. Once you factor in: “here’s Mercury, here’s what it’s doing, here’s the gravity from the Sun.. it’s still not behaving. Here’s the gravity from Jupiter… it’s still not behaving. Here’s the gravity from Saturn…” Once you factor in the tugs and pulls of every object that is big enough to be significant, we still can’t fully explain Mercury’s motion.

Mercury moves about 574 arc seconds per century in terms of how its position furthest from the Sun moves.

Fraser: Can you translate that?

Pamela: Okay, so one arc second is about the width of a piece of normal human hair held out at arm’s length. So each century, we can observe Mercury’s position when it’s furthest from the Sun, move 574 hair strands at arms length. It’s not a huge amount.

Fraser: I can see us calculating that with Hubble, today, but how could they have figured that out then?

Pamela: Ancient astronomers made some of the most amazing measurements ever. One of the most remarkable things I ever read about was how Hipparchus, back in 150 BC was going through star maps and was comparing his observations of the sky with the observations of someone named Timarchus, who worked 169 years before him.

In comparing their two maps, he realised that the North Pole was slightly different for both of them. They weren’t both marking the North Pole as being exactly dead on with the star Polaris. Hipparchus was able to realise that the entire sky is slowly precessing. The point that the North Pole is located on the sky is slowly changing.

Hipparchus figured out this change is about 0.0127 degrees per year. This is a hundredth of the width of your Sun a year, and he was doing this in 150BC. That’s a really amazing measurement, and it’s only about 0.02 off of today’s modern measurement. His measurement was 0.0127 degrees, today we know that this precession is 0.0139 degrees.

They could measure things. They had dark skies, not a lot of things to distract them – there was no YouTube, no Google… so they measured, and they thought and they went through libraries. All these things were handwritten and they were still going back and using records that were hundreds of years old. We’re lucky to use things more than 10 years old, because if they’re more than 10 years old, they’re not in PDF on the internet.

Fraser: Okay, so we know that Mercury had a strangely changing orbit. How does that tie into relativity?

Pamela: Newton came along and looked at planets doing this and tried to calculate how much motion all these perturbations add up to. We’d observed 574 arc seconds per century, and Newton was able to come up with 531 arc seconds per century, using his calculations.

So there was this gap of about 46 arc seconds per century that we couldn’t explain. People tried making up new planets. There was a theory that there was another planet inside of Mercury’s orbit. We even named it – it was called Vulcan. We tried finding it. People claimed they found it, other people claimed it didn’t exist.

There is no Vulcan, they were bad observations. If you look at the Sun, you’re going to see spots.

Fraser: I see, so they thought that some of the Sunspots they saw were actually the planet inside. I could see you would have a difficult time finding a planet so close to the Sun. Mercury’s already so hard to observe, so finding a planet that’s even inside that orbit should be even harder to observe, but you could probably see it going across the face of the Sun with your telescopes.

Pamela: There’s also the literal problem of if you look at the Sun, you see spots, in terms of your vision just can’t cope. So you’re looking at the field way too close to the Sun, and you’re going to see things that are just chemical reactions in your eyes from seeing the Sun, rather than things that are actually there.

Fraser: Then after a while you won’t see anything.

Pamela: Yeah, that’s another bad side effect as well.

So they looked and eventually figured out there’s no Vulcan. So we’re left with this 46 arc seconds per century that Mercury’s moving that we couldn’t account for.

Finally in the 1900s, Einstein came along and worked on his theory of general relativity and special relativity. In working on these theories, he hoped, hoped, hoped that this would account for Mercury and it did within 3 arc seconds. It turns out the last 3 arc seconds is because the Sun is not a sphere, it’s kind of flat and that affects things.

So from general relativity, we get a correction of 43 arc seconds per century. On top of that we get this extra correction called the Dickey-Goldberg correction that comes from the Sun not being a sphere.

When you add together Newtonian perturbations (because we have Jupiter, Saturn and everything else pulling on Mercury), a general relativity correction and then correct for the fact that the Sun is not a sphere, you can completely account for all of Mercury’s motion.

Pamela: Math. It works!

All right, next time I think we’ll proceed, though we might need to stick another questions show in there. We will work our way through the planets in the solar system, so Venus will be next.


