Sterrekunde

Hoe het antieke sterrekundiges geweet dat hulle die planete van die son na aan die verste van die son moes bestel?

Hoe het antieke sterrekundiges geweet dat hulle die planete van die son na aan die verste van die son moes bestel?


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As ons 'n geosentriese model van die heelal aanneem (soos antieke sterrekundiges gedoen het), hoe sou ons ooit (soos ou antieke sterrekundiges) die regte afstande van die planete kon ontdek? Hoe het hulle byvoorbeeld geweet dat Jupiter verder as Mars is, as Jupiter helderder is?


Volgens die Cambridge Concise History of Astronomy (p. 33 van my uitgawe), het die Grieke in wese die (nie onredelike nie) mening gehuldig dat die planete wat stadiger beweeg verder weg was en om groter sfere wentel.

Dit is natuurlik nie dieselfde as om voor te stel dat hulle 'die regte afstande' na die planete ken nie, maar net die orde. Hulle het wel 'n wiskundig gesonde manier ontwikkel om die afstand na die maan te skat, alhoewel hul waarnemingsgegewens hulle in die steek gelaat het.

Hulle het ook gedink dat die son in die vierde sfeer rondom die aarde was (om die waargenome gedrag van Mercurius en Venus te verklaar).

Nie alle Grieke het gedink dat die son om die aarde draai nie, en diegene wat vir 'n heliosentriese heelal aangevoer het, het gesê dat die redes waarom die sterre nie soos die aarde beweeg het nie, was omdat hulle baie, baie ver weg was.


Hoe het antieke sterrekundiges geweet dat hulle die planete van die son na aan die verste van die son moes bestel? - Sterrekunde

Begin van Sterrekunde

Belangrike punte: Waarom antieke mense sterrekunde en planete op die hemelruim na astronomie nagestreef het, voorbeelde van groot astronomiese monumente

Om die tyd van die jaar ten opsigte van die seisoene te ken, is van kardinale belang om te bepaal wanneer u gewasse moet plant. Dit was duidelik dat die maand was oor 30 dae en die jaar was oor 365 dae of 12 maande, maar nie een van hierdie verhoudings is presies nie, dus moes 'n telskema & quotreset & quot wees sodat dit nie sou dryf nie.

Die eerste sterrewagte wat ons ken, het hierdie probleem aangespreek

Stonehenge is 'n bekende voorbeeld. Dit is onder meer gebruik om die eerste dag van die somer, die kwotsomer-sonstilstand, te identifiseer. & Quot Antieke sterrekundiges kon dit gebruik soos 'n reuse-gunsight om vas te stel wanneer die son in 'n sekere rigting opkom. (Van Getty Trust, http://www.getty.edu/artsednet/images/I/stone2.html)
Hierdie prentjie gee u 'n idee hoe dit in sy beste tyd kon lyk. Dit is van 'n gerekonstrueerde Druïde seremonie in Stonehenge (Van C. Witcombe http://witcombe.sbc.edu/earthmysteries/EMStonehengeB.html)

'N Nog vroeër voorbeeld kan gevind word by Newgrange, net noord van Dublin

Deur sulke waarnemings het die antieke sterrekundiges kalenders ontwikkel wat uiteindelik ontwikkel het tot die een wat ons vandag gebruik

Die vroeë mens het ook geweet watter sterre gesien kan word en hoe dit lyk asof hulle gedurende die nag beweeg, hang af van waar hy op die aarde was:

Hoe verder 'n mens noord beweeg, hoe hoër verskyn die Noord-ster in die lug en dit lyk asof die sterre nader aan parallel aan die horison beweeg.

Hier is hoe die bewegings van Tucson lyk (as ons aanneem dat ons suid kyk en supergroothoekoë het wat van oos na wes kan sien) (animasie deur G. Rieke), moet u dalk herlaai om dit te begin.
en hier is hoe hulle vanaf die Noordpool sou lyk (as ons aanvaar dat ons supergroothoekoog die paalster bo-aan die animasie inneem) (deur G. Rieke) .

Dit is die gedrag wat voorgestel het dat die sterre en planete op groot kristalbolletjies geplaas is wat op die aarde gesentreer en daaromheen gedraai is.

