Sterrekunde

Is daar 'n tydwoord vir die baan van 'n maan?

Is daar 'n tydwoord vir die baan van 'n maan?


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Vir 'n planeet kan ons die rotasietydperk as 'n 'dag' en sy rewolusietydperk rondom sy ouerster as 'n 'jaar' noem. Sommige wêrelde het unieke terme, soos dat Marsdae "sols" genoem word, maar die beginsel is dieselfde: die een woord om die rotasie om sy as, die ander vir die omwenteling rondom die ster.

Is daar 'n ekwivalente woord vir die periode van 'n maan se rewolusie rondom 'n planeet? Op Aarde is dit ongeveer 'n 'maand', maar multimaanplanete het verskillende tydperke vir elke maan.

As ek die tydsberekening gehad het vir 'n sending na Europa of Ganymedes, watter term sou ek gebruik om te verwys na die tydsduur vir die maan om een ​​wentelbaan van Jupiter te voltooi?


U moet een van die presies omskrewe terme van spesifieke maatreëls van die wentelperiode gebruik, wat op hierdie Wikipedia-bladsy beskryf word.

U kan wegkom met net 'n orbitale periode as u dit nie gebruik in 'n konteks wat presies vereis nie.

Vir die lengte van die dag is daar 'n sinodiese dag sowel as 'n aardsperiode. Daar is 'n bespreking oor die verskil hierin.


As u by stephenG se antwoord voeg, kan die verwante Wikipedia-bladsy Maanmaand gebruik word om 'n idee te kry van wat die verskillende terme beteken.

Ek moet wel saamstem dat 'n mens waarskynlik moet probeer om weg te kom deur net die woord maand te gebruik terwyl jy die persoon met wie jy praat, aflei (hoes of iets laat val) en dan vinnig aanbeweeg, want enige term wat meer presies is, het 'n baie spesifieke definisie, en as iemand sê: "Wag, bedoel jy nie a nie blah blah blah maand? "sal jy moet ophou om uit te vind of jy dit doen of nie!

soort raam - of - kriteria Aarde se maan (dae) sideriese traagheid wrt sterre 27.321661 sinodies draaiend met Son-aarde 29.530588 drakoniese baanvlak (voorgangers) 27.212220 (of drakoniese) tyd tussen stygende knope (of nodale) anomalistiese tyd tussen periapses 27.554551 tropiese tyd tussen belyning van 27.321582 maan se as met die planeet-maan lyn

Tydperk (sterrekunde)

In die sterrekunde word 'n tydvak of verwysingstydperk is 'n tydstip wat as verwysingspunt gebruik word vir 'n astronomiese hoeveelheid wat wissel in tyd. Dit is nuttig vir die hemelkoördinate of orbitale elemente van 'n hemelliggaam, aangesien dit onderhewig is aan versteurings en mettertyd wissel. [1] Hierdie tydvariërende astronomiese groottes kan byvoorbeeld die gemiddelde lengte- of gemiddelde anomalie van 'n liggaam, die knooppunt van sy wentelbaan ten opsigte van 'n verwysingsvlak, die rigting van die apogee of aphelie van sy baan of die grootte insluit. van die hoofas van sy baan.

Die belangrikste gebruik van astronomiese hoeveelhede wat op hierdie manier gespesifiseer word, is om ander relevante bewegingsparameters te bereken om toekomstige posisies en snelhede te voorspel. Die toegepaste gereedskap van die dissiplines van die hemelmeganika of die subveld orbitale meganika (vir die voorspelling van baanpaaie en posisies vir liggame in beweging onder die gravitasie-effekte van ander liggame) kan gebruik word om 'n efemeris te genereer, 'n tabel met waardes wat die posisies en snelhede gee van astronomiese voorwerpe in die lug op 'n gegewe tydstip of tye.

Astronomiese hoeveelhede kan op verskillende maniere gespesifiseer word, byvoorbeeld as 'n polinoomfunksie van die tydsinterval, met 'n tydvak as 'n tydelike oorsprongspunt (dit is 'n algemene manier om 'n tydvak te gebruik). Alternatiewelik kan die tydvariërende astronomiese hoeveelheid uitgedruk word as 'n konstante, gelyk aan die maat wat dit in die tydvak gehad het, sodat die variasie oor tyd op 'n ander manier gespesifiseer kan word - byvoorbeeld deur 'n tabel, soos gewoonlik tydens die 17de en 18de eeu.

