Astronomija

Ali obstaja časovna beseda za kroženje lune?

Ali obstaja časovna beseda za kroženje lune?


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Za planet lahko pogovorno poimenujemo njegovo obdobje vrtenja kot "dan" in njegovo obdobje revolucije okoli matične zvezde kot "leto". Nekateri svetovi imajo edinstvene izraze, na primer Marsovske dni imenujejo "sols", vendar je načelo enako: ena beseda za vrtenje okoli svoje osi, druga za revolucije okoli zvezde.

Ali obstaja enakovredna beseda za obdobje lunine revolucije okoli planeta? Na Zemlji se približno ujema z "mesecem", vendar bi imeli planeti z več lunami različna obdobja za vsako luno.

Če bi bil zadolžen za merjenje časa v misiji v Evropo ali Ganimed, s katerim izrazom bi označil časovno obdobje, ki je potrebno Luni, da opravi eno orbito Jupitra?


Uporabiti morate enega od natančno določenih izrazov posebnih meritev orbitalnega obdobja, ki so opisani na tej strani Wikipedije.

Morda se boste izognili zgolj "orbitalnemu obdobju", če ga ne uporabljate v kontekstu, ki zahteva natančnost.

Za dolžino dneva obstaja sinodični dan in zvezdno obdobje. Tu je razprava o razliki med njimi.


Če odgovorimo na @ stephenG, lahko na sorodno stran Wikipedije Lunin mesec dobimo idejo o tem, kaj različni izrazi pomenijo.

Moram pa se strinjati, da bi se verjetno morali poskusiti izogniti samo uporabi besede mesec, medtem ko motečega, s katerim se pogovarjate, odvrnete (kaj zakašljate ali spustite) in nato hitro nadaljujete, kajti vsak natančnejši izraz bo imel zelo in če nekdo reče "Počakaj, ali ne misliš a bla-bla-bla mesec? "boste morali nehati ugotavljati, ali boste to storili ali ne!

vrsta okvira - ali - merila Zemljina luna (dnevi) zvezde vztrajnostne zvezde wrt 27.321661 sinodično vrtenje z Soncem-Zemljo 29.530588 drakonitna orbitalna ravnina (precesi) 27.212220 (ali drakonski) čas med naraščajočimi vozlišči (ali nodalni) anomalistični čas med periapsami 27.554551 tropski čas med poravnave osi Lune 27.321582 s črto planet-luna

Epoha (astronomija)

V astronomiji je an epoha ali referenčna doba je trenutek v času, ki se uporablja kot referenčna točka za neko časovno spremenljivo astronomsko količino. Uporabna je za nebesne koordinate ali orbitalne elemente nebesnega telesa, saj so predmet motenj in se s časom spreminjajo. [1] Te astronomske količine, ki se spreminjajo v času, lahko vključujejo na primer srednjo dolžino ali srednjo anomalijo telesa, vozlišče njegove orbite glede na referenčno ravnino, smer apogeja ali afelije njegove orbite ali velikost glavne osi svoje orbite.

Tako določena astronomska veličina je glavna za izračun drugih ustreznih parametrov gibanja za napovedovanje prihodnjih položajev in hitrosti. Uporabljena orodja disciplin nebesne mehanike ali mehanike njenega podpolja (za napovedovanje orbitalnih poti in položajev teles, ki se gibljejo pod gravitacijskimi učinki drugih teles) lahko uporabimo za ustvarjanje ephemeris, tabele vrednosti, ki podaja položaje in hitrosti astronomskih predmetov na nebu v določenem času ali časih.

Astronomske veličine je mogoče določiti na katerega koli od več načinov, na primer kot polinomsko funkcijo časovnega intervala, z epoho kot časovno točko izvora (to je običajni trenutni način uporabe epohe). Astronomsko količino, ki se spreminja v času, lahko izrazimo kot konstanto, ki je enaka meri, ki jo je imela v tej epohi, pri čemer ostane njeno spreminjanje skozi čas določeno na drug način - na primer s tabelo, kot je bilo običajno med 17. in 18. stoletja.

Beseda epoha je bil v starejši astronomski literaturi pogosto uporabljen na drugačen način, npr. v 18. stoletju v povezavi z astronomskimi tabelami. Takrat je bilo običajno, da se kot "epohe" označujejo ne standardni datum in čas nastanka za časovno spreminjajoče se astronomske količine, temveč vrednosti na ta datum in čas teh časovno različnih količin. [2] V skladu s to alternativno zgodovinsko uporabo bi se izraz, kot je "popravljanje obdobij", nanašal na prilagoditev vrednosti tabelaričnih astronomskih količin, ki veljajo za določen standardni referenčni datum in čas, običajno z majhno količino (in ne, kot bi lahko pričakovali od trenutne uporabe, na spremembo enega referenčnega datuma in časa na drug datum in uro).


Vsebina

Lastnosti orbite, opisane v tem oddelku, so približne. Lunina orbita okoli Zemlje ima veliko sprememb (motenj) zaradi gravitacijske privlačnosti Sonca in planetov, katerih proučevanje (lunina teorija) ima dolgo zgodovino. [10]

Eliptična oblika Uredi

Lunina orbita je skoraj krožna elipsa okoli Zemlje (polvečna in polminora sta 384.400 km in 383.800 km: razlika le 0,16%). Enačba elipse daje ekscentričnost 0,0549 in razdaljo perigeja in apogeja 362.600 km oziroma 405.400 km (razlika 12%).

Ker so bližnji predmeti večji, se navidezna velikost Lune spreminja, ko se premika proti in od opazovalca na Zemlji. Dogodek, imenovan "super luna", se zgodi, ko je polna Luna najbližje Zemlji (perigej). Največji možni navidezni premer Lune je enak za 12% večji (kot razdalja med perigejem in apogejem) kot najmanjša navidezna površina je za 25% večja, prav tako pa tudi količina svetlobe, ki jo odbija proti Zemlji.

Variacija lunine orbitalne razdalje ustreza spremembam tangencialne in kotne hitrosti, kot je zapisano v drugem Keplerjevem zakonu. Povprečno kotno gibanje glede na namišljenega opazovalca v barcentru Zemlja – Luna je 13,176 ° na dan proti vzhodu (J2000,0 epoha).

Raztezek Uredi

Lunin raztezek je njegova kotna razdalja kadar koli vzhodno od Sonca. Pri novi luni je nič in Luna naj bi bila v povezavi. Pri polni luni je raztezek 180 ° in naj bi bil v nasprotju. V obeh primerih je Luna v sizigiji, torej Sonce, Luna in Zemlja so skoraj poravnane. Kadar je raztezek bodisi 90 ° bodisi 270 °, naj bi bila Luna v kvadraturi.

Urejanje precesije

Usmerjenost orbite ni fiksno določena v vesolju, ampak se s časom vrti. Ta orbitalna precesija se imenuje apsidalna precesija in je vrtenje Lunine orbite znotraj orbitalne ravnine, to je osi elipse spreminjajo smer. Glavna os Lunine orbite - najdaljši premer orbite, ki se povezuje z njenimi najbližjimi in najbolj oddaljenimi točkami, perigejem oziroma apogejem - naredi eno popolno revolucijo vsakih 8,85 zemeljskih let ali 3232,6054 dni, saj se počasi vrti v isto smer kot Luna sama (neposredno gibanje) - kar pomeni, da se premakne proti vzhodu za 360 °. Lunina apsidalna precesija se razlikuje od nodalne precesije njene orbitalne ravnine in aksialne precesije same Lune.