Die afhanklikheid van oënskynlike posisies aan die hemel van die noord-suid-posisie het gelei tot die gebruik van sterre vir navigasie. Gereconstrueerde Griekse Trireme, Olympios, (van Perseus Encyclopedia, http://www.perseus.tufts.edu/cgi-bin/image?lookup=1989.02.0009)
& quot Glorieryke Odysseus, gelukkig met die wind, sprei seile en neem kunstig sit met die stuurspaan wat hy op haar koers hou ['haar' is 'n vlot wat hy gebou het om weg te vaar van die nimf Kalypso], en slaap het ook nooit neergedaal nie sy ooglede terwyl hy die Pleiades ['n sterretros] en die laat-sitende Bootes ['n konstellasie, 'n identifiseerbare patroon van sterre in die lug] en die Beer [die konstellasie wat nou bekend staan ​​as die Groot Dipper], hou mans gee ook die naam van die wa, wat op 'n vaste plek draai en na Orion kyk ('n ander konstellasie wat maklik geïdentifiseer kan word), en sy alleen word nooit in die was van die oseaan gedompel nie. Want so het Kalypso, helder onder godinne, vir hom gesê om oor die see te gaan en die Beer aan sy linkerhand te hou. & Quot

Sommige voorwerpe blyk ligpunte te wees wat met betrekking tot die sterre beweeg. Die Grieke het hierdie tipe voorwerpe die naam & quotplanet & quot vir swerwer gegee. Kwik, Venus, Mars, Jupiter en Saturnus is al sedert die prehistoriese tyd bekend.

Hierdie komplikasies het gelei tot gevorderde stelsels van sterrekunde / astrologie. Om dramatiese hemelse gebeure te voorspel, het die sterrekykers / priesters / sterrekykers akkurate langtermynkalenders en -tekste ontwikkel.

Die Maya's van Meso-Amerika bied 'n voorbeeld van groot prestasies in die sterrekunde wat hulle beliggaam in godsdienstige / seremoniële aspekte van hul kultuur.

Om sulke groot monumente te bou, het dus aansienlike beleggings in sterrekunde sowel as argitektuur vereis 'N Voorbeeld is El Caracol, 'n Maya-sterrewag in Chichen Itza, nie ver van El Castillo nie (G. Rieke )
Tempels in Tikal, Guatemala, is bo die boomtoppe verhef om die sterre onbelemmerd te besigtig. Tempel IV by Tikal, gebou in ongeveer 470 nC, styg tot 'n hoogte van 212 voet bo die oerwoudvloer . (G. Rieke)
Modern (dae) Maya in dae
Maan (sinodiese) maand 29.53059 29.53086
Sinodiese periode van Venus 583.93 583.92027
Sinodiese periode van Mars 779.94 780
Son (tropiese) jaar 365.24198 365.242

Hierdie prestasie is egter meer gerig op die voorspelling van gunstige tye vir heersers om tot aksie oor te gaan, dit wil sê om die toekoms te voorspel. Dit hoort meer op die gebied van 'n baie gesofistikeerde astrologie as in die sterrekunde. Die mislukking van die Mayas om onderliggende oorsake te soek vir die prosesse wat hulle so presies gemeet het, het beteken dat hul benadering wetenskaplik 'n doodloopstraat was.

Nie alle antieke samelewings was so suksesvol in die sterrekunde soos die Maya's nie. Byna almal het egter mites wat verband hou met die hemel en die aarde, veral skeppingsmites In die algemeen word sulke mites nie verfyn deur middel van gedetailleerde metings nie (soos in die Maya-tabel hierbo) en kan dit nie eers bygewerk word nie, aangesien nuwe astronomiese kennis opgedoen word. Dit is dus basies nie voorbeelde van 'n wetenskaplike benadering nie.


1. Die planete wentel om die Son

Enkele eeue later was daar baie vordering. Aristarchus van Samos (310 vC tot 230 vC) het aangevoer dat die son die 'sentrale vuur' van die kosmos was en hy het al die destydse bekende planete in hul regte volgorde van afstand daaroor geplaas. Dit is die vroegste bekende heliosentriese teorie van die sonnestelsel.

Ongelukkig het die oorspronklike teks waarin hy hierdie argument voer, verlore gegaan vir die geskiedenis, dus kan ons nie seker weet hoe hy dit uitgewerk het nie. Aristarchus het geweet dat die son veel groter was as die aarde of die maan, en hy het moontlik vermoed dat dit dus die sentrale posisie in die sonnestelsel moes wees.

Nietemin is dit 'n skokkende bevinding, veral as u van mening is dat dit eers in die 16de eeu herontdek is, deur Nicolaus Copernicus, wat selfs Aristarchus erken het tydens die ontwikkeling van sy eie werk.


Wat is die orde van die planete in die sonnestelsel?