Die woord tydvak is dikwels op 'n ander manier in ouer sterrekundige literatuur gebruik, bv. gedurende die 18de eeu, in verband met sterrekundige tabelle. Op daardie tydstip was dit gebruiklik om nie as die "tydperke" die standaarddatum en -tyd van oorsprong vir tydvariërende astronomiese hoeveelhede aan te dui nie, maar eerder die waardes op daardie datum en tyd van daardie hoeveelhede wat self wissel. [2] In ooreenstemming met die alternatiewe historiese gebruik, sou 'n uitdrukking soos 'die korrigeer van die tydperke' verwys na die aanpassing, gewoonlik met 'n klein hoeveelheid, van die waardes van die astronomiese hoeveelhede in tabelvorm wat van toepassing is op 'n vaste standaarddatum en verwysingstyd (en nie, soos verwag kan word van die huidige gebruik, na 'n verandering van een datum en tyd van verwysing na 'n ander datum en tyd nie).


Inhoud

Die eienskappe van die baan wat in hierdie afdeling beskryf word, is benaderings. Die wentelbaan van die Maan om die Aarde het baie variasies (versteurings) as gevolg van die aantrekkingskrag van die son en planete, waarvan die studie (maanteorie) 'n lang geskiedenis het. [10]

Elliptiese vorm Wysig

Die baan van die Maan is 'n byna sirkelvormige ellips rondom die Aarde (die as- en halfas is onderskeidelik 384.400 km en 383.800 km: 'n verskil van slegs 0.16%). Die vergelyking van die ellips lewer 'n eksentrisiteit van 0,0549 en perigee- en apogee-afstande van onderskeidelik 362 600 km en 405 400 km ('n verskil van 12%).

Aangesien nader voorwerpe groter lyk, verander die maan se skynbare grootte namate dit na en van 'n waarnemer op die aarde beweeg. 'N Gebeurtenis wat' supermaan 'genoem word, vind plaas wanneer die volmaan die naaste aan die aarde (perigeum) is. Die grootste moontlike skynbare deursnee van die Maan is dieselfde 12% groter (as perigee versus apogee-afstande) as die kleinste is die skynbare oppervlakte 25% meer, en so ook die hoeveelheid lig wat dit na die aarde weerkaats.

Die variansie in die baanafstand van die maan stem ooreen met veranderinge in die tangensiële en hoeksnelheid, soos in die tweede wet van Kepler gesê. Die gemiddelde hoekbeweging relatief tot 'n denkbeeldige waarnemer by die Aard – Maan-barisent is 13.176 ° per dag na die ooste (J2000.0-tydperk).

Verlenging Wysig

Die verlenging van die maan is te eniger tyd sy hoekafstand oos van die son. By nuwe maan is dit nul en word gesê dat die maan in samewerking is. By volmaan is die verlenging 180 ° en word daar gesê dat dit in opposisie is. In albei gevalle is die maan sysigig, dit wil sê die son, maan en aarde is amper gelyk. As die verlenging 90 ° of 270 ° is, word gesê dat die maan in kwadratuur is.

Precession Edit

Die oriëntasie van die baan is nie vas in die ruimte nie, maar draai mettertyd. Hierdie baanpresessie word apsidale presessie genoem en is die rotasie van die baan van die Maan binne die baanvlak, dit wil sê die asse van die ellips verander van rigting. Die hoofas van die maanbaan - die langste deursnee van die baan, wat onderskeidelik die naaste en verste punte, die perigeum en die apogee, verbind - maak elke 8.85 Aardejare, of 3 232,6054 dae, 'n volledige omwenteling, aangesien dit stadig draai in dieselfde rigting as die Maan self (direkte beweging) - beteken voorgangers 360 ° ooswaarts. Die apsidale presessie van die Maan is anders as die nodale presessie van sy baanvlak en aksiale presessie van die maan self.

Helling Wysig

Die gemiddelde helling van die maanbaan tot die ekliptiese vlak is 5.145 °. Teoretiese oorwegings toon dat die huidige helling ten opsigte van die ekliptiese vlak ontstaan ​​het deur gety-evolusie vanaf 'n vroeëre aarde-baan met 'n redelike konstante helling relatief tot die aarde se ewenaar. [11] Dit sou 'n helling van hierdie vroeëre baan van ongeveer 10 ° na die ewenaar vereis om 'n huidige helling van 5 ° tot die ekliptika te lewer. Daar word vermoed dat die helling van die ewenaar oorspronklik naby nul was, maar dat dit tot 10 ° kon verhoog word deur die invloed van planeetdiere wat naby die maan beweeg terwyl dit na die aarde geval het. [12] As dit nie sou gebeur nie, sou die maan nou baie nader aan die verduistering lê en sou die verduistering baie meer gereeld voorkom. [13]