Naklon Uredi

Povprečni naklon lunine orbite do ravnine ekliptike je 5,145 °. Teoretični premisleki kažejo, da je sedanji naklon glede na ekliptično ravnino nastal zaradi plimske evolucije iz prejšnje orbite blizu Zemlje s precej konstantnim naklonom glede na ekvator Zemlje. [11] Zahteval bi naklon te prejšnje orbite približno 10 ° do ekvatorja, da bi ustvaril sedanji naklon 5 ° do ekliptike. Menijo, da je bil prvotno nagib do ekvatorja blizu ničle, vendar bi ga lahko povečali na 10 ° z vplivom planetezimal, ki so šli blizu Lune, medtem ko so padali na Zemljo. [12] Če se to ne bi zgodilo, bi Luna zdaj ležala veliko bližje ekliptiki in mrki bi bili veliko pogostejši. [13]

Vrtilna os Lune ni pravokotna na njeno orbitalno ravnino, zato lunin ekvator ni v ravnini svoje orbite, ampak je nagnjen k njej s konstantno vrednostjo 6,688 ° (to je poševnost). Kot je odkril Jacques Cassini leta 1722, se rotacijska os Lune prekisuje z enako hitrostjo kot njena orbitalna ravnina, vendar je 180 ° izven faze (glej Cassinijeve zakone). Zato je kot med ekliptiko in luninim ekvatorjem vedno 1,543 °, čeprav rotacijska os Lune glede na zvezde ni fiksna. [14]

Urejanje vozlišč

Vozlišča so točke, v katerih Lunina orbita prečka ekliptiko. Luna prečka isto vozlišče vsakih 27.2122 dni, interval imenovan drakonski mesec ali drakonitski mesec. Linija vozlišč, sečišče med obema ravninama, ima retrogradno gibanje: za opazovalca na Zemlji se vrti proti zahodu vzdolž ekliptike z obdobjem 18,6 leta ali 19,3549 ° na leto. Če jih gledamo z nebesnega severa, se vozlišča gibljejo okoli Zemlje v smeri urnega kazalca, nasproti Zemljinemu vrtljaju in njegovi revoluciji okoli Sonca. Lunin ali Sončev mrk lahko nastopi, ko se vozlišča poravnajo s Soncem, približno vsakih 173,3 dni. Nagib lunine orbite tudi določa, da se mrki prekrižajo, ko vozlišča sovpadajo s polno in novo luno, ko se Sonce, Zemlja in Luna poravnajo v treh dimenzijah.

To dejansko pomeni, da je "tropsko leto" na Luni dolgo le 347 dni. To se imenuje drakonsko leto ali leto mrka. V to obdobje se ujemajo "letni časi" na Luni. Približno polovico tega drakonskega leta je Sonce severno od Luninega ekvatorja (vendar največ 1,543 °), drugo polovico pa je južno od Luninega ekvatorja. Očitno je učinek teh letnih časov majhen v primerjavi z razliko med Lunino nočjo in Luninim dnevom. Na luninih polih bo Sonce namesto običajnih luninih dni in noči približno 15 zemeljskih dni 173 dni "gor", saj bo "dol" polarni sončni vzhod in sončni zahod vsako leto trajal 18 dni. "Zgoraj" tukaj pomeni, da je središče Sonca nad obzorjem. [15] Lunini polarni sončni vzhodi in zahodi se pojavijo v času mrkov (sončni ali lunin). Na primer, ob Sončevem mrku 9. marca 2016 je bila Luna blizu padajočega vozlišča, Sonce pa blizu točke na nebu, kjer ekvator Lune prečka ekliptiko. Ko Sonce doseže to točko, središče Sonca zaide na Lunin severni pol in se dvigne na Lunin južni pol.

Nagib k ekvatorju in mirovanje lune Uredi

Vsakih 18,6 let kot med Lunino orbito in Zemljinim ekvatorjem doseže največ 28 ° 36 ′, vsoto ekvatorialnega nagiba Zemlje (23 ° 27 ′) in naklona Lune (5 ° 09 ′) proti ekliptiki. To se imenuje večji Lunin zastoj. Približno v tem času se bo Lunina deklinacija spreminjala od -28 ° 36 ′ do + 28 ° 36 ′. Nasprotno pa 9,3 leta kasneje kot med Lunino orbito in Zemljinim ekvatorjem doseže najnižjih 18 ° 20 ′. To se imenuje manjši Lunin zastoj. Zadnji Lunin zastoj je bil manjši zastoj oktobra 2015. Takrat se je padajoče vozlišče postavilo v vrsto z enakonočjem (točka na nebu, ki ima nič vzpenjanja in nič deklinacije). Vozlišča se gibljejo proti zahodu za približno 19 ° na leto. Sonce vsako leto prečka dano vozlišče približno 20 dni prej.

Ko je naklon Lunine orbite do Zemljinega ekvatorja najmanj 18 ° 20 ′, bo središče Luninega diska vsak dan nad obzorjem s širin, manjših od 70 ° 43 '(90 ° - 18 ° 20' - 57 'paralaksa) severno ali južno. Ko je naklon največ 28 ° 36 ', bo središče Luninega diska vsak dan nad obzorjem le od zemljepisnih širin, manjših od 60 ° 27' (90 ° - 28 ° 36 '- 57' paralakse) severno oz. južno.

Na višjih zemljepisnih širinah bo vsaj en dan vsak mesec, ko Luna ne bo vstajala, bo pa tudi vsaj en dan vsak mesec, ko Luna ne bo zašla. To je podobno sezonskemu vedenju Sonca, vendar s časom 27,2 namesto 365 dni. Upoštevajte, da je lahko točka na Luni dejansko vidna, ko je približno 34 ločnih minut pod obzorjem zaradi loma zraka.

Zaradi naklona Lunine orbite glede na Zemljin ekvator je Luna skoraj vsak teden vsak mesec skoraj dva tedna nad obzorjem na severnem in južnem tečaju, čeprav je Sonce pod obzorjem šest mesecev naenkrat. Obdobje od vzhoda do vzhoda lune na polih je tropski mesec, približno 27,3 dni, kar je povsem blizu zvezdičnemu obdobju. Ko je Sonce najbolj oddaljeno od obzorja (zimski solsticij), bo Luna polna, ko bo na najvišji točki. Ko je Luna v dvojčkih, bo nad obzorjem na severnem tečaju, ko bo v strelcu, pa na južnem polu.

Lunino svetlobo zooplanktoni uporabljajo na Arktiki, ko je Sonce mesece pod obzorjem [16], in mora biti v pomoč živalim, ki so živele v arktičnih in antarktičnih regijah, ko je bilo podnebje toplejše.

Urejanje modela v merilu

Model v lestvici sistema Zemlja - Luna: Velikosti in razdalje se merijo. Predstavlja srednjo razdaljo orbite in srednji polmer obeh teles.

Približno leta 1000 pred našim štetjem so bili Babilonci prva človeška civilizacija, za katero je znano, da je dosledno beležila lunina opazovanja. Na glinastih tablicah iz tistega obdobja, ki so jih našli na ozemlju današnjega Iraka, so v klinopisu zapisani časi in datumi luninih vzhodov in zahodov lune, zvezde, ki jih je Luna prešla blizu, in časovne razlike med vzponom zahajanje Sonca in Lune okoli polne lune. Babilonska astronomija je odkrila tri glavna obdobja gibanja Lune in z analizo podatkov ustvarila lunine koledarje, ki so segali tudi v prihodnost. [10] To uporabo podrobnih, sistematičnih opazovanj za napovedovanje na podlagi eksperimentalnih podatkov lahko označimo kot prvo znanstveno študijo v zgodovini človeštva. Vendar se zdi, da Babilonci niso imeli nobene geometrijske ali fizične interpretacije svojih podatkov in niso mogli napovedati prihodnjih Luninih mrkov (čeprav so bila "opozorila" izdana pred verjetnimi časi mrkov).