Die afgelope 60 jaar het mense ons sonnestelsel ernstig begin verken. Van die eerste lanserings in die laat 1950's tot vandag toe het ons sondes, wentelbane, landers en selfs rovers (soos NASA se Perseverance Rover wat in Februarie 2021 op Mars geraak het) na elke planeet in ons sonnestelsel gestuur. Maar kan u al agt van die planete noem? (Ja, daar is net agt - nie nege nie. Pluto het in 2006 & quot; verlaag & quot;) En kan u dit in die regte volgorde plaas?

As u 'n bietjie verroes is, bespreek ons ​​'n paar algemene maniere om die planete te bestel, asook 'n paar truuks om u te help om dit vorentoe te onthou. Kom ons begin met afstand van die son af.

Die orde van die planete op afstand

Die mees algemene manier om die planete te orden, is op afstand van die son. Met behulp van hierdie metode word die planete in die volgende volgorde gelys:

  • Mercurius - 0,39 AU van die son af
  • Venus - 0,72 AU
  • Aarde - 1,00 AU
  • Mars - 1,52 AU
  • Jupiter - 5.20 AU
  • Saturnus - 9.54 AU
  • Uranus - 19.20 AU
  • Neptunus - 30.06 AU

AU staan ​​vir astronomiese eenhede - dit is gelykstaande aan die gemiddelde afstand van die aarde tot die son (daarom is die aarde 1 AE van die son). Dit is 'n algemene manier waarop sterrekundiges afstande in die sonnestelsel meet, wat die groot skaal van hierdie afstande uitmaak. Om dit anders te stel, Mercurius, wat die naaste is, is 35,98 miljoen myl van die son af, terwyl Neptunus, die verste, 2,79 is miljard kilometers van die son af. Die aarde is 92,96 miljoen myl van die son af.

Hoe om die orde van die planete te onthou

Daar is baie handige uitdrukkings om die volgorde van die planete te onthou. Dit is gewoonlik geheueherinnerings wat die eerste letter van elke planeet se naam gebruik om 'n frase uit te dink wat makliker is om te onthou.

  • My baie uitstekende ma het net vir ons noedels (of Nachos) bedien
  • My baie maklike metode versnel net name
  • My baie duur Malamute-skip het noord gespring

In elk geval staan ​​& quotM & quot vir & quotMercury, & quot & quotV & quot vir & quotVenus, & quot, ensovoorts. U kan dit ook met 'n paar rymverse probeer onthou:

Ten slotte, as u musikaal geneig is, is daar 'n paar liedjies wat u kan help om dit te onthou. Twee gewildes is Mr. R's Planet Song en The Planet Song van Kids Learning Tube.

U kan die planete op ander maniere bestel

Terwyl die meeste mense die volgorde van die planete op afstand wil ken, is daar ander maniere om die planete te orden waaroor u nuuskierig kan wees.

As u byvoorbeeld die planete volgens grootte (radius) van die grootste tot die kleinste bestel, dan is die lys:

  • Jupiter (43.441 myl / 69.911 kilometer)
  • Saturnus (58,232 km / 58,232 km)
  • Uranus (25 362 km)
  • Neptunus (24.222 km)
  • Aarde (6.359 km)
  • Venus (6,052 km)
  • Mars (3 360 km)
  • Kwik (2.440 km)

Of u kan die planete volgens gewig (massa) bestel. Dan sal die lys van die massiefste tot die minste massiewe wees: Jupiter (1.8986 x 10 27 kilogram), Saturnus (5.6846 x 10 26 kg), Neptunus (10.243 x 10 25 kg), Uranus (8.6810 x 10 25 kg), Aarde (5.9736 x 10 24 kg), Venus (4.8685 x 10 24 kg), Mars (6.4185 x 10 23 kg) en Mercurius (3.3022 x 10 23 kg). Dit is interessant dat Neptunus meer massa het as Uranus, alhoewel Uranus groter is! Wetenskaplikes kan nie 'n planeet op 'n skaal plaas nie. Om massa te bepaal, kyk hulle hoe lank dit nabygeleë voorwerpe neem om die planeet te wentel en hoe ver die voorwerpe van die planeet is. Hoe swaarder die planeet is, hoe sterker trek hy na nabygeleë voorwerpe.

Ten slotte is 'n prettige manier om die planete te bestel volgens die aantal mane wat hulle het. Ons begin met die planeet wat die meeste het:

  • Saturnus (82)
  • Jupiter (79)
  • Uranus (27)
  • Neptunus (14)
  • Mars (2)
  • Aarde (1)
  • Venus en Mercurius (albei nul)

(Let op dat hierdie getalle voorlopige mane bevat wat nog deur sterrekundiges bevestig word.)

