Sterrekunde

Maan se pad vanaf die aarde gesien

Maan se pad vanaf die aarde gesien


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Soos waargeneem vanaf die aardoppervlak: sal die maan ooit dieselfde baan (baan) herhaal? Indien ja, wat is die periode van herhaling? En sal die maan oor 'n tydperk die naghemel met sy pad heeltemal bedek?

Meer verduideliking: ek het die maan vanuit my venster waargeneem en ek het gewonder of die maan dieselfde pad (baan) sal volg as voorheen, en as ek foto's neem van al die verskillende posisies van die maan oor 'n lang tydperk het dit die naghemel bedek?


Daar is 'n pragtige saamgestelde prentjie wat dit presies demonstreer op http://www.twanight.org/

Dit is 'n samestelling van 90 foto's oor die vier dae, sodat u maklik kan sien hoe ver die maan elke dag beweeg (ongeveer 13 grade)

Van https://cseligman.com/

gemiddeld kruis die maan die lug een keer elke 24 uur en 49 minute (53 minute langer as die ster "dag"). As gevolg daarvan styg dit (en verstel) dit elke dag later en later, totdat dit na ongeveer 27 dae, wanneer dit een keer om die lug gegaan het relatief tot die sterre, weer in sy oorspronklike posisie is, op sy oorspronklike tyd styg en sak. (s).


Die hemelse sfeer


Die rooi "Ecliptic" is die pad van die son. Die son is rondom 21 Maart by die lente-ewening en beweeg ooswaarts (toenemende regter hemelvaart).

Net die hemelsfeer plus die ekliptika, met sonstilstande en eweninge gemerk.


Dit is getrek vir noordelike breedtegrade, dit is die paadjies wat die son op die eweninge en sonstilstand oor die lug neem. Kan u sien dat die somerpad langer is (en dat die somerson dus langer in die lug bly)?

Hierdie figuur illustreer dat, afhangend van u breedtegraad, sommige sterre 'sirkumpolêr' sal wees en nooit sal ondergaan nie. Onthou: u breedtegraad = die hoogte van die noordelike hemelpool.

Voorbeelde wat verband hou met koördinate van die waarnemer (hoogte) met hemelse koördinate (deklinasie) vir verskillende breedtegrade op aarde. Ons beskou slegs maksimum hoogtes, dit wil sê punte op die meridiaan. Waarnemer se breedtegraad Hoogte van die noordelike hemelpool (Az. = 0) Hoogte van die suidelike hemelpool (Az. = 180) Hoogte van hemelse ewenaar (Az. = 0 of 180) Afwyking van die Noord-horison Afname van die Suid-horison Deklinasie van Zenith 0 (Ecuador) 0 0 90 90 -90 0 30 (Caribbean) 30 -30 60 (Az. 180) 60 (dws 30 grade verder as 90) -60 30 60 (Kanada) 60 -60 30 (Az. 180) 30 -30 60 90 (Noordpool) 90 -90 0 (dit wil sê die horison is gelyk aan die hemelse ewenaar) 0 0 90

  • hoogte van NCP = breedtegraad van die waarnemer
  • hoogte van SCP = - (waarnemer se latutude)
  • maksimum hoogte van die hemelse ewenaar = 90 - (breedtegraad van die waarnemer)
  • Desember van die noordelike horison = 90 - (breedtegraad van die waarnemer)
  • Desember van die suidelike horison = -90 + (breedtegraad van die waarnemer)
  • Desember van seniet = breedtegraad van die waarnemer

Dit werk ook suid van die ewenaar, maar u moet al die "noorde" met die "suide" oorskakel. Die laaste punt hieroor is dat hierdie korrespondensies tussen breedtegraad / deklinasie / hoogte altyd waar is, maar dat korrespondensies tussen lengte en regs ophang afhang van die uur van die dag en ook die seisoen.


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 orbitale presessie word apsidale presessie genoem en is die rotasie van die maan se baan 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 sy 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 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 dit kon verhoog word tot 10 ° 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 word 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 genaamd die drakoniese maand of drakoniese 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 voorkom 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 byna 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 die tydperk, wat oor die gebied van die huidige Irak gevind is, is met spykerskrif geskryf waarop die tye en datums van maan- en maanopgange, die sterre wat die maan naby verbygaan, 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 deurloop 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 snelheid 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 veroorsaak dat die Aarde – Maan-afstand ongeveer 38 millimeter per jaar vermeerder. [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 'n nabye resonansie tussen natuurlike oseaanfrekwensies en getyfrekwensies. [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 rotasie om sy as, en dit skep 'n perspektief-effek wat ons toelaat om tot agt grade lengte 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 (2792 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 gravitasie-effek van die Son op die maan is meer as twee keer die van die aarde op die maan. Die baan van die maan is dus altyd konveks [28] [29] (soos gesien as ons van groot afstand na die hele Son – Aarde – Maanstelsel kyk) buite die aarde – maan-sonbaan), en is nêrens konkaaf nie (vanuit dieselfde perspektief) of lus. [26] [28] [30] Dit wil sê, die gebied wat deur die maan se wentelbaan van die Son omsluit is 'n konvekse stel.


Die maan en verduisterings

Na die son is die maan die opvallendste hemelse voorwerp. Sy bewegings, fases en af ​​en toe verduisterings gee ons hemel 'n pragtige verskeidenheid. Dit is ook die naaste astronomiese voorwerp.