Die rotasie-as van die Maan is nie loodreg op sy wentelvlak nie, dus is die maan-ewenaar nie in die vlak van sy baan nie, maar is hy geneig met 'n konstante waarde van 6,688 ° (dit is die skuins). Soos deur Jacques Cassini in 1722 ontdek is, het die rotasie-as van die maan dieselfde tempo as sy wentelvlak, maar is 180 ° buite fase (sien Cassini se wette). Daarom is die hoek tussen die ekliptika en die maanekwator altyd 1.543 °, al is die rotasie-as van die maan nie vas ten opsigte van die sterre nie. [14]

Knope wysig

Die knope is punte waarop die maan se baan die ekliptika kruis. Die maan kruis elke 27.2122 dae dieselfde knoop, 'n interval wat die genoem word drakoniese maand of drakonitiese maand. Die knooppuntlyn, die kruising tussen die twee onderskeie vlakke, het 'n retrograde beweging: vir 'n waarnemer op die aarde draai dit weswaarts langs die ekliptika met 'n tydperk van 18,6 jaar of 19,3549 ° per jaar. As ons vanuit die hemelse noorde kyk, beweeg die knope kloksgewys om die Aarde, teenoor die Aarde se eie draai en sy omwenteling rondom die Son. 'N Verduistering van die maan of son kan plaasvind wanneer die knope ongeveer elke 173,3 dae met die son in lyn is. Die hellings van die maanbaan bepaal ook die verduistering van skaduwees wanneer die nodusse saamval met die volmaan en die nuwe maan wanneer die son, aarde en maan in drie dimensies in lyn is.

Dit beteken in werklikheid dat die 'tropiese jaar' op die maan net 347 dae lank is. Dit word die drakoniese jaar of verduisteringsjaar genoem. Die 'seisoene' op die maan pas in hierdie tydperk. Die son is ongeveer die helfte van hierdie drakoniese jaar noord van die maanekwator (maar hoogstens 1,543 °), en vir die ander helfte is dit suid van die maanekwator. Dit is duidelik dat die effek van hierdie seisoene gering is in vergelyking met die verskil tussen maan nag en maan dag. In plaas van gewone maandae en nagte van ongeveer 15 Aardae, sal die son 173 dae "op" wees, aangesien dit 'n 'onder' pool-sonsopkoms sal hê en die sonsondergang neem elke jaar 18 dae. Hier bo beteken dat die middel van die son bo die horison is. [15] Maanpoolopgange en -ondergange kom om die tyd van verduistering (son of maan) voor. Byvoorbeeld, tydens die sonsverduistering van 9 Maart 2016, was die maan naby sy dalende knoop, en die son was naby die punt in die lug waar die ewenaar van die maan die ekliptika kruis. Wanneer die son daardie punt bereik, sak die middelpunt van die son by die maan-noordpool en styg dit op by die maan-suidpool.

Helling tot die ewenaar en stilstand van die maan Edit

Elke 18,6 jaar bereik die hoek tussen die baan van die maan en die aarde se ewenaar 'n maksimum van 28 ° 36 ', die som van die aarde se ekwatoriale kanteling (23 ° 27') en die hellingsbaan van die maan (5 ° 09 ') na die ekliptika. Dit word genoem groot maan stilstand. Rondom hierdie tyd sal die afname van die maan wissel van -28 ° 36 'tot + 28 ° 36'. Omgekeerd bereik die hoek tussen die maan se baan en die aarde se ewenaar 9,3 jaar later sy minimum van 18 ° 20 ′. Dit word a genoem geringe maan stilstand. Die laaste stilstand van die maan was 'n geringe stilstand in Oktober 2015. Op daardie tydstip was die dalende knooppunt in lyn met die ekwinox (die punt in die lug met 'n regte hemelvaart nul en deklinasie nul). Die knope beweeg ongeveer 19 ° per jaar weswaarts. Die Son kruis elke jaar ongeveer 20 dae vroeër 'n gegewe knooppunt.

As die hellingsbaan van die maan tot die aarde se ewenaar minimaal 18 ° 20 ′ is, sal die middelpunt van die maanskyf elke dag bo die horison wees vanaf breedtegrade minder as 70 ° 43 '(90 ° - 18 ° 20') - 57 'parallaks) noord of suid. As die helling maksimum 28 ° 36 'is, sal die middelpunt van die maanskyf elke dag bo die horison wees, net vanaf breedtegrade minder as 60 ° 27' (90 ° - 28 ° 36 '- 57' parallaks) noord of suid.