Starogrški astronomi so bili prvi, ki so uvedli in analizirali matematične modele gibanja predmetov na nebu. Ptolemej je lunin gib opisal z uporabo natančno določenega geometrijskega modela epiciklov in evekcije. [10]

Sir Isaac Newton je prvi razvil popolno teorijo gibanja, mehaniko. Opazovanja luninega gibanja so bila glavni preizkus njegove teorije. [10]

Ime Vrednost (dnevi) Definicija
Zvezdniški mesec 27.321 662 glede na oddaljene zvezde (13,36874634 prehodov na sončno orbito)
Sinodični mesec 29.530 589 glede na Sonce (lunine faze, 12,36874634 prehodov na sončno orbito)
Tropski mesec 27.321 582 glede na pomladno točko (procesi v

Z lunino orbito je povezanih več različnih obdobij. [17] Zvezdniški mesec je čas, ki je potreben za eno popolno orbito okoli Zemlje glede na fiksne zvezde. To je približno 27,32 dni. Sinodični mesec je čas, potreben Luni, da doseže isto vizualno fazo. Ta se skozi leto močno razlikuje [18], v povprečju pa znaša približno 29,53 dni. Sinodično obdobje je daljše od zvezdnega obdobja, ker se sistem Zemlja-Luna premika po svoji orbiti okoli Sonca v vsakem zvezdnem mesecu, zato je potrebno daljše obdobje, da se doseže podobna poravnava Zemlje, Sonca in Lune. Anomalni mesec je čas med perigeji in je približno 27,55 dni. Ločitev Zemlja-Luna določa moč sile lunine plima.

Drakonski mesec je čas od naraščajočega vozlišča do naraščajočega vozlišča. Čas med dvema zaporednima prehodoma enake ekliptične dolžine se imenuje tropski mesec. Slednja obdobja se nekoliko razlikujejo od zvezdnega meseca.

Povprečna dolžina koledarskega meseca (dvanajstega leta) je približno 30,4 dni. To ni lunino obdobje, čeprav je koledarski mesec v preteklosti povezan z vidno lunino fazo.

Gravitacijsko privlačnost, ki jo Luna izvaja na Zemlji, je vzrok za plimovanje tako v oceanu kot na trdni Zemlji Sonce ima manjši plimski vpliv. Trdna Zemlja se hitro odzove na kakršno koli spremembo v plimovanju, izkrivljanje ima obliko elipsoida z visokimi točkami približno pod Luno in na nasprotni strani Zemlje. To je posledica velike hitrosti potresnih valov v trdni Zemlji.

Vendar hitrost potresnih valov ni neskončna in skupaj z učinkom izgube energije znotraj Zemlje to povzroči rahlo zamudo med prehodom največjega siljenja zaradi Lune čez in največjo plimovanje Zemlje. Ko se Zemlja vrti hitreje, kot da Luna potuje po svoji orbiti, ta majhen kot ustvari gravitacijski navor, ki upočasni Zemljo in pospeši Luno v njeni orbiti.

V primeru morskih plim in osek je hitrost plimskih valov v oceanu [19] veliko počasnejša od hitrosti Luninega plimovanja. Kot rezultat, ocean ni nikoli v ravnovesju s plimsko silo. Namesto tega siljenje ustvari dolge oceanske valove, ki se širijo okoli oceanskih bazenov, dokler s turbulenco ne izgubijo energije, bodisi v globokem oceanu bodisi na plitvih celinskih policah.

Čeprav je odziv oceana bolj kompleksen od obeh, je oceanske plime in oseke mogoče razdeliti na majhen elipsoidni izraz, ki vpliva na Luno, in drugi izraz, ki nima učinka. Oceanski elipsoidni izraz upočasni tudi Zemljo in pospeši Luno, toda ker ocean razsipa toliko energije plimovanja, imajo sedanje oceanske plime za red veliko večji učinek kot plime in oseke na trdni Zemlji.

Zaradi plimovalnega navora, ki ga povzročajo elipsoidi, se del Zemljinega kotnega (ali rotacijskega) zagona postopoma prenaša na vrtenje para Zemlja-Luna okoli njunega medsebojnega masnega središča, imenovanega barycentre. Za podrobnejši opis glej pospešek plime in oseke.

Ta nekoliko večji orbitalni kotni moment povzroči, da se razdalja med Zemljo in Luno poveča na približno 38 milimetrov na leto. [20] Ohranjanje kotnega momenta pomeni, da se osna rotacija Zemlje postopoma upočasnjuje in se zaradi tega njen dan vsako leto podaljša za približno 24 mikrosekund (brez ledeniškega odboja). Obe številki veljata samo za trenutno konfiguracijo celin. Plimski ritmi izpred 620 milijonov let kažejo, da se je Luna v stotinah milijonih let umikala s povprečno hitrostjo 22 mm (0,87 palca) na leto (2200 km ali 0,56% ali razdalja med Zemljo in Luno na sto milijonov let) in dan se je podaljšal s povprečno hitrostjo 12 mikrosekund na leto (ali 20 minut na sto milijonov let), kar je približno polovica njunih trenutnih vrednosti.

Sedanja visoka stopnja je lahko posledica skoraj resonance med naravnimi oceanskimi frekvencami in frekvencami plimovanja. [21] Druga razlaga je, da se je Zemlja v preteklosti vrtela veliko hitreje, dan pa je verjetno trajal le 9 ur na zgodnji Zemlji. Nastali plimovalni valovi v oceanu bi bili takrat precej krajši in bi bilo težje, če bi plimovanje z dolgimi valovi vzbujalo plimovanje s kratkimi valovi. [22]

Luna se postopoma umika od Zemlje v višjo orbito in izračuni kažejo, da bi se to nadaljevalo približno 50 milijard let. [23] [24] Do takrat bi bila Zemlja in Luna v medsebojni resonanci z vrtljivo orbito ali plimskim zaklepanjem, pri čemer bi Luna krožila okoli Zemlje v približno 47 dneh (trenutno 27 dni), in tako Luna kot Zemlja bi se istočasno vrteli okoli svojih osi, vedno obrnjeni drug proti drugemu z isto stranjo. To se je že zgodilo z Luno - ista stran je vedno obrnjena proti Zemlji - in počasi se dogaja tudi z Zemljo. Vendar se upočasnitev vrtenja Zemlje ne dogaja dovolj hitro, da bi se vrtenje podaljšalo na mesec dni, preden drugi učinki spremenijo situacijo: približno 2,3 milijarde let od zdaj bo povečanje sončnega sevanja povzročilo izhlapevanje zemeljskih oceanov, [25 ] odstranjevanje glavnine plimovanja in trenja ter pospeševanja.

Luna je v sinhroni rotaciji, kar pomeni, da ima ves čas isti obraz proti Zemlji. To sinhrono vrtenje v povprečju velja le, ker ima Lunina orbita določeno ekscentričnost. Posledično se kotna hitrost Lune spreminja, ko kroži okoli Zemlje, zato ni vedno enaka hitrosti vrtenja Lune, ki je bolj konstantna. Ko je Luna v perigeju, je njeno orbitalno gibanje hitrejše od vrtenja. Takrat je Luna nekoliko naprej v svoji orbiti glede na vrtenje okoli svoje osi, kar ustvarja perspektivni učinek, ki nam omogoča, da vidimo do osem stopinj zemljepisne dolžine njene vzhodne (desne) skrajne strani. Nasprotno pa, ko Luna doseže svoj apogej, je njeno orbitalno gibanje počasnejše od vrtenja in razkrije osem stopinj zemljepisne dolžine njene zahodne (leve) skrajne strani. To se imenuje optična libracija v dolžini.

Lunina os vrtenja je nagnjena za skupaj 6,7 ° glede na normalno ravnino ekliptike. To vodi do podobnega perspektivnega učinka v smeri sever – jug, ki se imenuje optična libracija v zemljepisni širini, ki omogoča, da vidimo skoraj 7 ° zemljepisne širine onkraj pola na skrajni strani. Nazadnje, ker je Luna le približno 60 zemeljskih polmerov oddaljena od masnega središča Zemlje, se opazovalec na ekvatorju, ki Luno opazuje vso noč, premika bočno za en premer Zemlje. To povzroča dnevna libracija, ki omogoča ogled dodatne lunarne dolžine za eno stopinjo. Iz istega razloga bi lahko opazovalci na obeh zemeljskih geografskih polih videli še eno stopnjo vrednosti nihanja na zemljepisni širini.

Poleg teh "optičnih knjižnic", ki jih povzroča sprememba perspektive za opazovalca na Zemlji, obstajajo tudi "fizične knjižnice", ki so dejanske nutacije smeri vrtenja pola Lune v vesolju, vendar so te zelo majhne.