The Moon's Motions

Die beweging van die maan deur ons lug is soortgelyk aan dié van die son, maar anders:

  • Soos die son, kom die maan in die ooste op en sak in die weste (met uitsondering vir waarnemers in die uiterste noordelike en suidelike streke van die aarde).
  • Soos die son, beweeg die maan nie so vinnig soos ons sterre oor ons lug nie.
  • Terwyl die son slegs een graad per dag ten opsigte van die sterre ooswaarts kruip, beweeg die maan ongeveer 13 grade per dag ooswaarts ten opsigte van die sterre (sien illustrasie hieronder). Dit beteken dat as u aandag skenk, u maklik die maan se beweging ten opsigte van die sterre gedurende 'n enkele nag kan opmerk.
  • As gevolg hiervan beweeg die maan ooswaarts met betrekking tot die son met ongeveer 12 grade per dag (13 minus 1). Dit beteken dat die maan elke opeenvolgende dag later en later opkom, soms in die oggend, soms in die middag en soms in die nag. Net so sak die maan later elke dag en kan dit op enige tyd van die dag of nag sak, afhangend van waar dit is met betrekking tot die son.
  • Teen 'n tempo van 12 grade per dag voltooi die maan 'n volle sirkel, ten opsigte van die son, ongeveer een keer elke 30 dae (eintlik 29,5). Hierdie tydperk is oorspronklik een maand genoem (raai waar die woord vandaan kom), hoewel ons moderne kalender nie direk aan die maan gekoppel is nie en dit verskillende maande bepaal om verskillende lengtes te hê.
  • Die maan word altyd naby die ekliptika gesien, maar sy beweging dra dit gedurende die loop van elke maand ongeveer vyf grade na weerskante.

Hierdie gesimuleerde beeld met veelvuldige blootstelling toon die posisies van die son en die maan ten opsigte van die sterre oor 'n periode van nege dae. Terwyl die son slegs een graad per dag ooswaarts (van regs na links) beweeg, beweeg die maan met 13 grade per dag ooswaarts. Die geel lyn is die ekliptika, waaruit die maan nooit meer as ongeveer vyf grade afdwaal nie. (Die grootte van beide die son en die maan is oordrewe vir nadruk.)

Vraag: Gestel, vir die doel van hierdie vraag, is 'n maand presies 30 dae. Hoeveel minute later, op elke opeenvolgende dag, sou u verwag dat die maan gemiddeld sou opkom? Onthou dat die maan na 'n volle maand (30 dae) op dieselfde tyd as oorspronklik moet opkom (die seisoenale variasies moet verwaarloos word).

Die maan se fases

Behalwe vir die lugbeweging, is die maan ook bekend vir sy fases: sy verskeidenheid skynbare vorms, van halfmaan tot half tot gibbous tot vol. Daar is ook 'n "nuwe" fase wanneer ons die maan vir 'n paar dae glad nie sien nie. Wat veroorsaak hierdie opvallende veranderinge in die voorkoms van die maan?

  • Eerstens gaan die maan presies een keer elke (maan) maand deur sy volledige siklus van fases, dit wil sê in dieselfde tydperk as sy beweging ten opsigte van die son.
  • Tweedens, die maan se fase is altyd nuut as dit naby die son in ons lug is, en vol as dit oorkant die son is, met tussenfases onder tussenhoeke (tussen die maan en die son).
  • Derdens, as die maan in sy halfmaanfase is en die lug genoeg donker is, kan jy soms die res van die maan se verligte skyf uitmaak. Dit vertel ons dat die maan nie regtig van vorm verander nie, maar dat dit net baie helderder is as die res.
  • Ten slotte, in elke fase tussen nuut en vol, kan u seker maak dat die verligte kant van die maan altyd die kant is wat na die son kyk.

Die eenvoudige verklaring van al hierdie waarnemings is dat die maan skyn deur weerkaatsde sonlig. Dit is bolvormig, met die helfte van die sfeer wat op enige tydstip deur die son verlig word. Hoeveel van hierdie helfte ons sien, hang egter af van ons rigting. As die maan en die son naby mekaar in ons lug is, kyk ons ​​na die donker kant van die maan sodat ons dit glad nie kan sien nie ('nuwemaan'). Wanneer die maan oorkant die son in ons lug is, kyk ons ​​na die maan se verligte kant sodat ons 'n volmaan sien. In ander hoeke sien ons die deel van die maan se verligte kant, maar nie almal nie. Die onderstaande illustrasie wys hoe u die maanfases kan simuleer met behulp van 'n bal en 'n sterk rigtingligbron om die son te simuleer.

Simulasie van maanfases met behulp van 'n piepschuimbal wat deur 'n sterk, rigtinggewende ligbron verlig word. Die vier foto's is met verskillende hoeke tussen die "maan" en die ligbron geneem.

Die nuwe en sekelmaanfases van die maan vind vanuit ons perspektief plaas wanneer die son min of meer agter die maan is. Deur net na hierdie fases te kyk, sê ons dus onmiddellik dat die maan nader aan ons moet wees as die son. Daar is selfs 'n manier om te skat hoeveel nader dit is, soos ek hieronder sal verduidelik.

Vraag: Gestel jy woon op 'n middel-noordelike breedtegraad, waar sou jy in die lug kyk om 'n sekelmaan met sonsondergang te sien?

Vraag: Hoe laat van die dag of nag kom die maan op as dit vol is?

Vraag: Baie pragtige foto's van die maan word vervals deur twee afsonderlike foto's te kombineer. Dink aan maniere waarop u kan sien wanneer die maan kunsmatig by 'n foto gevoeg is.