Op hoër breedtegrade sal daar elke maand 'n periode van ten minste een dag wees wanneer die Maan nie opkom nie, maar daar sal ook 'n periode van ten minste een dag elke maand wees wanneer die Maan nie sak nie. Dit is soortgelyk aan die seisoenale gedrag van die son, maar met 'n tydperk van 27,2 dae in plaas van 365 dae. Let daarop dat 'n punt op die maan eintlik sigbaar kan wees as dit ongeveer 34 boogminute onder die horison is, as gevolg van atmosferiese breking.

Vanweë die neiging van die baan van die Maan ten opsigte van die aarde se ewenaar, is die maan elke maand vir bykans twee weke bo die horison by die Noord- en Suidpool, alhoewel die son ses maande op 'n slag onder die horison is. Die tydperk vanaf maanopkoms tot maanopkoms op die pole is 'n tropiese maand, ongeveer 27,3 dae, baie naby aan die sestertydperk. As die son die verste onder die horison is (wintersonstilstand), sal die maan vol wees as dit op sy hoogste punt is. As die maan in Tweeling is, sal dit bo die horison op die Noordpool wees, en wanneer dit in die Boogskutter is, sal dit op die Suidpool wees.

Die maan se lig word deur soöplankton in die Noordpoolgebied gebruik as die son maande onder die horison is [16] en moes die diere wat in die Arktiese en Antarktiese gebiede gewoon het, behulpsaam gewees het toe die klimaat warmer was.

Skaalmodel Wysig

Skaalmodel van die Aarde – Maanstelsel: Groottes en afstande is volgens skaal. Dit stel die gemiddelde afstand van die baan en die gemiddelde radius van albei liggame voor.

Ongeveer 1000 vC was die Babiloniërs die eerste menslike beskawing waarvan bekend was dat hulle 'n konstante verslag van maanwaarnemings gehou het. Kleitablette uit daardie tydperk, wat oor die gebied van die huidige Irak gevind is, is met spykerskrif geskryf waarop die tye en datums van maanopkoms en maanondergang, die sterre wat die Maan naby geslaag het, en die tydsverskille tussen stygende en die ondergang van beide die son en die maan rondom die volmaan. Babiloniese sterrekunde ontdek die drie hoofperiodes van die maan se beweging en gebruik data-analise om maankalenders op te stel wat tot in die toekoms strek. [10] Hierdie gebruik van gedetailleerde, sistematiese waarnemings om voorspellings te maak op grond van eksperimentele data, kan as die eerste wetenskaplike studie in die mensegeskiedenis geklassifiseer word. Dit lyk egter asof die Babiloniërs geen geometriese of fisiese interpretasie van hul gegewens het nie, en hulle kon nie toekomstige maansverduisterings voorspel nie (hoewel 'waarskuwings' voor waarskynlike verduisteringstye uitgereik is).

Antieke Griekse sterrekundiges was die eerste wat wiskundige modelle van die beweging van voorwerpe in die lug bekendgestel en ontleed het. Ptolemeus het die maanbeweging beskryf deur 'n goed gedefinieerde geometriese model van fietse en ontduiking te gebruik. [10]

Sir Isaac Newton was die eerste wat 'n volledige teorie van beweging, meganika, ontwikkel het. Die waarnemings van die maanbeweging was die belangrikste toets van sy teorie. [10]

Naam Waarde (dae) Definisie
Sideriese maand 27.321 662 ten opsigte van die verre sterre (13.36874634 passeer per sonbaan)
Sinodiese maand 29.530 589 met betrekking tot die son (fases van die maan, 12.36874634 passeer per sonbaan)
Tropiese maand 27.321 582 met betrekking tot die randpunt (voorgangers in

Daar is verskillende periodes wat verband hou met die maanbaan. [17] Die sideriese maand is die tyd wat dit neem om een ​​volledige baan om die aarde te maak met betrekking tot die vaste sterre. Dit is ongeveer 27,32 dae. Die sinodiese maand is die tyd wat dit die maan neem om dieselfde visuele fase te bereik. Dit wissel veral gedurende die jaar, [18] maar is gemiddeld ongeveer 29,53 dae. Die sinodiese periode is langer as die sideriese periode, omdat die Aarde-Maan-stelsel gedurende elke sideriese maand in sy wentelbaan om die Son beweeg, en daarom is 'n langer periode nodig om 'n soortgelyke belyning van die Aarde, die Son en die Maan te bereik. Die anomalistiese maand is die tyd tussen perigees en is ongeveer 27,55 dae. Die Aarde – Maan-skeiding bepaal die sterkte van die verhogingskrag van die maan.

Die drakoniese maand is die tyd van stygende knoop tot stygende knoop. Die tyd tussen twee opeenvolgende passasies van dieselfde ekliptiese lengte word die tropiese maand genoem. Laasgenoemde periodes verskil effens van die sterre-maand.