Ko gledamo s severnega nebesnega pola (tj. Iz približne smeri zvezde Polaris), Luna kroži okoli Zemlje v nasprotni smeri urnega kazalca, Zemlja pa okoli Sonca, Luna in Zemlja pa se vrtita na svoji osi v nasprotni smeri urnega kazalca.

Desno pravilo lahko uporabimo za označevanje smeri kotne hitrosti. Če palec desne roke kaže na severni nebesni pol, se njegovi prsti zvijejo v smeri, da Luna kroži okoli Zemlje, Zemlja kroži okoli Sonca, Luna in Zemlja pa se vrtita na lastnih oseh.

V predstavitvah Osončja je običajno narisati Zemljino trajektorijo s stališča Sonca in Lunino pot s stališča Zemlje. To bi lahko dalo vtis, da Luna kroži okoli Zemlje tako, da gre včasih nazaj, če jo gledamo iz Sončeve perspektive. Ker pa je orbitalna hitrost Lune okoli Zemlje (1 km / s) majhna v primerjavi z orbitalno hitrostjo Zemlje okoli Sonca (30 km / s), se to nikoli ne zgodi. V Sončevi orbiti Lune ni nobenih zank.

Če upoštevamo sistem Zemlja-Luna kot binarni planet, je njegovo težišče znotraj Zemlje, približno 4.671 km (2.902 mi) [27]> ali 73,3% Zemljinega polmera od središča Zemlje. To težišče ostaja na črti med središči Zemlje in Lune, ko Zemlja zaključi svoje dnevno kroženje. Pot sistema Zemlja-Luna v njegovi sončni orbiti je opredeljena kot gibanje tega vzajemnega težišča okoli Sonca. Posledično se Zemljino središče v vsakem sinodičnem mesecu spreminja znotraj in zunaj sončne orbitalne poti, ko se Luna premika po svoji orbiti okoli skupnega težišča. [28]

Sončev gravitacijski učinek na Luno je več kot dvakrat večji od Zemljinega na Luni, zato je Lunina pot vedno izbočena [28] [29] (kot je videti, če gledamo proti Soncu na celoten sistem Sonce-Zemlja-Luna z velike razdalje zunaj sončne orbite Zemlja – Luna) in ni nikjer konkavno (iz iste perspektive) ali zanke. [26] [28] [30] To pomeni, da je območje, zaprto z Lunino orbito Sonca, konveksna množica.


Ali obstaja časovna beseda za kroženje lune? - astronomija

Posledice plimnega trenja

Oceanske plime so ne edini učinek teh plimovanja. Tudi trdno telo Zemlje se na ta način rahlo izboči. Vsakodnevno upogibanje Zemlje (tako v trdnem telesu kot tudi v oceanih) povzroča izgubo energije vrtenja Zemlje zaradi trenja. Ta energija gre v toploto, s čimer se poveča notranja temperatura Zemlje. Izguba rotacijske energije pomeni, da se Zemlja upočasni s hitrostjo vrtenja, trenutno za približno 0,002 sekunde na stoletje.

Kot si lahko predstavljate, Zemlja na Luno izvaja tudi plimovanje. Dejansko so plimovalne sile Zemlje na Luni približno 20-krat večje od sil na Luni na Zemlji. Upoštevajte, kaj se zgodi, ko se vrteče telo plimno popači. Linija popačenja se nenehno obrača stran od črte med obema telesoma, zaradi česar izbokline rahlo vodijo. Nato obstaja neto navor, ki nasprotuje smeri vrtenja in tako upočasni obe telesi. Ta navor obstaja, dokler upočasnjeno vrtenje ne povzroči, da se orbitalno obdobje telesa izenači z obdobjem vrtenja. Ko se to enkrat zgodi, naj bi bilo telo plimovanje zaklenjeno , navor in odvajanje plimskih sil preneha. V tem trenutku je Luna v plimovanju z Zemljo, toda Zemlja ni z Luno. Zato Luna drži isti obraz do Zemlje. V oddaljeni prihodnosti se bo upočasnjena Zemlja sčasoma plimsko zaklenila z Luno in nadaljnji razvoj sistema ne bo prišel.

Ko se to zgodi, kako bo izgledal sistem Zemlja / Luna? Vodilna izboklina Zemlje tudi dodatno vleče Luno v svoji orbiti, kar daje rahel pospešek vzdolž orbite in poveča njeno orbitalno hitrost. To pomeni, da se Luna počasi umika od Zemlje.

Kviz predavanj # 1

Vzdušje je plast plina, ki obdaja nekatere planete. Kot morda že veste, segrevanje plina povzroči njegovo širjenje (tlak se poveča). Ogromen planet, kot je Jupiter, ima tako močno gravitacijo, da lahko zadrži pline in ne morejo pobegniti v vesolje. Zemlja se lahko drži težjih plinov, kot sta kisik in ogljikov dioksid, ne pa tudi najlažjih plinov, kot sta vodik in helij. Vsako telo ima hitrost pobega , to je, kako hitro se mora objekt premakniti, da pobegne. Hitrost pobega na zemeljskem površju je 11,2 km / s. Ko se plin segreje, se njegovi delci premikajo hitreje, zato je, ali se bo plin zadrževal v zemeljski atmosferi, odvisno od tega, kako vroče je in ali se molekule premikajo hitreje od hitrosti uhajanja. Toda vse molekule plinov se ne premikajo z enako hitrostjo. Govorimo o povprečni hitrosti, ki je odvisna od temperature, a tudi takrat, ko se večina molekul giblje z nižjo povprečno hitrostjo, bo nekaj najhitrejših še vedno preseglo hitrost pobega in odneslo v vesolje. Bližje kot je povprečna hitrost hitrosti pobega, več molekul se izgubi in hitreje bo atmosfera ušla.

Luna

Ali ima Luna ozračje? Da bi to ugotovili, primerjamo hitrost pobega Lune s hitrostjo delcev plina, ki tvorijo njeno atmosfero. Če se delci plina ne gibljejo veliko (faktor 10) počasneje od hitrosti uhajanja, bodo delci sčasoma pronicali iz ozračja. Lunina hitrost pobega je le 2,32 km / s, hitrost kisika (0 2 ) na površini Lune je 0,78 km / s. S o hitrost ubeža je le 3-krat večja od povprečne hitrosti. Luna bo izgubila ves kisik, ki bi lahko nastal, že po nekaj sto letih.

Vsi atomi, ki se zadržujejo okoli Lune in nastanejo zaradi izpuščanja ali razmikanja kamnin, trajajo le kratek čas in jih je treba nenehno obnavljati. Lunino ozračje je neverjetno dober vakuum, le 10 - 14 atm.

Zemlja

Ko je bila Zemlja prvič oblikovana, bi bilo njeno ozračje večinoma H in He, vendar ga je izgubilo zaradi hitrosti teh delcev, ki jim je sčasoma uspelo pobegniti. Novo, težje vzdušje H 2 O, o 2 , N 2 in CO 2 je bil izpuščen iz vulkanizma ali pa so ga sem pripeljali kometi.

Tlak zemeljske atmosfere (in vseh atmosfer) pada z višino, zato je daleč večina ozračja na najnižjih višinah. Tudi temperatura pada z višino, vsaj blizu površine. Zato se tako ohladi in na vrhu gore je tako malo zraka. To isto splošno vedenje velja za vsa ozračja, tudi ozračja zvezd! A opazite, kaj se dogaja na spodnji sliki, ki prikazuje ploskev spremembe temperature z višino v zemeljski atmosferi.

Struktura atmosferske temperature:

Zakaj se temperatura ozračja spusti na približno 10 km, nato pa začne spet naraščati v stratosferi? To je posledica absorpcije ultravijolične (UV) svetlobe s Sonca, ki v tej atmosferi odloži energijo in jo segreje. To območje se imenuje stratosfera, ker je stabilno za gibanje navzgor (inverzijska plast), kar pomeni, da se oblaki ne dvigajo v stolpcih, temveč se razprostirajo v tankih plasteh, kot sloji. Območje največjega segrevanja z ultravijolično svetlobo (Stratopavza) je tudi lokacija ozonske plasti O 3, ki je v veliki meri odgovorna za absorpcijo UV in zaščito pred škodljivim sevanjem.