Sonsverduisterings

By 'n ongelooflike toeval is die maan s'n oënskynlike grootte in ons lug is amper presies dieselfde as die son s'n: ongeveer 'n halwe graad. Dit beteken dat dit moontlik is dat die maanskyf die son heeltemal kan bedek as die nuwemaan by die son verbygaan tydens sy maandelikse rit om die ekliptika. As dit wel gebeur, word ons lug vir 'n paar minute donker en kan ons die dun, warm gasse rondom die son sien, wat die sonkorona genoem word. Hierdie dramatiese gebeurtenis word 'n totale sonsverduistering genoem.

Links: Algehele sonsverduistering, wat die sonkorona en verskeie rooi prominensies om die rand wys (Luc Viatour). Sentrum: Gedeeltelike sonsverduistering (Michael Mortensen). Regs: Ringvormige sonsverduistering wanneer die maan te ver is om die son heeltemal te bedek (Sancho Panza).

'N Sonsverduistering vind plaas wanneer die maan tussen die son en die aarde beweeg. Slegs 'n klein gedeelte van die aarde word bedek deur die maan se skaduwee. (Diagram nie volgens skaal nie.)

Onthou egter dat die maan tot vyf grade na weerskante van die ekliptika kan dwaal. In die meeste maande mis die nuwe maan die son (vanuit ons perspektief) 'n paar grade, na die een of die ander kant, as dit verbygaan. Maar die maan se pad kruis die verduistering ongeveer twee keer elke maand, en 'n sonsverduistering vind plaas wanneer die son toevallig tydens die kruising is. Daar is ongeveer een keer elke ses maande 'n goeie kans hiervoor. Maar selfs dan vereis die perfekte belyning dat u êrens langs 'n smal paadjie oor die aardoppervlak is, gewoonlik minder as 300 km breed. En selfs dan, die maan se skyf is nie altyd groot genoeg om die son te bedek nie, want albei se skynbare groottes verskil, aangesien hul afstande van die aarde effens verskil. As die maan te ver is en / of die son te naby is, maar tog die belyning goed is, noem ons die gebeurtenis 'n ringverduistering. Andersins, as die maan net 'n gedeelte van die son bedek, noem ons dit 'n gedeeltelike sonsverduistering.

Of die verduistering totaal (of ringvormig) is of nie, 'n gedeeltelike sonsverduistering is gewoonlik ongeveer twee keer per jaar oor 'n wye gebied sigbaar. Ek het deur die jare verskeie gedeeltelike sonsverduisterings gesien, en as u oud genoeg is, het u dit waarskynlik ook. Ek was nog nooit op die regte plek om 'n totale sonsverduistering te sien nie, maar ek sien uit na die een wat op 21 Augustus 2017 deur 'n groot deel van die VSA sal kom.

Maansverduisterings

'N Ewe interessante, indien minder dramatiese gebeurtenis, vind plaas wanneer die son en die maan in ons lug presies teenoor mekaar staan. Dit is wanneer die maan normaalweg vol is en baie helder is. Maar as die belyning presies genoeg is, sal die aarde die sonlig verhoed om die maan te bereik. Hierdie gebeurtenis word 'n maansverduistering genoem. Soos met sonsverduisterings, is daar 'n goeie kans dat 'n maansverduistering ongeveer een keer in die ses maande plaasvind. Sommige is slegs gedeeltelik, en die aarde se skaduwee bedek nooit meer as 'n gedeelte van die maan nie, omdat die belyning onvolmaak is.

'N Maansverduistering vind plaas wanneer die maan en son weerskante van die aarde is. Dikwels word die hele maan deur die aarde se skaduwee bedek. (Diagram nie volgens skaal nie.)

'N Reeks van drie foto's van die totale maansverduistering van 16 tot 17 Augustus 1989, geneem oor 'n tydperk van ongeveer 'n halfuur. Die blootstellingstyd was die kortste vir die helder gedeeltelike fase (links) en die langste vir die donkerrooi totale fase (regs). Die middelste foto wys hoe die verduisterde gedeelte van die maan is veel darker than the small sliver that's receiving direct sunlight.

Your chances of having seen a total lunar eclipse are pretty good, because they're visible from everywhere on the night side of the earth, and the totality can last over an hour. Clouds can get in the way, of course, and you sometimes need to be willing to get up in the middle of the night. The next lunar eclipse as of this writing will be a total one on December 10, 2011 best viewed from the Asia-Pacific region, it will also be visible in the morning, as the moon is setting, from western North America. For the next two years the lunar eclipses will be only partial, with the next total lunar eclipse not occurring until April 15, 2014.

Although you might expect the moon to become totally dark during a lunar eclipse, in fact a little sunlight still gets to it&mdashby bending around the earth, through earth's atmosphere. The same thing happens to us every evening after the sun sets, when we still see light in the western sky. And just as with sunsets, it's the rooi component of the sunlight that best penetrates earth's atmosphere and reaches the eclipsed moon.

The Moon's Size

The earth's shadow, at the moon's location, is nearly three times as wide as the moon itself.

Besides just being a cool sight to watch, a lunar eclipse gives us a way to determine the moon's true size.

As shown at right, the curvature of earth's shadow on the partially eclipsed moon tells us the size of earth's shadow, in comparison to the moon. Specifically, the diameter of earth's shadow is a little less than three times the diameter of the moon. In other words, the moon's diameter is a little more than one third that of the shadow.