Die gemiddelde lengte van 'n kalendermaand ('n twaalfde van 'n jaar) is ongeveer 30,4 dae. Dit is nie 'n maanperiode nie, hoewel die kalendermaand histories verband hou met die sigbare maanfase.

Die gravitasie-aantrekkingskrag wat die Maan op die Aarde uitoefen, is die oorsaak van getye in beide die oseaan en die vaste aarde wat die Son beïnvloed. Die vaste aarde reageer vinnig op enige verandering in die getyforse, die vervorming neem die vorm aan van 'n ellipsoïde met die hoogtepunte ongeveer onder die maan en aan die oorkant van die aarde. Dit is die gevolg van die hoë snelheid van seismiese golwe binne die vaste aarde.

Die snelheid van seismiese golwe is egter nie oneindig nie, en tesame met die effek van energieverlies binne die Aarde, veroorsaak dit 'n effense vertraging tussen die verloop van die maksimum dwang as gevolg van die maan en die maksimum aardgety. Aangesien die aarde vinniger draai as wat die maan om sy baan beweeg, lewer hierdie klein hoek 'n swaartekrag wat die aarde vertraag en die maan in sy baan versnel.

In die geval van die getye van die oseaan is die snelheid van die getygolwe in die oseaan [19] baie stadiger as die spoed van die maan se getyforse. As gevolg hiervan is die oseaan nooit in ewewig met die getyforse nie. In plaas daarvan genereer die dwang die lang seegolwe wat versprei rondom die oseaanbekkens totdat hulle uiteindelik hul energie verloor deur onstuimigheid, hetsy in die diep oseaan of op vlak kontinentale rakke.

Alhoewel die reaksie van die oseaan die meer komplekse van die twee is, is dit moontlik om die getye van die oseaan in 'n klein ellipsoïede term te verdeel wat die maan beïnvloed plus 'n tweede term wat geen effek het nie. Die ellipsoïede term van die oseaan vertraag ook die aarde en versnel die maan, maar omdat die oseaan soveel gety-energie versprei, het die huidige oseaan-getye 'n groter orde as die vaste getye van die aarde.

Vanweë die getywringkrag, wat veroorsaak word deur die ellipsoïede, word sommige van die Aarde se hoekige (of roterende) momentum geleidelik oorgedra na die rotasie van die Aarde / Maan-paar rondom hul onderlinge massamiddelpunt, die barycentre genoem. Sien getyversnelling vir 'n meer gedetailleerde beskrywing.

Hierdie effens groter wentelmomentum laat die afstand tussen die aarde en die maan met ongeveer 38 millimeter per jaar styg. [20] Die behoud van die hoekmomentum beteken dat die Aksiale rotasie geleidelik verlangsaam, en dat sy dag dus jaarliks ​​met ongeveer 24 mikrosekondes verleng word (uitgesonderd gletser-rebound). Albei syfers is slegs geldig vir die huidige opset van die vastelande. Getijrytmiete van 620 miljoen jaar gelede toon dat die maan oor honderde miljoene jare gemiddeld 22 mm (0,87 in) per jaar (2200 km of 0,56% of die aarde-maanafstand per honderd miljoen jaar) teruggesak het. en die dag het gemiddeld 12 mikrosekondes per jaar (of 20 minute per honderd miljoen jaar) verleng, albei ongeveer die helfte van hul huidige waardes.

Die huidige hoë tempo kan te wyte wees aan byna resonansie tussen natuurlike oseaanfrekwensies en getygroepe. [21] 'n Ander verklaring is dat die aarde vroeër baie vinniger gedraai het, 'n dag wat moontlik net 9 uur op die vroeë aarde geduur het. Die gevolglike getygolwe in die oseaan sou dan baie korter gewees het en dit sou moeiliker gewees het vir die gety met lang golflengte om die kort golflengte op te wek. [22]

Die maan is geleidelik besig om van die aarde af in 'n hoër baan terug te trek, en berekeninge dui daarop dat dit ongeveer 50 miljard jaar sou voortduur. [23] [24] Teen daardie tyd sou die aarde en die maan in 'n wedersydse draai-resonansie of getyvergrendeling verkeer, waarin die maan binne ongeveer 47 dae (tans 27 dae) om die aarde sal wentel, en beide die maan en die aarde sou op dieselfde tyd om hul asse draai, altyd met dieselfde kant na mekaar gerig. Dit het al met die maan gebeur - dieselfde kant kyk altyd na die aarde - en gebeur ook stadig met die aarde. Die verlangsaming van die Aarde se rotasie vind egter nie vinnig genoeg plaas sodat die rotasie tot 'n maand kan verleng voordat ander gevolge die situasie verander nie: ongeveer 2,3 miljard jaar van nou af sal die toename van die sonstraling veroorsaak het dat die Aarde se oseane verdamp het, [25 ] die grootste gedeelte van die getywrywing en versnelling verwyder.