Do Mezopavze temperatura spet upada, nato pa se v termosferi spet dvigne zaradi absorpcije rentgenskih žarkov s Sonca. Ta "atmosfera" je le malo pod višino satelitov, ki krožijo okoli Zemlje, kot je Space Shuttle, ki kroži približno 200 km navzgor, in ta del ozračja je tako tanek, da je skoraj popoln vakuum.

Zemeljska atmosfera je približno 1/5 O 2 in 4/5 N 2 , s sledovi drugih plinov, kot je ogljikov dioksid (CO 2 ) in vodo (H 2 O). Vendar pa je količina CO 2 se je v zadnjih 200 letih znatno povečal, deloma kot posledica človekove dejavnosti (kurjenje fosilnih goriv itd.). Ogljikov dioksid je a toplogrednih plinov , tako imenovani, ker deluje kot rastlinjak pri ohranjanju Zemlje na toplem. Kako deluje učinek tople grede? Sončno sevanje prehaja skozi ozračje v vidnem območju spektra in segreva tla, ki nato izžareva energijo v vesolje, ampak zdaj v infrardeči povezavi del spektra. Toplogredni plini blokirajo uhajanje toplote z absorpcijo infrardečega sevanja. Posledično obstajajo zaskrbljujoči znaki, da se podnebje na Zemlji segreva. Zemeljsko podnebje je sčasoma postajalo hladnejše in toplejše v presledkih, kar je povzročalo ledeniška in medledeniška obdobja, zato še ni mogoče ugotoviti, ali so za ogrevanje podnebja krivi predvsem ljudje. Kljub temu pa mi lahko pomagajo pri obvladovanju problema z zmanjšanjem emisij toplogrednih plinov.

Vprašanje predavanja št

Zemlja je tisti planet, ki ga lahko zelo podrobno preučujemo. Veliko vemo o zgradbi notranjosti Zemlje, vendar se moramo zavedati, da se lahko drugi planeti v osnovi razlikujejo. We must take what we learn about Earth and compare and contrast it with the other planets.

The mean ali bulk density of a planet is an easily measured quantity that can tell us a great deal about what the planet is made of. Earth's bulk density is 5520 kg/m 3 (compare with the density of water, 1000 kg/m 3 ). But the density of the Earth's surface (density of silicate rocks) is only 2800 kg/m 3 , so the interior must be much more dense than the surface. The structure of the interior of the Earth has been pieced together from a number of clues, such as what the surface is made of, what we see coming to the surface in volcanoes, and most of all what we learn from earthquakes .

The earthquakes, adjustments of the Earth's crust due to internal stresses, launch two types of seismic waves, longitudinal compression (P) waves (sound waves), and transverse distortion (S) waves . The P waves travel faster, and so the letter P could stand for Pressure or Prompt waves. The slower S waves arrive later, and so the letter S could stand for Secondary, or Slow waves. An important difference between longitudinal and transverse waves is that longitudinal waves can travel through liquid, but transverse waves cannot. Both kinds of waves refract due to density gradients in the Earth's interior. Due to the refraction of both kinds of waves, and the lack of propagation of S waves, we learn that the interior has a liquid layer, and also can measure such quantities as temperature and density as a function of depth. See this web page to see some drawings of the interior of the Earth. If you ever wondered how Earthquake epicenters and magnitudes are determined, take this short virtual earthquake lesson.

The Moon's interior is quite different from the Earth's. From seismic experiments left from the Moon landings, we know that t he Moon appears to be made entirely of the crustal material of the same sort as Earth's surface. It is strange that it does not have metals in its core. It is thought that late in the formation of the Earth, a giant impactor (planetesimal) struck the Earth. Such a collision would have destroyed the impactor, the metals of the impactor would sink to the center of the Earth, and much of the outer crust of the Earth could have been torn off to later come together to form the Moon. This may explain the unusually thin crust of the Earth's oceans. Without the impact, the Earth might not have plate tectonics, as we discuss in the next section.

Lecture Quiz #3


Is there a timekeeping word for the orbit of a moon? - astronomija

What's the date today? If you're not sure, what do you do? Easy. Look on the calendar. Ampak which calendar do you use?

The Gregorian calendar
If your year begins on January 1, and ends twelve months later on December 31, then you're using the Gregorian calendar. It was named after Pope Gregory VIII (1502-1585) who effected calendar reform in the 16th century. After initial resistance, even the Protestant countries in Europe adopted the new calendar, though the whole process took about 150 years. Today it's the most widely-used calendar in the world. Even countries with their own calendars use it for business purposes.

The Gregorian calendar is a solar calendar, as are, for example, the Baha’i, Hindu and Iranian calendars. Solar calendars have years of 365 days and are related to the changing position of the Earth in its journey around the Sun.

What is a day?
Having said that a year usually has 365 days, what is a day?

Following the ancient Egyptian custom, a day is 24 hours - the time it takes Earth to turn once on its axis. But when does the day begin? It starts at sunrise in the Hindu calendar and at sunset in the Jewish and Islamic calendars. Yet our days now start in the middle of the night, just after midnight local time. Faster travel and communications made it necessary to standardize civil time internationally.

Leap years
Unfortunately for timekeeping, Earth's orbit of the Sun isn't related to its rotation on its axis. Therefore you can't get an even number of days by dividing the year by 365. It comes out close to 365 and a quarter days. The astronomer Sosigenes (first century BCE), who advised Julius Caesar on the Julian calendar, solved this problem by adding a day every four years. Solar calendars continue to use the device of the leap year.

Leap years are a good idea, but Sosigenes's proposal wasn't a complete solution. Earth doesn't actually take 365 days plus six hours to orbit the Sun. It takes 365 days plus five hours and nearly 49 minutes. The eleven-minute difference sounds trivial unless you start adding it up over many centuries.

Problems with the Julian calendar and the seasons
In fact, by the sixteenth century, the calendar was ten days out of synch with the seasons. Thus the equinoxes and solstices were occurring ten days after the traditional calendar date. This is why the pope was concerned. Many church observances are related to the date of Easter, which itself is related to the March equinox. (Easter is the first Sunday following the first full moon on or after the equinox.)

The Gregorian calendar makes use of leap years, but not every four years. In order to stop accumulating extra days, if the end of a century is divisible by 400, there is no leap year. The year 2000 was not a leap year, but 1900, 1800 and 1700 were.

Months
Months are a relic of lunar calendars. The root of the word month is moon. Many calendars were originally based on the phases of the Moon, as this is an easily-observed and regular set of changes. It takes 29 and a half days for the Moon to go once around the Earth and return to the same position, and therefore the same phase. This is known as a lunar month or a lunation.

However, the Moon's orbit of the Earth is independent of Earth's orbit of the Sun, so the number of days in a year doesn't divide into an even number of lunar months. There are twelve lunations with eleven days left over.

The Islamic calendar is a purely lunar calendar and the calendar months alternate in length between 29 and 30 days. It's also linked to the Moon's phases. This does mean that religious observances move through the seasons as the years go on, unless the calendar is adjusted.

The Gregorian calendar has twelve months completely unrelated to Moon's phases and varying in length from 28 to 31 days. However lunisolar calendars, such as the Chinese calendar, incorporate information on both the position of the Earth in its orbit and the Moon's phases.

The week
There is one more calendar unit: the week. The ancient Greeks had ten-day weeks, but seven days was the convention in the Middle East. The Greeks and the Romans named the days of the week for planets, which themselves were named for gods. Although many European countries have day names based on the Latin ones, English has followed the Germanic tradition and named days of the week after Norse gods.

We tend to take a calendar for granted, but it is quite an exciting object when you think that behind it lie the traditions of many cultures and nearly three thousand years of history.

References:
(1) "The Calendar" https://www.rmg.co.uk/discover/explore/time/seasons-calendars
(2) "The Gregorian Calendar" https://galileo.rice.edu/chron/gregorian.html

Content copyright © 2021 by Mona Evans. All rights reserved.
This content was written by Mona Evans. If you wish to use this content in any manner, you need written permission. Contact Mona Evans for details.