If earth's shadow were the same size as the earth itself, we could immediately conclude that the moon's diameter is a little over a third that of the earth. But it's not that simple, because earth's shadow tapers off, getting narrower as you get farther and farther away (as shown in the lunar eclipse diagram above). To take this tapering into account, you have to do a rather intricate calculation using several principles of geometry and algebra. The bottom line is that the earth's shadow, at the moon's location, is about 3/4 as wide as the earth itself. Therefore, since the moon is a little over 1/3 as wide as the shadow, it must be a little over 1/4 as wide as the earth. In miles, the earth's diameter is about 8000 so the moon's diameter is about 2000.

This ingenious method of determining the moon's size was devised by the Greek astronomer Aristarchus of Samos, in the third century B.C. Although the method is absolutely sound, Aristarchus used rather inaccurate measurements so his numerical answer for the moon's size wasn't especially good. But later Greek astronomers repeated the calculation, using better measurements.

Even without doing any calculations, however, you can just look at a partial lunar eclipse and see that the moon is probably just a few times smaller than the earth. Of course, you can also see that earth's shadow is curved. Aristotle, the great Greek philosopher, used this observation to argue that the earth must be a sphere.

The Distance to the Moon

The closer an object is, the greater its angular size.

Knowing the moon's true diameter (1/4 that of the earth), we can use its measured angular diameter (half a degree) to determine its distance. Qualitatively, the idea is this: For a given true size, a larger angular diameter would mean that the moon must be closer whereas a smaller angular diameter would mean that the moon must be farther away. Because we weet the moon's angular diameter, we can calculate just how far it is.

The calculation takes a few steps, and requires some effort and practice to fully understand. But similar calculations come up so often in astronomy that it's a good investment to start practicing now.

Big circle diagram for calculating the moon's distance. (The half-degree angle has been greatly exaggerated.)

A useful trick for visualizing the calculation is to sketch a big, imaginary circle, centered on you (the observer), passing through the moon. Then label the angle that represents the moon's measured angular width (half a degree in reality, but it's fine to exaggerate it in the picture as I have). Also label the moon's known diameter, 1/4 (in units of earth's diameter).

The calculation proceeds in three steps:

  1. Ask yourself how many hypothetical moons it would take, placed side by side, to reach all the way around the circle. Since a full circle is 360° but each moon takes up only 1/2°, the answer is 360 divided by 1/2, or 720 hypothetical moons.
  2. Now calculate the circumference of the circle. Since each moon has a width of 1/4 e.d., and it takes 720 of them to go all the way around, the answer is 720 times 1/4 e.d., or 180 earth diameters. (You could use units of miles instead of earth diameters if you prefer, but then the numbers get a little more cumbersome: 720 × 2000 = 1440000.)
  3. The distance to the moon is the radius of the circle. But for any circle, the circumference is 2&pi times the radius, where &pi is a number that's equal to a little more than 3. If you don't need an especially accurate answer, it's fine to pretend that &pi equals 3 and just say that the circumference is six times the radius. Then the radius (or distance) is simply the circumference divided by 6, or in this case, 180 e.d. divided by 6, or 30 earth diameters.

I like to call these kinds of calculations . The general idea is that if you know the angular size and the true size of an object, you can calculate its distance by drawing a big circle and going through these three steps. A slight variation, which comes up just as often, is when you already know the distance and the angle, and want to calculate the true size. Please make sure you understand each step of the calculation, and practice until you can put them together and do a whole big circle problem on your own.

Vraag: Imagine that you live on an alien world with a moon that appears only half as wide as ours: 1/4 degree. You also know that the true diameter of this moon is 500 miles. Sketch a big circle, centered on you and passing through the moon, and label it with the angle and true diameter. How many hypothetical moons would fit around the full circle?

Next step: What is the circumference of the circle, in miles?

Final step: Using the approximation &pi = 3, what is your estimate of the distance to this moon, in miles?

The next two questions are harder, so you may want to skip them the first time your read this. Be sure to come back and work them, for practice, when you have time!

Vraag: Bruno the Bully is chasing you across the playground. Being the nerdy type, you decide to stop running away and figure out just how far from you he is (so you can then calculate how long you have to live). You know that Bruno is 6 feet tall, and, holding up your outstretched fist, you estimate his angular height to be five degrees. Using the approximation &pi = 3, what is your estimate of Bruno's distance in feet?

Vraag: According to your map, the mountain on the horizon is 30 kilometers away. Holding out your fist, you estimate its angular height to be three degrees. How tall is the mountain, in kilometers? (Again, please use the approximation &pi = 3.)

Coming back to the moon, our calculation gave a distance of 30 earth diameters (or 60 earth radii, or about a quarter million miles). Here is a picture of the earth and moon, to scale:

Scale drawing of the earth and the moon, showing that the moon is 1/4 the earth's diameter, and 30 earth diameters away.

Comprehending this enormous distance is a challenge even today, when automobile odometer mileages often exceed a quarter million. But imagine how astonishing this distance must have been to the ancient Greeks, who traveled only by the power of muscle and wind. Equally astonishing is that the Greeks could determine this distance without the benefit of space travel or even a telescope. As we'll see over and over again, the human intellect can reach out to measure things that are far beyond our physical reach.

The Distance to the Sun

Besides calculating the moon's distance, Aristarchus also devised an equally ingenious method to estimate the distance to the sun.

When the moon is half full, the sun must be somewhere directly to its side. By measuring the observed angle between the moon and the sun, you can tell that the sun is many times farther away than the moon.

Wait until the moon is exactly half full, then look up at it. If the illuminated half of the moon's sphere is on your right, this means that the sunlight must be coming directly from the right, at a 90-degree angle to your line of sight. So on the diagram, you know that the sun must lie somewhere along the dashed orange line.