Dark tears

Because they orbit closer to the sun than Earth, Mercury and Venus are the only planets that can make solar transits from our perspective. With its swift 88-day orbit, Mercury passes between Earth and the sun every four months or so. But the planet’s orbit is tilted compared to the plane of Earth’s orbit, so most of the time, the tiny world passes above or below the sun's disk from our line of sight.

That orbital configuration means Mercury transits happen just 13 to 14 times a century, with the most recent prior event taking place in 2016. Venusian transits are even rarer, happening on average only once a century. The last transit of Venus occurred in 2012, and we won’t see another one until 2117.

No one on Earth will see Mercury cross the sun again until November 2032. North Americans will have an even longer dry spell to contend with, as they will have to wait until 2049 for the next Mercury transit visible from their part of the globe.

One interesting sight to watch for during a transit is the so-called black drop effect, an optical illusion that happens when the planet either just enters or starts to leave the sun’s disk. When Mercury’s leading edge first touches the sun, the planet will appear to grow a narrow neck connecting it to the edge of the sun, making the silhouette look like a teardrop. This strange apparition happens again just as Mercury becomes engulfed by the sun’s disk. (Here’s why science fiction icon Kim Stanley Robinson is inspired by Mercury.)

Seeing a planet sail across our sun also offers a chance to witness a crucial method astronomers use to find planets beyond our solar system. NASA’s now retired Kepler mission was able to successfully identify and confirm 2,662 of these exoplanets across the galaxy using transit events like the one we will see up close on November 11.

In many instances, our line of sight is aligned so that telescopes on Earth can detect the tiny dips in starlight as an exoplanet transits its host star. From this data, astronomers can then calculate the size, orbit, and even some physical properties of these alien worlds.

Even if you are clouded out, in the wrong place, or don’t have the right gear, this transit can be enjoyed worldwide via live webcasts that will showcase the entire event. Virtual Telescope promises to have coverage from Earth-based telescopes, while NASA’s sun-watching satellite SDO will offer a dramatic perspective on the transit via its own livestream from space.


How was disproving Vulcan’s existence central to Einstein’s General Theory of Relativity?

Einstein’s theory of relativity explains the same phenomenon, but does so with a completely different kind of structure or picture than Isaac Newton. In Newton’s theory, gravity is a property that leaps across space between two bodies a force that pulls on you. Einstein didn’t just say, my numbers are better. He said, you have to fundamentally change that picture of gravity, that understanding of what the properties of reality are.

The core of General Relativity is that space and time are not static, but dynamic and can change. The way they change is by the presence and motion of matter and energy. A vast mass like the sun creates curves in spacetime, which means that things don’t go straight. A ray of light passing close to the sun will travel a curving path.


First Photo of Mercury From Orbit Comes Tuesday: What Will We See?

NASA's Messenger spacecraft, the first probe ever to orbit Mercury, will snap its first photo of the planet from orbit tomorrow morning (March 29).

The first photo of Mercury from orbit will feature previously unseen terrain near the small, rocky planet's south pole. Messenger will begin taking pictures Tuesday at 3:40 am EDT (0740 GMT), and NASA plans to release the first image in the afternoon. [Photos of Mercury From the Messenger Probe]

Messenger entered orbit around Mercury March 17, becoming the first spacecraft to ever do so. NASA launched the $446 million Messenger mission in August 2004. The spacecraft is expected to continue orbiting Mercury for at least one Earth year.

Messenger's extreme orbit brings it within 124 miles (200 kilometers) of Mercury at the closest point and retreats to more than 9,300 miles (15,000 km) away at the farthest, researchers said.

The probe is still in a commissioning phase, which allows scientists to check the spacecraft's systems after the orbital insertion maneuvers. As part of this phase, Messenger's Mercury Dual Imaging System (MDIS) will take 1,549 images.

NASA plans to have the Messenger spacecraft start by taking 364 images over six hours on Tuesday.

After the checkout phase, Messenger is set to begin its orbital science operations April 4. These will involve snapping around 75,000 images of the entire surface of Mercury to study the planet's geology, formation and history. [Video: Messenger's Mercury Orbit Arrival]

Messenger is the first mission ever to orbit around Mercury, though the spacecraft did fly by the planet three times before settling into orbit. The first probe ever to visit Mercury was NASA's Mariner 10 spacecraft, which flew by the planet three times in the mid-1970s.


Poglej si posnetek: Ortuť (December 2022).