Astronomy Science Project

Showing Moon Phases

  • An orange (or a Styrofoam ball of a size similar to an orange)
  • A pencil
  • A desk lamp (or any lamp with a removable shade)
  • A room that can easily be made dark
  • An adult’s help
  1. Get an adult to help you push the sharp end of a pencil halfway through the orange push it far enough to keep it stable when you hold the unsharpened end.
  2. Find a room that you can make dark by turning off the lights and closing shades. If you can’t make it dark enough, do the experiment when it is dark outside or use blankets to cover windows.
  3. Set the lamp on a table or dresser so it is about the same level as your head when you’re standing. Turn the lamp on and remove the shade or turn the lamp so that the bulb is facing toward you (if you’re using a desk lamp).
  4. Stand about 3 feet in front of the lamp and hold the pencil with the orange attached to it out at arm’s length. The orange should be between you and the lamp. For this activity, you represent Earth, the lamp is the sun, and the orange is the moon.
  5. To see the moon’s phases, slowly turn your whole body to the left, keeping your arm straight out in front of you with the orange at eye level. This is how the moon orbits the Earth. Keep turning in the same direction until you have gone in a full circle and are facing the lamp again. Keep your eyes on the orange and watch the shadows on it very carefully to see the phases of the moon as we see them from Earth.

It takes around 29 days for the moon to orbit the Earth once and the same amount of time for the moon to spin around one complete time on its axis. That means that we always see the same side of the Moon! However, we do see the moon changing as it goes through its phases.

While facing the lamp (sun), the surface of the orange (moon) facing you (Earth) was dark, even though the other half of the orange, facing toward the lamp was bright. This is the first phase of the moon, called new moon. We can’t see the moon at all during this phase!

As you began to turn away from the lamp, a shadow still covered most of the orange, but you probably saw a small crescent shape of light on the right side of the orange. This phase is called waxing crescent.

The next phase is called the first quarter: the light (sun) shone on the half of the orange (moon) facing it. From Earth, we see half of the light side and half of the dark side during this phase so sometimes it is called a “half moon.”

As you continued to turn to the left, the light shone on more of the side of the orange you could see, lighting up all of the orange except for a small crescent. This is the waxing gibbous phase.

Once you had turned halfway around so that the lamp was directly behind you, the light (sun) shone directly on the orange (moon) making the whole side facing you bright. This is a full moon. During a full moon, the side facing away from Earth is dark. This phase is the exact opposite of new moon.

(Note: if the orange isn’t fully illuminated, try moving your head or shoulders so you aren’t blocking the lamp. If you are blocking it, you’ve created a lunar eclipse – which happens when the Earth blocks the sun’s light from hitting the moon. Normally, the moon is just above or just below Earth so an eclipse doesn’t happen every time there is a full moon.)

At this point, the amount of the light side of the moon that we can see begins to decrease, or wane. The next phase is called waning gibbous. Most of the moon is still light during this phase.

Next is the last quarter (also called third quarter) where only half of the illuminated side of the moon is visible. This phase is opposite of first quarter. Notice that your back is facing toward the direction you were facing when you saw the first quarter phase!

The last visible phase is the waning crescent, where only a sliver of light is visible. This phase is opposite the waxing crescent. After this, you will be facing toward the lamp (sun) again, and the orange (moon) will be back to the new moon phase!

If you’re having difficulty remembering the difference between waxing and waning moon phases, these rhymes might help:

Waxing: “Moon on the right, getting bigger every night.” (Leading to a full moon.)
Waning: “When the moon is waning, it is fading to the left until there’s no moon remaining.” (Leading to a new moon.)

Rhymes taken from this article. Project adapted from this article.


Is March’s full moon a supermoon?

A full supermoon moon rising on December 3, 2017, as captured by Peter Lowenstein in Mutare, Zimbabwe. Supermoons don’t appear noticeably larger than other full moons, but they do appear noticeably brighter! Thank you, Peter!

The crest of this month’s full moon falls on March 28, 2021 at 18:48 UTC (2:48 p.m. Eastern translate UTC to your time). This full moon comes just two days before the March 30 lunar perigee, when the moon is swinging into the near part of its orbit with respect to Earth. This month’s full moon will be the 4th-closest (and therefore 4th-largest and 4th-brightest) of the 12 full moons of 2021. Should it be dubbed a supermoon? Some experts say yes, and others no … here’s why.

First know that the word supermoon has arisen in popular culture. There’s no official definition for the term. The International Astronomical Union (IAU) is the group generally recognized for naming and defining things in astronomy. But the IAU has been, so far, silent on the subject of supermoons, which professional astronomers might prefer to call perigean full moons.

Here are three different sources of supermoon info in 2021.

First, the excellent website TimeandDate.com says:

There are no official rules as to how close or far the moon must be to qualify as a supermoon or a micro moon. Different outlets use different definitions. Due to this, a full moon classified as a supermoon by one source may not qualify as a super full moon by another.

TimeandDate goes on to give its own definition of a supermoon:

A full or new moon that occurs when the center of the moon is less than 360,000 kilometers (ca. 223,694 miles) from the center of Earth.

By TimeandDate’s defintion, only the full moons of April and May count as full supermoons in 2021.

Our second source is Fred Espenak, the go-to astronomer on all things related to lunar and solar eclipses. He lists the full moon of March 28, 2021, as a supermoon in his post Full Moon at Perigee. You’ll also find a table in that post, showing his list of supermoons for the 21st century. Fred Espenak lists four full supermoons for 2021:

2021 Mar 28
2021 Apr 27
2021 May 26
2021 Jun 24

Now here’s a third source: the astrologer Richard Nolle. Whatever your thoughts or feelings about astrology may be, Nolle is, after all, the person who coined the term supermoon. On his supermoon list for the 21st century. Richard Nolle’s list agrees with TimeandDate.com that there are only two full moon supermoons for 2021:

Why are the various lists different? It all goes back to the definition of the word supermoon.

Here’s one thing we all can agree on. Supermoons are based on lunar perigee in apogee. Each month, the moon comes closest to Earth at perigee and swings farthest away at apogee.

In his original definition, Richard Nolle defined a supermoon as:

… a new or full moon which occurs with the moon at or near (within 90% of) its closest approach to Earth in a given orbit.

If a new or full moon aligns with apogee, then it’s at 0% of its closest approach to Earth. On the other hand, if a new or full moon aligns with perigee, then it’s at 100% of its closest approach to Earth.

Although we can all agree on that, the phrase 90% of perigee is ambiguous. Read on.

A 2013 supermoon, as captured by EarthSky Facebook friend Anthony Lynch in Dublin, Ireland.

Nolle’s 90% is based on 2021’s closest perigee and farthest apogee. Looking at his list for all the supermoons in the 21st century, it appears that Nolle might base his 90% figure on the year’s closest perigee and farthest apogee. Let’s take the year 2021. Based on the year’s closest perigee and farthest apogee, any new or full moon coming closer than 224,865 miles (361,885 km) would qualify as a supermoon.

Here are the distances of the four closest full moons in 2021:

Full moon (March 28, 2021): 225,042 miles or 362,170 km
Full moon (April 27, 2021): 222,212 miles or 357,615 km
Full moon (May 26, 2021): 221,851 miles or 357,462 km
Full moon (June 24, 2021): 224,652 miles or 361,558 km

This year, in 2021, the moon swings farthest away from Earth on May 11 (252,595 miles or 406,512 km), and then sweeps closest to Earth on December 4 (221,702 miles or 356,794 km). That’s a difference of 30,935 miles or 49,785 km. Ninety percent of the difference corresponds to 27,842 miles or 44,807 km. Presumably, any new or full moon coming closer than 224,791 miles (361,766 km) would be “at or near (within 90% of) its closest approach to Earth.”