Die maan is in sinchrone rotasie, wat beteken dat dit te alle tye dieselfde gesig na die aarde toe hou. Hierdie sinchrone rotasie is gemiddeld slegs waar omdat die baan van die maan 'n besliste eksentrisiteit het. As gevolg hiervan, wissel die hoeksnelheid van die maan namate dit om die aarde wentel en is dit dus nie altyd gelyk aan die konstante rotasiesnelheid van die maan nie. As die maan sterk is, is sy baanbeweging vinniger as die rotasie. Op daardie stadium is die maan 'n bietjie voor in sy wentelbaan ten opsigte van sy draai rondom sy as, en dit skep 'n perspektief-effek wat ons toelaat om tot agt grade lengtelyn van sy oostelike (regter) verste kant te sien. Omgekeerd, as die maan sy hoogtepunt bereik, is sy wentelbeweging stadiger as sy rotasie, wat die agt lengtegraad van sy westelike (linker) verste kant openbaar. Dit word na verwys as optiese librasie in lengte.

Die rotasie-as van die maan word in totaal 6,7 ° geneig ten opsigte van die normaal tot die vlak van die ekliptika. Dit lei tot 'n soortgelyke perspektief-effek in die noord-suid-rigting, waarna verwys word optiese librasie in breedtegraad, waarmee 'n mens amper 7 ° breedtegraad anderkant die paal aan die ander kant kan sien. Ten slotte, omdat die maan slegs ongeveer 60 radiusse van die aarde se massamiddelpunt is, beweeg 'n waarnemer by die ewenaar wat die maan dwarsdeur die nag waarneem, sywaarts met een Aarde-deursnee. Dit gee aanleiding tot a daglibrasie, wat 'n mens toelaat om 'n addisionele maandelengte van een graad te sien. Om dieselfde rede sou waarnemers aan albei die Aarde se geografiese pole in staat wees om 'n addisionele graad se librasie in breedtegraad te sien.

Behalwe hierdie "optiese librasies" wat veroorsaak word deur die perspektiefverandering vir 'n waarnemer op Aarde, is daar ook "fisiese librasies" wat werklike nutasies is van die rigting van die rotasiepool van die Maan in die ruimte: maar dit is baie klein.

Vanuit die noordelike hemelpool (d.w.z. vanaf die benaderde rigting van die ster Polaris) wentel die Maan antikloksgewys en die Aarde wentel antikloksgewys, en die Maan en Aarde draai op hul eie as linksom.

Die regterkantse reël kan gebruik word om die rigting van die hoeksnelheid aan te dui. As die duim van die regterhand na die noordelike hemelpool wys, krul sy vingers in die rigting waarop die maan om die aarde wentel, die aarde om die son wentel en die maan en die aarde op hul eie as draai.

In weergawes van die sonnestelsel is dit algemeen om die baan van die Aarde vanuit die oogpunt van die Son en die baan van die Maan vanuit die oogpunt van die Aarde te teken. Dit kan die indruk wek dat die maan so om die aarde wentel dat dit soms agteruit gaan as dit vanuit die son se perspektief gesien word. Omdat die wentelsnelheid van die maan rondom die aarde (1 km / s) klein is in vergelyking met die wentelsnelheid van die aarde rondom die son (30 km / s), gebeur dit egter nooit nie. Daar is geen agterste lusse in die maan se sonbaan nie.

Met inagneming van die Aarde – Maanstelsel as 'n binêre planeet, is sy swaartepunt binne die Aarde, ongeveer 4,671 km (2 902 mi) [27]> of 73,3% van die Aarde se radius vanaf die middelpunt van die Aarde. Hierdie swaartepunt bly op die lyn tussen die middelpunte van die Aarde en die Maan, terwyl die Aarde sy dagrotasie voltooi. Die pad van die aarde – maanstelsel in sy sonbaan word gedefinieer as die beweging van hierdie onderlinge swaartepunt om die son. Gevolglik draai die Aarde se middelpunt gedurende elke sinodiese maand binne en buite die sonbaan, terwyl die maan in sy wentelbaan om die gemeenskaplike swaartepunt beweeg. [28]

Die swaartekrag-effek op die maan is meer as twee keer dié van die aarde op die maan. Daarom is die baan van die maan altyd konveks [28] [29] (soos gesien as ons van groot afstand na die hele Son – Aarde – Maanstelsel kyk) buite die sonbaan van die aarde – maan), en is nêrens konkaaf nie (vanuit dieselfde perspektief) of lus. [26] [28] [30] Dit wil sê, die gebied wat deur die maan se baan van die son omring is, is 'n konvekse stel.