To determine waar along the dashed orange line, all you have to do is measure the angle between the moon and the sun, from your perspective. Point to the moon with one hand and the sun with the other, and estimate the angle between your arms. Then, on the diagram, measure out the same angle and extend a line from you toward the sun. Draw the sun where that line intersects the dashed orange line.

If you actually try this procedure, though, you'll find that the angle is indistinguishable from 90°. It has to be slightly less, of course, but you can't tell how much less because it's impossible to tell when the moon is exactly half full. But you can see, at least, that since the angle is close to 90°, the sun must be many times farther away than the moon.

Aristarchus wrote that the angle between the half-moon and the sun was 87°, and from this, estimated that the sun is about 19 times as far away as the moon. What he probably should have said is that the sun is at least 19 times as far away. And since the sun and moon have about the same angular size, this means that the sun's true diameter is at least 19 times that of the moon. Thus, Aristarchus was able to prove that the sun must be much bigger than the earth&mdasha result that must have been hard for anyone to believe at the time.

We now know that the sun is almost 400 times as far away as the moon&mdashan even more astonishing result. Because this distance is so large, nobody was able to measure it until after the invention of the telescope, 1900 years after the time of Aristarchus. But even without a telescope, jy can use the method of Aristarchus to see that the sun must be many times farther away than the moon. Next time you see a half-full moon, look at where the sun is and then try to visualize them in three dimensions!

Vraag: Suppose you see a half-full moon in the southern sky, about half-way between the horizon and zenith. As you look toward it, the illuminated half is on the right. What time of day or night is it?


Moon's path as seen from Earth - Astronomy

I heard in the TV that moon is moving away from the earth towards the sun. Why is that happening? And when was this exactly discovered?

The Moon's orbit (its circular path around the Earth) is indeed getting larger, at a rate of about 3.8 centimeters per year. (The Moon's orbit has a radius of 384,000 km.) I wouldn't say that the Moon is getting closer to the Sun, specifically, though--it is getting farther from the Earth, so, when it's in the part of its orbit closest to the Sun, it's closer, but when it's in the part of its orbit farthest from the Sun, it's farther away.

The reason for the increase is that the Moon raises tides on the Earth. Because the side of the Earth that faces the Moon is closer, it feels a stronger pull of gravity than the center of the Earth. Similarly, the part of the Earth facing away from the Moon feels less gravity than the center of the Earth. This effect stretches the Earth a bit, making it a little bit oblong. We call the parts that stick out "tidal bulges." The actual solid body of the Earth is distorted a few centimeters, but the most noticable effect is the tides raised on the ocean.

Now, all mass exerts a gravitational force, and the tidal bulges on the Earth exert a gravitational pull on the Moon. Because the Earth rotates faster (once every 24 hours) than the Moon orbits (once every 27.3 days) the bulge tries to "speed up" the Moon, and pull it ahead in its orbit. The Moon is also pulling back on the tidal bulge of the Earth, slowing the Earth's rotation. Tidal friction, caused by the movement of the tidal bulge around the Earth, takes energy out of the Earth and puts it into the Moon's orbit, making the Moon's orbit bigger (but, a bit pardoxically, the Moon actually moves slower!).

The Earth's rotation is slowing down because of this. One hundred years from now, the day will be 2 milliseconds longer than it is now.

This same process took place billions of years ago--but the Moon was slowed down by the tides raised on it by the Earth. That's why the Moon always keeps the same face pointed toward the Earth. Because the Earth is so much larger than the Moon, this process, called tidal locking, took place very quickly, in a few tens of millions of years.

Many physicists considered the effects of tides on the Earth-Moon system. However, George Howard Darwin (Charles Darwin's son) was the first person to work out, in a mathematical way, how the Moon's orbit would evolve due to tidal friction, in the late 19th century. He is usually credited with the invention of the modern theory of tidal evolution.

So that's where the idea came from, but how was it first measured? The answer is quite complicated, but I've tried to give the best answer I can, based on a little research into the history of the question.

There are three ways for us to actually measure the effects of tidal friction.

* Measure the change in the length of the lunar month over time.

This can be accomplished by examining the thickness of tidal deposits preserved in rocks, called tidal rhythmites, which can be billions of years old, although measurements only exist for rhythmites that are 900 million years old. As far as I can find (I am not a geologist!) these measurements have only been done since the early 90's.

* Measure the change in the distance between the Earth and the Moon.

This is accomplished in modern times by bouncing lasers off reflectors left on the surface of the Moon by the Apollo astronauts. Less accurate measurements were obtained in the early 70's.

* Measure the change in the rotational period of the Earth over time.

Nowadays, the rotation of the Earth is measured using Very Long Baseline Interferometry, a technique using many radio telescopes a great distance apart. With VLBI, the positions of quasars (tiny, distant, radio-bright objects) can be measured very accuarately. Since the rotating Earth carries the antennas along, these measurements can tell us the rotation speed of the Earth very accurately.

However, the change in the Earth's rotational period was first measured using eclipses, of all things. Astronomers who studied the timing of eclipses over many centuries found that the Moon seemed to be accelerating in its orbit, but what was actually happening was that the Earth's rotation was slowing down. The effect was first noticed by Edmund Halley in 1695, and first measured by Richard Dunthorne in 1748--though neither one really understood what they were seeing. I think this is the earliest discovery of the effect.

This page was last updated on January 28, 2019.

Oor die skrywer

Britt Scharringhausen

Britt studies the rings of Saturn. She got her PhD from Cornell in 2006 and is now a Professor at Beloit College in Wisconson.


Moon's path as seen from Earth - Astronomy

I heard in the TV that moon is moving away from the earth towards the sun. Why is that happening? And when was this exactly discovered?