Farthest apogee (2021): 252,595 miles (406,512 km)
Closest perigee (2021): 221,702 miles (356,794 km)
Difference (2021): 30,893 miles (49,718 km)

90% x 30,893 miles (49,718 km) = 27,804 miles (44,746 km)

90% of moon’s closest distance to Earth = 252,595 miles (406,512 km) – 27,804 miles (44,746 km) = 224,791 miles (361,766 km)

Thus, figuring out 󈭊% of the moon’s closest approach to Earth” by the year’s closest perigee and farthest apogee, any new or full moon coming closer than 224,791 miles (361,766 km) to Earth, as measured from the centers of the Earth and moon, counts as a supermoon in 2021.

Since the full moon on March 28, 2021, only comes to within 225,042 miles (362,170 km) of Earth, it doesn’t count as a supermoon on Richard Nolle’s list. But we’re not quite sure why the full moon of June 24, 2021, didn’t make his list.

July 2014 supermoon. Image via Evgeny Yorobe Photography.

Espenak’s 90% based on perigee and apogee of each month’s orbit. Ironically, Fred Espenak’s full supermoon list might more strictly adhere to Nolle’s definition (at least as it is written) than Nolle himself does.

Once again, Nolle describes a supermoon as:

… a new or full moon which occurs with the moon at or near (within 90% of) its closest approach to Earth in a given orbit.

If a “given orbit” can be taken to mean current monthly orbit, then the March full moon comes to within 95.9% of its closest approach to Earth relative to the most recent apogee and the upcoming perigee.

March 18, 2021 apogee: 252,434 miles (405,253 km)
March 30, 2021 perigee: 223,886 miles (360,309 km)
Difference: 28,548 miles (44,944 km)

March 18, 2021 apogee: 252,434 miles (405,253 km)
March 28, 2021 full moon: 225,042 miles (362,170 km)
Difference: 27,392 miles (43,083 km)

27,392/28,548 = 0.959 (95.9%) = distance of the March 2021 full moon relative to the most recent apogee and upcoming perigee.

Depending on what meaning we give to the words in a given orbit, we could say the March 18 apogee = 0% of the moon’s closest approach to Earth for this orbit, and the March 30 perigee = 100% of the moon’s closest approach to Earth.

That being the case, then the March full moon comes to within 95.5% of its closest approach to Earth for the month.

Super cool super-moonrise composite captured by Fiona M. Donnelly in Ontario, during the August 2014 supermoon.

March full moon’s distance relative to 2021’s closest perigee/farthest apogee. However, if we compute the percentage distance of the March full moon relative to the year’s farthest apogee and closest perigee, then the March full moon only comes to within 88.8% of its closest approach to Earth:

Farthest apogee (2021): 252,595 miles (406,512 km)
Closest perigee (2021): 221,702 miles (356,794 km)
Difference: 30,893 miles (49,718 km)

Farthest apogee (2021): 252,595 miles (406,512 km)
March full moon (2021): 225,042 miles (362,170 km)
Difference (2020): 27,553 miles (44,342 km)

27,553/30,893 = 0.892 (89.2%) = distance of the March full moon relative to the year’s farthest apogee and closest perigee.

Another contrast of a full supermoon (full moon at perigee) with a micro-moon (full moon at apogee). Image via Stefano Sciarpetti/ APOD.

Is the March full moon a supermoon? Depends on which perigee/apogee distances you choose. The moon’s perigee and apogee distances vary throughout the year, so it appears that the limiting distance for the supermoon depends on which perigee and apogee distances are being used to compute 90% of the moon’s closest approach to Earth.

If we choose the year’s closest perigee and farthest apogee, as Nolle did, we narrow the definition of supermoon.

If we choose the perigee and apogee for a given monthly orbit, as Espenak did, then we broaden the definition of supermoon.

Given the narrower definition, the full moon on March 28, 2021, is not a supermoon, but given the broader one, it is.

The moon’s apparent size in our sky depends on its distance from Earth. The supermoon of March 19, 2011 (right), compared to an average moon of December 20, 2010 (left). Image via Marco Langbroek/ Wikimedia Commons.


Astronomy and Space Quiz Part 2

38. One rotation of the moon takes about:

39. Mare means sea, but are found on rocky planets and the moon. True or False?

40. Craters on the moon&rsquos surface are formed by:

41. Approximately how many high and low tides are there in a period of 24 hours?

42. Centaurs are half asteroid-half comet objects in orbits between Jupiter and Neptune. True or False?

43. In what order of alignment are the sun, the earth and the moon in a solar eclipse?

44. To find an object in the sky, which two coordinates are needed?

Answer: Altitude and azimuth.

45. The tilt of the earth on its axis causes:

Answer: Day and night to be of different lengths in different parts of the world.

46. ____ can be described as large dirty, icy snowballs.

47. The constellations that travel directly overhead in the same path as the sun are the:

Answer: Zodiac constellations.

48. The number of degrees that a star is positioned above the horizon is its:

49. The colour of the coolest stars is:

50. The most likely final form of our sun in its life cycle is a:

51. The light and heat generated by stars is the result of:

Answer: The angle of sunlight at different parts of the world.

53. A lunar eclipse occurs when people on earth cannot see the:

54. The tides which cause the most damages to our beaches and occur at full and new moon phases are:


Thread: Moon in orbit

What forces are acting on the moon to keep it in the orbit around the Earth?

The well known one is "earth's gravity on moon minus moon's gravity on earth"

The gravitational force between the Earth-Moon System, which tries to pull the moon towards the former, as it orbits about the Earth is known as Centripetal force. This force is balanced by Centrifugal force, which pulls on the Earth keeping the moon in motion. The balance between Centripetal and Centrifugal force are what keeps the Moon orbiting the Earth.
This reasoning provides an understanding for how the Moon stays in orbit around the Earth. However you could also look at Einstein's Theory of GR to explain zakaj the Moon orbits the Earth in the way it does. GR provides that objects with mass curve the spacetime within their vicinity and it is this curvature which influences the motions of other objects. The greater the objects mass and density, the larger the curvature of spacetime will be. It follows therefore that the Moon orbits the Earth because of the Earth's curvature of spacetime within the vicinity of the Moon. This relationship between mass and curvature cause the gravitational and Centripetal forces to exist, causing the Moon to orbit Earth.

That's the force that the Zemlja 'feels'. The original question was about forces acting on the Luna and those are, as already mentioned, the centripetal and the centrifugal forces. From the mechanics point of view it's the same as if you would start spinning around holding a string with a rock attached to the other end. The only differance is that with gravity you don't see the agent keeping the two objects together. Hope this clarifies a bit.

Keep in mind that the Newtonian way of looking at this is that there is only one force involved: gravity.

Of course, force = mass times acceleration. So the single force (gravity) is accelerating the moon*.

The acceleration of the moon does not change its speed instead, the acceleration changes the direction of the moon's travel. Thus the moon travels in an approximate circle around the earth instead of flying off.

Good thing too, or we wouldn't have all of these Moon Hoax threads around here.

*Of course, gravity is accelerating the Earth toward the moon, also, but we're just talking about the moon so far in this thread.

I think you're asking for an example, rather than an analogy, maybe?

The point of the analogy is that the marble is not really attracted to the cannonball by the cannonball (at least, to the extent that it is), but is a result of the configuration of the path. But what you're modeling is the curvature of spacetime, and it's pretty hard to model curved time without actually doing it.

The gravitational force between the Earth-Moon System, which tries to pull the moon towards the former, as it orbits about the Earth is known as Centripetal force. This force is balanced by Centrifugal force, which pulls on the Earth keeping the moon in motion. The balance between Centripetal and Centrifugal force are what keeps the Moon orbiting the Earth.

Centrifugal force doesn't exist. It is a mathematical result and in only found inside an accelerating frame of referance. ie, if you are sitting in a car as it rounds a corner, you experinece a "force" on you making you press up against the door of the car. In reality though the force is being applied to the car making it turn (accelerate towards the centre of the bend), you are attempting to travel in a straight line and the car door intercepts you and applies a force on you to change your direction (accelerate you) to match its own new velocity. (remember acceleration is a change in velocity, and velocity has two components, magnitude (speed) and direction. A change of direction without a change in magnitude is still an acceleration.)