Kortom, daar is 'n aantal maniere om die planete te orden en te orden op grond van verskillende feite daaroor, solank jy onthou dat daar altesaam agt is, dit is wat tel. (Jammer, Pluto!)

Van Pluto gepraat, wat is die saak? Na sy ontdekking in 1930 is Pluto as 'n planeet geklassifiseer. In 2006 het die Internasionale Astronomiese Unie Pluto afgegradeer van & quotplanet & quot tot & quotdwarf planeet. & Quot Dit is omdat die definisie van 'n planeet beteken dat hy sy baan van ander voorwerpe skoongemaak het (wat Pluto nog nie gedoen het nie, omdat hy sy ruimte met baie mense deel. Kuiper-gordel-voorwerpe). Pluto is een van vyf dwergplanete in ons sonnestelsel - en dit is nie eens die grootste een nie (dit is Eris).


2.2 Antieke Sterrekunde

Kom ons kyk nou kort terug in die geskiedenis. 'N Groot deel van die moderne Westerse beskawing is op die een of ander manier afgelei van die idees van die antieke Grieke en Romeine, en dit geld ook in die sterrekunde. Baie ander ou kulture het egter ook gesofistikeerde stelsels ontwikkel om die lug waar te neem en te interpreteer.

Sterrekunde regoor die wêreld

Antieke Babiloniese, Assiriese en Egiptiese sterrekundiges het die geskatte lengte van die jaar geken. Die Egiptenare van 3000 jaar gelede het byvoorbeeld 'n kalender aangeneem gebaseer op 'n jaar van 365 dae. Hulle het die stygende tyd van die helder ster Sirius in die vroeë hemelhemel deeglik dopgehou, wat jaarliks ​​'n siklus het wat ooreenstem met die oorstroming van die Nylrivier. Die Chinese het ook 'n werkende kalender gehad, en hulle het die lengte van die jaar op dieselfde tyd as die Egiptenare bepaal. Die Chinese het ook komete, helder meteore en donker kolle op die son aangeteken. (Baie soorte astronomiese voorwerpe is in die Wetenskap en die heelal bekendgestel: 'n kort toer. As u nie terme soos komete en meteore, wil u dalk die hoofstuk hersien.) Later het Chinese sterrekundiges sorgvuldig boekgehou van 'gaststerre' - diegene wat normaalweg te flou is om te sien, maar skielik opvlam om vir 'n paar weke of maande sigbaar te word. Ons gebruik nog steeds sommige van hierdie plate in die bestudering van sterre wat lank gelede ontplof het.

Die Maya-kultuur in Mexiko en Sentraal-Amerika het 'n gesofistikeerde kalender ontwikkel wat gebaseer was op die planeet Venus, en hulle het duisend jaar gelede astronomiese waarnemings gedoen vanaf webwerwe wat daaraan toegewy is. Die Polinesiërs het geleer om by honderde kilometers oop oseaan by die sterre te navigeer - 'n vaardigheid wat hulle in staat gestel het om nuwe eilande ver van die plek waar hulle begin het, te koloniseer.

In Brittanje, voor die wydverspreide gebruik van skrif, het antieke mense klippe gebruik om die bewegings van die son en die maan by te hou. Ons vind nog steeds enkele van die groot klipsirkels wat hulle vir hierdie doel gebou het, en dateer van so ver terug as 2800 VHJ. Die bekendste hiervan is Stonehenge, wat in die aarde, maan en lug bespreek word. 1

Vroeë Griekse en Romeinse kosmologie

Ons konsep van die kosmos - die basiese struktuur en oorsprong daarvan - word kosmologie genoem, 'n woord met Griekse wortels. Voordat die teleskope uitgevind is, moes mense afhanklik wees van die eenvoudige bewyse van hul sintuie vir 'n beeld van die heelal. Die ou mense het kosmologieë ontwikkel wat hul direkte siening van die hemel gekombineer het met 'n ryk verskeidenheid filosofiese en godsdienstige simboliek.

Ten minste 2000 jaar voor Columbus het opgeleide mense in die oostelike Mediterreense streek geweet dat die aarde rond is. Die geloof in 'n sferiese aarde het moontlik gespruit uit die tyd van Pythagoras, 'n filosoof en wiskundige wat 2500 jaar gelede geleef het. Hy het geglo dat sirkels en sfere 'perfekte vorme' was en het voorgestel dat die aarde dus 'n sfeer moes wees. As bewys dat die gode van sfere gehou het, noem die Grieke die feit dat die Maan 'n sfeer is, en gebruik bewyse wat ons later beskryf.