The Moon's orbit (its circular path around the Earth) is indeed getting larger, at a rate of about 3.8 centimeters per year. (The Moon's orbit has a radius of 384,000 km.) I wouldn't say that the Moon is getting closer to the Sun, specifically, though--it is getting farther from the Earth, so, when it's in the part of its orbit closest to the Sun, it's closer, but when it's in the part of its orbit farthest from the Sun, it's farther away.

The reason for the increase is that the Moon raises tides on the Earth. Because the side of the Earth that faces the Moon is closer, it feels a stronger pull of gravity than the center of the Earth. Similarly, the part of the Earth facing away from the Moon feels less gravity than the center of the Earth. This effect stretches the Earth a bit, making it a little bit oblong. We call the parts that stick out "tidal bulges." The actual solid body of the Earth is distorted a few centimeters, but the most noticable effect is the tides raised on the ocean.

Now, all mass exerts a gravitational force, and the tidal bulges on the Earth exert a gravitational pull on the Moon. Because the Earth rotates faster (once every 24 hours) than the Moon orbits (once every 27.3 days) the bulge tries to "speed up" the Moon, and pull it ahead in its orbit. The Moon is also pulling back on the tidal bulge of the Earth, slowing the Earth's rotation. Tidal friction, caused by the movement of the tidal bulge around the Earth, takes energy out of the Earth and puts it into the Moon's orbit, making the Moon's orbit bigger (but, a bit pardoxically, the Moon actually moves slower!).

The Earth's rotation is slowing down because of this. One hundred years from now, the day will be 2 milliseconds longer than it is now.

This same process took place billions of years ago--but the Moon was slowed down by the tides raised on it by the Earth. That's why the Moon always keeps the same face pointed toward the Earth. Because the Earth is so much larger than the Moon, this process, called tidal locking, took place very quickly, in a few tens of millions of years.

Many physicists considered the effects of tides on the Earth-Moon system. However, George Howard Darwin (Charles Darwin's son) was the first person to work out, in a mathematical way, how the Moon's orbit would evolve due to tidal friction, in the late 19th century. He is usually credited with the invention of the modern theory of tidal evolution.

So that's where the idea came from, but how was it first measured? The answer is quite complicated, but I've tried to give the best answer I can, based on a little research into the history of the question.

There are three ways for us to actually measure the effects of tidal friction.

* Measure the change in the length of the lunar month over time.

This can be accomplished by examining the thickness of tidal deposits preserved in rocks, called tidal rhythmites, which can be billions of years old, although measurements only exist for rhythmites that are 900 million years old. As far as I can find (I am not a geologist!) these measurements have only been done since the early 90's.

* Measure the change in the distance between the Earth and the Moon.

This is accomplished in modern times by bouncing lasers off reflectors left on the surface of the Moon by the Apollo astronauts. Less accurate measurements were obtained in the early 70's.

* Measure the change in the rotational period of the Earth over time.

Nowadays, the rotation of the Earth is measured using Very Long Baseline Interferometry, a technique using many radio telescopes a great distance apart. With VLBI, the positions of quasars (tiny, distant, radio-bright objects) can be measured very accuarately. Since the rotating Earth carries the antennas along, these measurements can tell us the rotation speed of the Earth very accurately.

However, the change in the Earth's rotational period was first measured using eclipses, of all things. Astronomers who studied the timing of eclipses over many centuries found that the Moon seemed to be accelerating in its orbit, but what was actually happening was that the Earth's rotation was slowing down. The effect was first noticed by Edmund Halley in 1695, and first measured by Richard Dunthorne in 1748--though neither one really understood what they were seeing. I think this is the earliest discovery of the effect.

This page was last updated on January 28, 2019.

Oor die skrywer

Britt Scharringhausen

Britt studies the rings of Saturn. She got her PhD from Cornell in 2006 and is now a Professor at Beloit College in Wisconson.


The Moon From the Other Side

A number of people who've seen the annual lunar phase and libration videos have asked what the other side of the Moon looks like, the side that can't be seen from the Earth. This video answers that question. (Update: The video was selected for the SIGGRAPH 2015 Computer Animation Festival.)

Just like the near side, the far side goes through a complete cycle of phases. But the terrain of the far side is quite different. It lacks the large dark spots, called maria, that make up the familiar Man in the Moon on the near side. Instead, craters of all sizes crowd together over the entire far side. The far side is also home to one of the largest and oldest impact features in the solar system, the South Pole-Aitken basin, visible here as a slightly darker bruise covering the bottom third of the disk.

The far side was first seen in a handful of grainy images returned by the Soviet Luna 3 probe, which swung around the Moon in October, 1959. Lunar Reconnaissance Orbiter was launched fifty years later, and since then it has returned hundreds of terabytes of data, allowing LRO scientists to create extremely detailed and accurate maps of the far side. Those maps were used to create the imagery seen here.

In the first of the two viewpoints, the virtual camera is positioned along the Earth-Moon line at a distance of 30 Earth diameters from the Moon and 60 ED from the Earth. The focal length is equivalent to a 2000 mm telephoto lens on a 35 mm SLR, making the horizontal field of view about one degree. The view is consistent with what you might see through an amateur telescope at these distances.

In the second view, the virtual camera is much closer to the Moon, only 1.2 ED, versus 31 ED from Earth. The camera focal length has been reduced to 80 mm, giving a 25° horizontal field. The result is an Earth that appears much smaller, more closely resembling the way it would look to the eye from the surface of the Moon.