Is daar 'n tydwoord vir die baan van 'n maan? - Sterrekunde

Gevolge van getywrywing

Die see getye is nie die enigste effek van hierdie getykragte. Die vaste liggaam van die aarde bult ook so effens uit. Die daaglikse buiging van die aarde (beide die vaste liggaam en die verswakking van die oseane) veroorsaak verlies aan energie deur die wrywing van die aarde. Hierdie energie gaan in hitte, wat die aarde se interne temperatuur verhoog. Die verlies aan rotasie-energie beteken dat die Aarde sy rotasiesnelheid vertraag, tans met ongeveer 0,002 sekondes per eeu.

Soos u dink, oefen die Aarde ook getykragte uit op die Maan. In werklikheid is die getyskragte van die Aarde op die Maan ongeveer 20 keer groter as dié van die Maan op die Aarde. Let op wat gebeur as 'n draaiende liggaam gety verwring word. Die vervormingslyn word voortdurend weggedraai van die lyn tussen die twee liggame af, wat die bultjies effens laat lei. Daar is dan 'n netto wringkrag wat teen die draairigting staan, wat albei liggame vertraag. Hierdie wringkrag bestaan ​​totdat die vertraagde rotasie veroorsaak dat die wentelperiode van die liggaam dieselfde is as die rotasieperiode. Sodra dit gebeur, word gesê dat die liggaam dit is gety gesluit , en die wringkrag en afvoer deur getykragte staak. Op hierdie oomblik is die maan getyd met die aarde opgesluit, maar die aarde is nie gety met die maan nie. Daarom hou die maan dieselfde gesig op die aarde. In die verre toekoms sal die verlangsame Aarde uiteindelik in die maan opgesluit bly en geen verdere evolusie van die stelsel sal plaasvind nie.

Hoe sal die aarde / maanstelsel daar uitsien as dit gebeur? Die voorste bult van die Aarde oefen ook 'n ekstra trek uit op die Maan in sy wentelbaan, wat 'n effense versnelling langs die baan gee, en die wentelsnelheid verhoog. Dit beteken dat die maan stadig van die aarde af wegdraai.

Lesingvasvra # 1

Die atmosfeer is die laag gas wat sommige planete omring. Soos u miskien weet, laat die verhitting van 'n gas dit uitbrei (die druk neem toe). 'N Massiewe planeet soos Jupiter het so 'n sterk swaartekrag dat dit sy gasse kan inhou, en hulle kan nie in die ruimte ontsnap nie. Die aarde kan swaarder gasse, soos suurstof en koolstofdioksied, vashou, maar nie die ligste gasse soos waterstof en helium nie. Enige liggaam het 'n ontsnap spoed , hoe vinnig 'n voorwerp moet beweeg om te ontsnap. Die ontsnapspoed op die aardoppervlak is 11,2 km / s. Wanneer 'n gas verhit word, beweeg die deeltjies vinniger, dus of 'n gas in die aarde se atmosfeer gehou sal word, hang af van hoe warm dit is en of die molekules vinniger beweeg as die ontsnapspoed. Maar nie alle gasmolekules beweeg met dieselfde snelheid nie. Ons praat oor die gemiddelde spoed, wat van die temperatuur afhang, maar selfs wanneer die meeste molekules teen 'n laer gemiddelde spoed beweeg, sal enkele van die vinnigste die ontsnappingsspoed oorskry en na die ruimte wegdryf. Hoe nader die gemiddelde snelheid aan die ontsnapspoed is, hoe meer molekules verlore gaan, en hoe vinniger sal die atmosfeer ontsnap.

Maan

Het die maan 'n atmosfeer? Om dit uit te vind, vergelyk ons ​​die ontsnapspoed van die Maan met die spoed van die gasdeeltjies wat sy atmosfeer vorm. Tensy die gasdeeltjies stadiger beweeg ('n faktor van 10) as die ontsnapspoed, sal die deeltjies mettertyd uit die atmosfeer syfer. Die ontsnapspoed van die maan is slegs 2,32 km / s en die suurstofsnelheid (0 2 ) aan die maanoppervlak 0,78 km / s. S o die ontsnapspoed is slegs 3 keer die gemiddelde spoed. Na slegs 'n paar honderd jaar verloor die maan enige suurstof wat geproduseer kan word.

Enige atome wat rondom die maan talm, wat geproduseer word as gevolg van uitgasing of spalasie van rotse, hou net 'n kort rukkie en moet voortdurend aangevul word. Die atmosfeer van die maan is 'n ongelooflike goeie vakuum, slegs 10 - 14 atm.