Die geskrifte van Aristoteles (384–322 VHJ), die tutor van Alexander die Grote, som baie van die idees van sy tyd saam. Hulle beskryf hoe die vordering van die maan se fases - sy skynbaar veranderende vorm - die gevolg is daarvan dat ons verskillende gedeeltes van die maan se sonlig-halfrond sien soos die maand verloop (sien Aarde, Maan en Hemel). Aristoteles het ook geweet dat die son verder van die aarde af moet wees as wat die maan is, want soms het die maan presies tussen die aarde en die son verbygegaan en die son tydelik weggesteek. Ons noem dit 'n sonsverduistering.

Aristoteles het oortuigende argumente aangehaal dat die aarde rond moet wees. Eerstens is die feit dat wanneer die maan tydens 'n verduistering van die maan uit die aarde se skaduwee binnekom of uitkom, die vorm van die skaduwee wat op die maan gesien word, altyd rond is (Figuur 2.9). Slegs 'n sferiese voorwerp produseer altyd 'n ronde skaduwee. As die aarde byvoorbeeld 'n skyf was, sou die sonlig dit op die randjie kon tref en sy skaduwee op die maan 'n lyn sou wees.

As 'n tweede argument het Aristoteles verduidelik dat reisigers wat 'n groot afstand suidwaarts gaan sterre kan sien wat nie verder noord sigbaar is nie. En die hoogte van die Noordster - die ster die naaste aan die noordelike hemelpool - neem af namate 'n reisiger suid beweeg. Op 'n plat aarde sou almal dieselfde sterre bokant sien. Die enigste moontlike verklaring is dat die reisiger oor 'n geboë oppervlak op die aarde moes beweeg het en sterre vanuit 'n ander hoek moes vertoon. (Sien die funksie Hoe weet ons dat die aarde rond is? Vir meer idees om te bewys dat die aarde rond is.)

Een Griekse denker, Aristarchus van Samos (310-230 v.G.J.), het selfs voorgestel dat die aarde om die son beweeg, maar Aristoteles en die meeste antieke Griekse geleerdes het hierdie idee verwerp. Een van die redes vir hul gevolgtrekking was die gedagte dat as die aarde om die son beweeg, hulle die sterre vanaf verskillende plekke langs die aarde sou sien. Namate die aarde beweeg, moet nabygeleë sterre hul posisies in die lug verskuif in vergelyking met sterre in die verte. Op 'n soortgelyke manier sien ons voorgrondvoorwerpe lyk asof dit teen 'n verre agtergrond beweeg wanneer ons beweeg. As ons met 'n trein ry, lyk dit asof die bome op die voorgrond hul posisie skuif ten opsigte van heuwels in die verte as die trein verbyrol. Onbewustelik gebruik ons ​​hierdie verskynsel om die afstande rondom ons te skat.

Die skynbare verskuiwing in die rigting van 'n voorwerp as gevolg van die beweging van die waarnemer word parallaks genoem. Ons noem die verskuiwing in die oënskynlike rigting van 'n ster as gevolg van die aarde se wentelbeweging sterre parallaks. Die Grieke het toegewyde pogings aangewend om die sterre-parallaks waar te neem en selfs die hulp van Griekse soldate met die duidelikste visie in te roep, maar dit was tevergeefs. Dit lyk asof die helderder (en vermoedelik nader) sterre net nie verskuif nie, aangesien die Grieke dit in die lente waargeneem het en dan weer in die herfs (wanneer die aarde aan die oorkant van die son is).

Dit het beteken dat die aarde nie beweeg het nie, of dat die sterre so ver moes wees dat die parallaksverskuiwing onmeetlik klein was. 'N Kosmos van so 'n enorme mate het 'n sprong van verbeelding vereis wat die meeste antieke filosowe nie bereid was om te maak nie; daarom het hulle teruggetrek na die veiligheid van die Aardgesentreerde beskouing, wat die Westerse denke vir byna twee millennia sou oorheers.

Basiese beginsels oor sterrekunde

Hoe weet ons dat die aarde rond is?