We know how the Moon looks from here on Earth. But what does it look like from the other side?

Well for one thing, we can also see the Earth.

The spinning Earth looms large in this time-lapse telescopic view, made possible by computer graphics. We're looking along the imaginary line connecting the Earth and the Moon. From this vantage point, the Moon will be full soon, but on Earth, it's a waning crescent.

The far side of the Moon has fewer of the smooth, dark spots, called maria, that cover the side that faces Earth. Instead, the far side is covered with craters of all sizes.

In this second perspective, we're much closer to the Moon, using a wide-angle lens that makes the distant Earth seem smaller. (music)

With our view fixed on the Moon, the rest of the solar system seems to dance and whirl around us. (music)

Before the Space Age, no one knew what was on the other side of the Moon. Since 2009, Lunar Reconnaissance Orbiter has been making some of the most detailed global maps of the Moon's surface, making it much easier for everyone to see what it's like on the other side.


What does Earth look like from outer space?

What does Earth look like from outer space? And … how far away from Earth can we be and see it still with our own eyes?

To find the answer to these questions, let’s take an imaginary trip through the solar system. Spacecraft exploring our solar system have given us marvelous views of Earth. Keep reading, and check out the photos on this page, to see how Earth looks from various other places in our own neighborhood of space.

First, imagine blasting off and being about 200 miles (300 km) above Earth’s surface. That’s about the height of the orbit of the International Space Station (ISS). From the window of the ISS, the surface of the Earth looms large. In the daytime, you can clearly see major landforms. At night, from Earth orbit, you see the lights of Earth’s cities.

Earth in daylight, from the International Space Station in 2012. The North American Great Lakes shine in the sun. Read more about this image. Earth at night, from the ISS in 2012. Ireland is in the foreground, and the United Kingdom in the back and to the right. A bright sunrise is in the background. Greens and purples show an aurora borealis along the rest of the horizon.

Let’s get farther away, say, the distance of the orbit of the moon.

As we pass the moon – some quarter million miles (about 380,000 km) away – Earth looks like a bright ball in space. It’s not terribly different from the way the moon looks to us.

The first images of the Earth from the moon came from the Apollo mission. Apollo 8 in 1968 was the first human spaceflight to leave Earth orbit. It was the first earthly spacecraft to be captured by and escape from the gravitational field of another celestial body, in this case the moon.

It was the first voyage in which humans visited another world and returned to return to Earth.

Earth seen from moon via Apollo 8 astronauts in 1968. Image via NASA.

In the decades since Voyager first began traveling outward, moon exploration has become more common. The robotic Kaguya spacecraft orbited around Earth’s moon in 2007. Launched by Japan, and officially named the Selenological and Engineering Explorer (SELENE), Kaguya studied the origin and evolution of the moon. The frame below is from Kaguya’s onboard HDTV camera.

Earth viewed from the moon by Kaguya in 2007. Image via SELENE Team JAXA/ NHK. Another image from Kaguya, which got footage and stills of Earth setting. Remember that, if you were on the moon, you would not see Earth rise or set. But spacecraft in orbit around the moon do experience this scene. Image via JAXA.

Now let’s keep moving outward until we can see both the Earth and moon together in space. The next picture was mind-blowing when first released. It shows a crescent-shaped Earth and moon – the first of its kind ever taken by a spacecraft – on September 18, 1977.

This picture of a crescent-shaped Earth and moon – 1st of its kind ever taken by a spacecraft – was recorded September 18, 1977, by Voyager 1 at a distance of 7.25 million miles (11.66 million km) from Earth. The moon is at the top of the picture and beyond the Earth as viewed by Voyager. Image via NASA.

Since 1977, many robot spacecraft have ventured outward into our solar system. The mosaic below shows images of Earth and the moon acquired by the multispectral imager on the Near Earth Asteroid Rendezvous Spacecraft (NEAR) on January 23, 1998, 19 hours after the spacecraft swung by Earth on its way to the asteroid 433 Eros. The images of both were taken from a range of 250,000 miles (400,000 km), approximately the same as the distance between the two bodies.

Earth and moon seen by NEAR spacecraft in 1998.

Speeding outward from the Earth and moon system, you pass the orbits of the planets Mars, Jupiter and Saturn. From all of these worlds, Earth looks like a star, which gets fainter as you get farther away.

Earth and moon, as seen from Mars by NASA’s Curiosity rover on January 31, 2014. Read more about this image. View larger. | Earth seen behind the rings of Saturn. See us in the lower right? Mars and Venus are in the upper left. Image via the Cassini spacecraft, July 19, 2013. This is the famous image known as Pale Blue Dot. It’s a photograph of Earth taken on February 14, 1990, by the Voyager 1 space probe from a record distance of about 6 billion kilometers (3.7 billion miles). Earth is the bluish-white speck approximately halfway down the brown band to the right.

The images above are from Saturn, the sixth planet outward in orbit around the sun. I’ve never seen any image of Earth from Uranus or Neptune or any other body beyond Saturn’s orbit. Only five spacecraft from Earth – the two Voyager spacecraft, the two Pioneers, and the New Horizons spacecraft, which passed Pluto in 2015 – have ever ventured that far. Those craft weren’t designed to look back at Earth, and, to my knowledge, they didn’t capture images of Earth from distances beyond Saturn.

But, speaking theoretically now, could Earth be seen from distances beyond Saturn?