Newton noted that "Any body set in motion will continue in that motion untill such time another foce acts upon it."

If we apply this to the moon, its forward motion is unchanged as it orbits (the magnitude portion of its velocity does change due to Kepler's Laws of Orbital Motion and rotational inertia.) It just wants to travel in a straight line at a constant speed, and would do so if there was no other force acting on it. Now we add Gravity. This will accelerate the moon directly towards the centre of mass of the Earth-Moon system, in other words, the gravity well acts like a force attempting to push (or pull) the moon to the centre of mass. However this is always occuring at an angle to the direction of motion of the moon, so what it does is change the veleocity of the moon by changing its direction rather than its magnitude (the magnitude will change slightly throughout the orbit due to the moon's orbit not being a perfect circle.)

This shows us that no other force is required to keep it in orbit, just that applied by gravity. There is no "balancing of Centipedal and Centifugal forces keeping the moon in position," there is just gravity acting on an object in motion by drawing the moon towards a centre point, and the moon missing it because it is moving forwards too fast.


Is there a timekeeping word for the orbit of a moon? - astronomija

Differential (Tidal) Forces, Precession and Nutation

Differential Gravitational Forces

  • Rings of Saturn
  • Volcanoes of Io
  • Earth ocean tides
  • The Moon keeping the same face toward Earth
  • The breakup of the comet Shoemaker-Levy 9 that crashed into Jupiter and crater chain on Ganymede.
  • The resonance between Saturn's moons, Titan and Hyperion
  • Accretion disks around black holes

We can expand the terms in rounded parentheses using the binomial expansion

to get a final expression for the difference in force from one side of a body to the other:

The minus sign means that the force is less on the more distant side. This expression is valid only for the two special points on either side of the body on the line joining the two bodies. In the text, a more general approach is used to get an expression for anywhere within the body. These differences in the force experienced within a body lead to tidal bulges , as shown in Figure 2, below.


Figure 2: Differential (tidal) forces on a body relative to the primary (left), and relative to its
own center (right). The forces relative to its center stretch the body along the line joining the
body and the primary, and compress the body along the perpendicular directions, to form a
football shape (prolate spheroid).

The figure on the left shows the forces relative to the Sun, and the figure on the right (obtained by subtracting the central force vector on the left from all of them) shows the forces relative to the center of the body. These relative forces tend to stretch the body laterally, and compress the body in the perpendicular direction, to form a football shape.

Both the Moon and the Sun exert tidal forces on the Earth. Let's calculate the relative magnitudes of those tidal forces. We will call the force due to the Moon D F Luna , and the force from the Sun D F Sonce . The ratio is not going to depend on R , the radius of the Earth, or on m , the mass element within the Earth, but will depend on M , the mass of the primary, since it is a different primary in the two cases. The ratio is:

Because the oceans, being liquid, are easily deformable, the most obvious response to these tidal forces is the ocean tides. As the Earth rotates, the continents pass through these tidal bulges once a day, causing the diurnal tides every 12 hours. When the Sun and the Moon line up (near new or full Moon), the forces add together and cause very high spring tides (the word spring is not related to the season!). When the Sun is 90 degrees from the Moon (near first and third quarter), the high and low tides are not as great--these are called neap tides.

  • What time of year should the very highest tides occur?
  • During some years, this highest tide is higher than others. Zakaj?
The ocean tides are ne the only effect of these tidal forces. The solid body of the Earth also bulges slightly in this way. The daily flexing of the Earth (both solid body and sloshing of the oceans) cause loss of energy of the Earth's rotation, due to friction. This energy goes into heat, increasing the Earth's internal temperature. The loss of rotational energy means that the Earth is slowing down in its rotation rate, currently by about 0.002 seconds per century.

As you might imagine, the Earth also exerts tidal forces on the Moon. In fact, the tidal forces of Earth on the Moon are about M Zemlja R Luna /M Luna R Zemlja

20 times larger than those from the Moon on the Earth. Note what happens when a rotating body is tidally distorted. The line of distortion is continually being rotated away from the line between the two bodies, causing the bulges to lead slightly. There is then a net torque opposing the direction of rotation, thus slowing down both bodies. This torque exists until the slowing rotation causes the body's orbital period to equal its rotational period. Once this happens, the body is said to be tidally locked , and the torque and dissipation by tidal forces ceases. At this moment in time, the Moon is tidally locked with the Earth, but the Earth is not tidally locked with the Moon. That is why the Moon keeps the same face to the Earth. In the distant future, the slowing Earth will eventually become tidally locked with the Moon, and no further evolution of the system will occur.

When this occurs, what will the Earth/Moon system look like? It is interesting to note that the leading bulge of the Earth also exerts an extra pull on the Moon in its orbit, giving a slight acceleration along the orbit, and therefore an increase to its orbital velocity, v q . This means the Moon's orbital angular momentum L = mrv q increases with time. In a beautiful confirmation of the law of conservation of angular momentum, we know that this has to come from somewhere else in the system. In fact, the rotational angular momentum lost by the Earth through this tidal interaction is exactly the orbital angular momentum gained by the Moon!

Do we expect the Moon then to come closer to Earth, or move farther away? We can answer this by comparing velocities in different orbits given by Kepler's third law (for a circular orbit), P 2 = kr 3 . The period is related to the orbital velocity and circumference of the orbit by v = 2 str r/P = 2 str r/kr 3/2

r - 1/2 , so the angular momentum is proportional to vr

  • The tidal forces of Earth on the Moon slow down the rotation of the Moon (while speeding up the rotation of the Earth).
  • The Moon eventually keeps the same face toward the Earth, becoming tidally locked.
  • The tidal forces of the Moon on the Earth slow down the rotation of the Earth, while speeding up the orbital motion of the Moon.
  • The Moon spirals away from the Earth, increasing its angular momentum, compensating for the lost angular momentum of the Earth rotation.
  • The Earth eventually keeps the same face toward the Moon, becoming tidally locked.
  • At this point, the system stops evolving and remains in this configuration forever (except as influenced by external forces).
We said before that the Earth is slightly oblate because of its rotation, and the resulting centrifugal force causing a change in shape of the rotating Earth. Because of the tilt of the Earth's rotation axis, this bulge is tilted relative to direction of forces from the Sun. The differential force of the Sun on one side of this bulge relative to the other side is such that the Earth is being pulling in the direction to decrease the tilt angle. Because the Earth is rotating, however, such a torque is not successful in righting the Earth, but rather causes a change in angular momentum perpendicular to the spin axis. This torque causes the Earth to precess, just as a leaning top would. This is just the precession we learned about in the previous lecture. The period of precession of the Earth is 26,000 years, and causes the direction of the pole to change in the sky, as well as causing the crossing point of the ecliptic and the celestial equator to move westward by about 50" per year.

Because the Moon's orbit is tilted slightly (about 5 degrees) from the ecliptic, and of course it orbits once per roughly 28 days, the direction and magnitude of the net torque on the Earth due to the Sun and Moon changes on monthly and yearly time scales. This causes a slight nodding of the axis on these time scales, so that the precession motion is not a smooth circle in the sky, but is a wiggly circle. This nodding of axis is called nutation . In the next lecture we will learn more about the Moon's orbital motion.

The differential gravity forces on a body, shown in Figure 2, stretch the body along a line between the body and the primary. This is due to the gravity gradient , which we can see is proportional to 1/d 3 , where d is the distance between the bodies. It follows that if a body approaches the primary too closely, the difference in force across the body's diameter can be greater than the forces holding the body together. When this occurs, the body is literally torn apart. For large bodies ( R > 500 km ), gravitation dominates all cohesive forces. For smaller bodies (e.g. a comet), the tensor strength of the material making up the body provides the dominant force.

For such larger bodies, Edouard Roche showed that a satellite will be torn apart by gravitational forces if it approaches the primary closer than a distance d = 2.44 ( r M / r m ) 1/3 R ,

Let's calculate the Roche Limit for an icy body ( r m

1 200 kg/m 3 ) around Saturn. From Table A3-3, the radius of Saturn is R S = 60,000 km , and the average density of Saturn is r M


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