Benewens die twee maniere (uit die geskrifte van Aristoteles) wat in hierdie hoofstuk bespreek word, kan u ook soos volg redeneer:

  1. Kom ons kyk hoe 'n skip sy hawe verlaat en op 'n helder dag in die verte vaar. Op 'n plat aarde sou ons sien dat die skip net al hoe kleiner word as dit wegseil. Maar dit is nie wat ons eintlik waarneem nie. In plaas daarvan sink skepe onder die horison, met die romp wat eers verdwyn en die mas 'n rukkie langer sigbaar bly. Uiteindelik kan slegs die bokant van die mas gesien word terwyl die skip om die kromming van die aarde vaar. Uiteindelik verdwyn die skip onder die horison.
  2. Die Internasionale Ruimtestasie sirkel die aarde so ongeveer 90 minute. Foto's wat van die pendeltuig en ander satelliete geneem word, wys dat die aarde vanuit elke perspektief rond is.
  3. Gestel jy het 'n vriend in elke tydsone van die aarde gemaak. U bel almal op dieselfde uur en vra: "Waar is die son?" Op 'n plat aarde sal elke oproeper u ongeveer dieselfde antwoord gee. Maar op 'n ronde aarde sou u sien dat die son vir sommige vriende hoog in die lug sou wees, terwyl dit vir ander sou opkom, ondergaan of heeltemal buite sig sou wees (en hierdie laaste groep vriende sou vir u ontsteld wees oor maak hulle wakker).

Meting van die aarde deur Eratosthenes

Die Grieke het nie net geweet dat die aarde rond was nie, maar ook die grootte daarvan kon meet. Die eerste redelik akkurate bepaling van die deursnee van die aarde is in ongeveer 200 v.G.J. deur Eratosthenes (276–194 vC), 'n Griek wat in Alexandrië, Egipte, woon. Sy metode was geometries, gebaseer op die waarnemings van die son.

Die son is so ver van ons af dat al die ligstrale wat ons planeet tref, ons oor wese parallelle lyne nader. Kyk na Figuur 2.10 om te sien waarom. Neem 'n ligbron naby die aarde — sê, op posisie A. Die strale tref verskillende dele van die aarde langs uiteenlopende paaie. Vanuit 'n ligbron by B of by C (wat nog verder weg is), is die hoek tussen strale wat teenoorgestelde dele van die aarde tref, kleiner. Hoe verder die bron, hoe kleiner is die hoek tussen die strale. Vir 'n bron wat oneindig ver is, beweeg die strale langs parallelle lyne.

Natuurlik is die son nie oneindig ver weg nie, maar gegewe sy afstand van 150 miljoen kilometer, lig ligstrale wat die aarde tref vanaf 'n punt op die son met mekaar af deur 'n hoek te veel om met die blote oog waargeneem te word. As gevolg hiervan, as mense regoor die aarde wat die son kon sien, daarna sou wys, sou hul vingers in wese almal parallel met mekaar wees. (Dieselfde geld ook vir die planete en sterre - 'n idee wat ons sal gebruik in ons bespreking van hoe teleskope werk.)

Eratosthenes is meegedeel dat sonlig op die eerste dag van die somer in Syene, Egipte (naby die moderne Aswan), die middaguur die bodem van 'n vertikale put getref het. Dit het aangedui dat die son direk oor die put was - wat beteken dat Syene op 'n direkte lyn vanaf die middelpunt van die aarde na die son was. Op die ooreenstemmende tyd en datum in Alexandrië het Eratosthenes die skaduwee gesien wat 'n kolom gemaak het en gesien dat die son nie direk bokant was nie, maar effens suid van die hoogtepunt, sodat die strale daarvan 'n hoek gemaak het met die vertikale gelyk aan ongeveer 1/50 van 'n sirkel (7 °). Omdat die sonstrale wat die twee stede tref, parallel met mekaar is, waarom sou die twee strale nie dieselfde hoek met die aarde se oppervlak maak nie? Eratosthenes het geredeneer dat die kromming van die ronde aarde beteken dat 'regop' in die twee stede nie dieselfde was nie. En die meting van die hoek in Alexandrië, besef hy, het hom in staat gestel om die grootte van die aarde te bepaal. Alexandria, het hy gesien, moet 1/50 van die aarde se omtrek noord van Syene wees (Figuur 2.11). Alexandrië is gemeet tot 5000 stadions noord van Syene. (Die stadion was 'n Griekse lengte-eenheid, afgelei van die lengte van die renbaan in 'n stadion.) Eratosthenes het dus bevind dat die aarde se omtrek 50 × 5000, of 250 000 stadia moet wees.

Dit is nie moontlik om die akkuraatheid van die Eratosthenes-oplossing presies te evalueer nie, want daar bestaan ​​twyfel oor watter van die verskillende soorte Griekse stadia hy as sy afstandseenheid gebruik het. As dit die algemene Olimpiese stadion was, is sy uitslag ongeveer 20% te groot. Volgens 'n ander interpretasie gebruik hy 'n stadion gelyk aan ongeveer 1/6 kilometer, in welke geval sy syfer binne 1% van die korrekte waarde van 40.000 kilometer was. Al was sy meting nie presies nie, was sy sukses om die grootte van ons planeet te meet deur slegs skaduwees, sonlig en die krag van menslike denke te gebruik, een van die grootste intellektuele prestasies in die geskiedenis.