Speaking only in terms of Earth’s brightness, the answer is yes. Our world doesn’t become too faint to see with the eye alone until far beyond Neptune’s orbit, at around 9 billion miles (14 billion km) from home. Now consider Pluto’s orbit. It’s highly elliptical, stretching from just 2.7 billion miles (4.4 billion km) to over 4.5 billion miles (7.3 billion km) from the sun. Pluto is within the limiting distance at which – if we just consider brightness alone, no other factors – we should be able to see Earth with the eye alone.

But there is another factor. As you go outward from Earth, our world appears closer and closer to the blazing sun. As you get farther away, the sun’s glare begins to overwhelm the view of Earth. From Pluto – even though Earth would be helder enough to see – you probably couldn’t see it in the sun’s glare.

So that is the answer to the question of how far you could be from Earth, and still see it with your own eyes. Although no one knows for sure because no one has tried it (and because human eyesight varies from person to person), the Earth would become impossible to see with the eye somewhere beyond Saturn’s orbit.

Now let’s change the game. Let’s say we could use instruments, and not just the eye alone. Suppose intrepid astronaut-astronomers went to Pluto. Suppose they took all the instruments they needed to view Earth in the sun’s glare. Could they use telescopes, obscuring disks, and other techniques to get a glimpse of Earth? Maybe!

But it still wouldn’t be easy.

Bottom line: How does Earth look from space? How far away in space could you view Earth with the eye alone? Considering enigste brightness, the answer is about 9 billion miles (14 billion km) away, about the distance of Neptune or Pluto. In practice, though, seeing it from that distance would be a challenge because the sun’s glare would overwhelm the view of Earth.


Planetary Motions

In Fall 2005 two planets -- Venus and Mars -- will outshine all others in the night sky. We will observe these planets and form a detailed picture of their -- and our -- orbital motion about the Sun.

As seen from the Earth, the Sun, Moon, and planets all appear to move along the ecliptic. More precisely, the ecliptic is the Sun's apparent path among the stars over the course of a year. (Of course, it's actually the Earth that moves about the Sun, and not the other way around, but because of our orbital motion, the Sun seems to move across the backdrop of distant stars.) The planets don't remain exactly on the ecliptic, but they always stay fairly close to it.

Unlike the Sun, however, the planets don't always make steady progress along the ecliptic. They usually move in the same direction as the Sun, but from time to time they seem to slow down, stop, and reverse direction! This retrograde motion was a great puzzle to ancient astronomers. Copernicus gave the correct explanation: all planets, including the Earth, move around the Sun in the same direction retrograde motion is an illusion created when we observe other planets from the moving planet Earth.

It's easiest to understand the retrograde motion of the inner planets, Mercury and Venus. These planets are closer to the Sun than we are, and they orbit the Sun faster than we do. From our point of view, the Sun trundles along the ecliptic (due, of course, to our orbital motion), while Mercury and Venus run rings around the Sun. So at some times we see these planets moving in the same direction as the Sun, while at other times we see them moving in the opposite direction.

For the outer planets, Mars, Jupiter, Saturn, and so on, the explanation is a bit more subtle. These planets are further from the Sun than we are, and they orbit the Sun more slowly than we do. From time to time we pass one of these planets, and when that happens, the planet seems to be moving backwards because we're moving faster than it is. At such times we naturally see the Sun and the planet in opposite parts of the sky the planet is said to be in opposition to the Sun. Opposition is a good time to observe an outer planet it's above the horizon all night, and relatively close to the Earth.

An outer planet's apparent motion is always retrograde for a month or more before and after opposition. The duration of retrograde motion depends on the planet it's shortest for Mars, and generally longest for Pluto. The moment when a planet's apparent motion changes direction is called a stationary point , because at that instant the planet appears to be more or less stationary with respect to the stars. An outer planet always has one stationary point before opposition, and another stationary point after opposition.

Venus and Mars are the two planets that come nearest to the Earth. As all three planets orbit the Sun, the view of our neighbors will constantly change in various ways. By watching the apparent motion, change in distance, and change in phase of these two planets, we can see that many different effects are explained by the one basic idea that all planets orbit the Sun.

THE BIG PICTURE

Fig. 1 shows the orbits of Venus, Earth, and Mars and their positions in Fall 2005. From this diagram we can predict several interesting observational results.

LAB REPORT

Once your observations of Venus and Mars are complete, please write a lab report on this project. If you cannot make enough observations because of bad weather, you can at least use the "planetarium" software to find the position of a planet at any time. Or, in the case of Mars, you can use the maps given in the book "Stars and Planets". Your report should contain an introduction to the problem of planetary motion, a brief section on equipment used, a description of your observations collecting all relevant measurements and plots, and a section describing your conclusions. In this last section, you should compare the predicted behavior of Venus and Mars with your actual observations do they agree?


Though the Moon is often thought of as a nighttime visitor, it&rsquos also visible during the day as a faint, pale presence. The best times to see a daytime Moon are perhaps during the first and last quarter phases, when the Moon is high enough above the horizon and at about 90 degrees from the Sun in the sky. This helps make the Sun&rsquos reflected light bright enough to see as it reflects off of the Moon. The Moon can be seen in the daylit sky at any phase except for the new moon, when it&rsquos invisible to us, and full moon, when it&rsquos below the horizon during the day. The crescent through quarter phases are high in the sky during the day, but the daytime gibbous phases can be glimpsed only just before the Sun sets.

Spend the next month getting to know the Moon and its phases by filling out your own observation journal.

Have you ever wondered when the next full moon will be? How about the first quarter moon? Put the dates and times for all the Moon's phases for the year at your fingertips by building your own Moon Phases Calendar and Calculator!

Moon phases can be hard to visualize. This simple activity uses a lamp, styrofoam ball and pencil to show how phases work.