Aarde

Toe die aarde vir die eerste keer gevorm is, sou sy atmosfeer meestal H en Hy begin wees, maar dit verloor het as gevolg van die spoed van hierdie deeltjies wat hulle mettertyd laat ontsnap het. 'N Nuwe, swaarder atmosfeer van H 2 O, O 2 , N 2 , en CO 2 is uit vulkanisme uitgegooi, of deur komete hierheen gebring.

Die druk van die Aarde se atmosfeer (en alle atmosfeer) val met hoogte, dus is verreweg die grootste deel van die atmosfeer op die laagste hoogtes. Die temperatuur daal ook met hoogte, ten minste naby die oppervlak. Daarom word dit so koud en is daar so min lug bo-op 'n berg. Dieselfde algemene gedrag geld vir alle atmosfeer, selfs die atmosfeer van sterre! Maar let op wat in die onderstaande figuur gebeur, wat 'n grafiek toon van die verandering van temperatuur met hoogte in die aarde se atmosfeer.

Atmosferiese temperatuurstruktuur:

Waarom daal die temperatuur van die atmosfeer tot ongeveer 10 km en begin dan weer in die Stratosfeer te styg? Dit is te wyte aan die absorpsie van ultraviolet (UV) lig van die son, wat energie in hierdie streek van die atmosfeer afsit en dit verhit. Hierdie streek word die Stratosfeer genoem omdat dit stabiel is vir opwaartse bewegings ('n inversielaag), wat beteken dat wolke nie in kolomme opkom nie, maar in dun lae versprei, soos lae. Die gebied van maksimum verhitting deur UV-lig (die Stratopause) is ook die ligging van die osoonlaag, O 3, wat grootliks verantwoordelik is vir die opname van die UV en om ons te beskerm teen die skadelike bestraling.

Tot by die Mesopauze daal die temperatuur weer, maar styg dan weer in die Thermosfeer as gevolg van die opname van X-strale uit die son. Hierdie 'atmosfeer' is net effens onder die hoogte van lae satelliete rondom die aarde, soos die ruimtetuig, wat ongeveer 200 km bo wentel, en hierdie deel van die atmosfeer is so dun dat dit 'n byna perfekte vakuum is.

Die aarde se atmosfeer is ongeveer 1/5 O 2 , en 4/5 N 2 , met spoorhoeveelhede van ander gasse soos koolstofdioksied (CO 2 ) en water (H 2 O). Die hoeveelheid CO 2 het die afgelope 200 jaar aansienlik gestyg, deels as gevolg van menslike aktiwiteit (verbranding van fossielbrandstowwe, ens.). Koolstofdioksied is 'n kweekhuisgasse , sogenaamd omdat dit soos 'n kweekhuis optree om die aarde warm te hou. Hoe werk die kweekhuiseffek? Straling vanaf die son gaan deur die atmosfeer in die sigbare spektrum van die spektrum en verhit die grond, wat dan herstel die energie uit in die ruimte, maar nou in die infrarooi deel van die spektrum. Kweekhuisgasse blokkeer die ontsnapping van die hitte deur die infrarooi bestraling op te neem. As gevolg hiervan is daar kommerwekkende aanduidings dat die aarde se klimaat warmer word. Die aarde se klimaat het mettertyd kouer en warmer geword, wat gletsertydperke en tussen-ystydperke veroorsaak, en dit is dus onmoontlik om te weet of mense hoofsaaklik die skuld het vir die klimaatsverhitting. Nietemin, ons kan help om die probleem onder beheer te hou deur kweekhuisgasvrystellings te verminder.

Lesingsvraag # 2

Die aarde is die een planeet wat ons in detail kan bestudeer. Ons weet baie oor die struktuur van die binnekant van die aarde, maar ons moet onthou dat ander planete op fundamentele maniere kan verskil. We must take what we learn about Earth and compare and contrast it with the other planets.

Die mean of 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? - Sterrekunde

What's the date today? If you're not sure, what do you do? Easy. Look on the calendar. But 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. Alle regte voorbehou.
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 en 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. Lees verder.

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 why 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 Aarde 'feels'. The original question was about forces acting on the Maan 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? - Sterrekunde

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 Maan , and the force from the Sun D F Son . 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 lente 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. Hoekom?
The ocean tides are nie 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 Aarde R Maan /M Maan R Aarde

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 bl r/P = 2 bl 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


Kyk die video: Sadi Jithe Lagi Ae Lagi Rehn De Part 1 By Gurdas Maan. Punjabiyan Di Shaan. Punjabi Sufiana (Desember 2022).