Hipparchus en presisie

Miskien was die grootste sterrekundige in die oudheid Hipparchus, gebore in Nicea in die huidige Turkye. Hy het 'n sterrewag op die eiland Rhodes opgerig omstreeks 150 VC, toe die Romeinse Republiek sy invloed in die hele Middellandse See-gebied uitgebrei het. Daar het hy die posisies van voorwerpe in die lug so akkuraat moontlik gemeet en 'n baanbrekende sterrekatalogus saamgestel met ongeveer 850 inskrywings. Hy het hemelkoördinate vir elke ster aangewys en die ligging in die lug gespesifiseer, net soos ons die posisie van 'n punt op die aarde spesifiseer deur die breedtegraad en lengtegraad te gee.

Hy het ook die sterre in skynbare groottes verdeel volgens hul skynbare helderheid. Hy het die helderstes 'sterre van die eerste grootte' genoem, die volgende helderste groep, 'sterre van die tweede grootte' en so meer. Hierdie taamlik arbitrêre stelsel, in gewysigde vorm, bly vandag nog gebruik (hoewel dit al hoe minder nuttig is vir professionele sterrekundiges).

Deur die sterre waar te neem en sy data met ouer waarnemings te vergelyk, het Hipparchus een van sy merkwaardigste ontdekkings gemaak: die posisie in die hemelruim van die noordelike hemelpool het gedurende die vorige eeu en 'n half verander. Hipparchus het korrek afgelei dat dit nie net gedurende die tydperk wat deur sy waarnemings gedek is, gebeur het nie, maar in werklikheid die hele tyd plaasgevind het: die rigting waarin die lug blyk te draai, verander stadig maar aanhoudend. Onthou uit die gedeelte oor hemelpale en die hemelse ewenaar dat die noordelike hemelpaal net die projeksie van die Aarde se Noordpool in die lug is. As die noordelike hemelpaal rondwoel, moet die aarde self wankel. Vandag verstaan ​​ons dat die rigting waarin die Aarde se as wys, wel stadig maar gereeld verander - 'n beweging wat ons presessie noem. As u ooit na 'n draaitol gekyk het, het u 'n soortgelyke beweging waargeneem. Die as van die bokant beskryf 'n pad in die vorm van 'n keël, terwyl die Aarde se swaartekrag dit probeer omver te werp (Figuur 2.12).

Omdat ons planeet nie 'n presiese sfeer is nie, maar 'n bietjie by die ewenaar uitbult, laat die son en die maan hom trek soos 'n bokant. Dit neem ongeveer 26 000 jaar voordat die aarde se as een sirkel van presessie voltooi het. As gevolg van hierdie beweging, verander die punt waar ons as in die lug wys na verloop van tyd. Terwyl Polaris vandag die ster is wat die naaste aan die noordelike hemelpool is (dit sal sy naaste punt bereik rondom 2100), sal die ster Vega in die konstellasie Lyra die Noordster oor 14 000 jaar wees.

Ptolemeus se model van die sonnestelsel

Die laaste groot sterrekundige van die Romeinse era was Claudius Ptolemeus (of Ptolemaeus), wat in ongeveer 140 jaar in Alexandrië floreer. Hy het 'n reuse-samestelling van astronomiese kennis geskryf, wat vandag sy Arabiese naam heet, Almagest (wat “Die Grootste” beteken). Almagest handel nie uitsluitlik oor Ptolemeus se eie werk nie, dit bevat 'n bespreking van die astronomiese prestasies van die verlede, hoofsaaklik dié van Hipparchus. Vandag is dit ons vernaamste bron van inligting oor die werk van Hipparchus en ander Griekse sterrekundiges.

Ptolemeus se belangrikste bydrae was 'n geometriese voorstelling van die sonnestelsel wat die posisies van die planete vir enige gewenste datum en tyd voorspel het. Hipparchus, wat nie genoeg data byderhand het om die probleem self op te los nie, het eerder waarnemingsmateriaal bymekaargemaak vir die nageslag. Ptolemeus het hierdie materiaal met nuwe waarnemings van sy eie aangevul en 'n kosmologiese model opgestel wat meer as duisend jaar geduur het, tot die tyd van Copernicus.

Die ingewikkelde faktor om die bewegings van die planete te verklaar, is dat hul skynbare dwaal in die lug die gevolg is van die kombinasie van hul eie bewegings met die Aarde se wentelbaanrevolusie. As ons die planete vanaf ons uitkykpunt op die bewegende Aarde dophou, is dit 'n bietjie soos om na 'n motorwedren te kyk terwyl u daaraan meeding. Soms gaan teenstanders se motors by jou verby, maar op ander tye gaan jy verby hulle, sodat dit lyk asof hulle 'n rukkie agteruit beweeg ten opsigte van jou.

Figuur 2.13 toon die beweging van die aarde en 'n planeet verder van die son — in hierdie geval, Mars. Die aarde beweeg om die son in dieselfde rigting as die ander planeet en in byna dieselfde vlak, maar sy wentelsnelheid is vinniger. As gevolg hiervan haal dit die planeet periodiek in, soos 'n vinniger renmotor op die binnebaan. Die figuur wys waar ons die planeet op verskillende tye in die lug sien. Die pad van die planeet tussen die sterre word in die sterveld aan die regterkant van die figuur geïllustreer.

Skakel na leer

Met die planetêre konfigurasiesimulator van Foothill AstroSims kan u die gewone progressie en af ​​en toe retrograde beweging van ander planete sien. U kan heen en weer wissel tussen die kykbeweging vanaf die aarde en Mars (sowel as ander planete).

Normaalweg beweeg planete oor die weke en maande ooswaarts in die lug as hulle om die Son wentel, maar van posisies B tot D in Figuur 2.13, terwyl die aarde in ons voorbeeld deur die planete beweeg, lyk dit asof dit agteruit dryf en weswaarts in die lug beweeg. Alhoewel dit eintlik na die ooste beweeg, het die vinniger bewegende aarde dit verbygesteek en lyk dit uit ons perspektief om dit agter te laat. Terwyl die aarde sy baan in die rigting van E draai, neem die planeet weer sy skynbare oostelike beweging in die lug op. Die tydelike skynbare weswaartse beweging van 'n planeet as die aarde tussen hom en die son swaai, word retrograde beweging genoem. Sulke agteruitgang is vir ons baie makliker om vandag te verstaan, noudat ons weet dat die Aarde een van die bewegende planete is en nie die middelpunt van die hele skepping nie. Maar Ptolemeus het voor die veel ingewikkelder probleem te staan ​​gekom om sulke bewegings te verklaar terwyl hy 'n stilstaande Aarde aanneem.

Verder, omdat die Grieke van mening was dat hemelse bewegings sirkels moes wees, moes Ptolemeus sy model slegs met behulp van sirkels konstrueer. Om dit te doen, het hy tientalle sirkels nodig gehad, sommige beweeg in ander kringe, in 'n komplekse struktuur wat 'n moderne kyker duiselig maak. Maar ons moenie toelaat dat ons moderne oordeel ons bewondering vir Ptolemeus se prestasie vertroebel nie. In sy tyd was 'n komplekse heelal wat op die aarde gerig was, heeltemal redelik en op sy eie manier redelik mooi. Soos berig is, het Alfonso X, die koning van Castilië, gesê nadat hy die Ptolemeïese stelsel van planeetbewegings aan hom laat verduidelik het: 'As die Here die Almagtige my geraadpleeg het voordat hy die Skepping aangepak het, sou ek iets eenvoudiger moes aanbeveel.'

Ptolemeus het die probleem opgelos om die waargenome bewegings van planete te verklaar deur elke planeet te laat draai in 'n klein baan wat 'n episiklus genoem word. Die middelpunt van die fiets het dan om die aarde gedraai in 'n sirkel genaamd a uitstel (Figuur 2.14). Wanneer die planeet op posisie is x in Figuur 2.14 op die wentelbaan wentel dit in dieselfde rigting as die middelpunt van die fiets vanaf die aarde, dit lyk asof die planeet ooswaarts beweeg. Wanneer die planeet by is yegter, die beweging daarvan is in die teenoorgestelde rigting van die beweging van die middelpunt van die e-fiets rondom die aarde. Deur die regte kombinasie van snelhede en afstande te kies, het Ptolemaeus daarin geslaag om die planeet teen die regte spoed en vir die regte tydsgewrig weswaarts te laat beweeg en sodoende die retrograde beweging met sy model te herhaal.

Skakel na leer

Gebruik die Ptolemaïese Stelselsimulator van Foothill AstroSims om te ondersoek hoe Ptolemeus se stelsel van afleidings en episiklusse die skynbare beweging van die planete verklaar.

Ons sal egter in wentelbane en swaartekrag sien dat die planete, soos die aarde, om die son beweeg in ellipse, nie sirkels nie. Hul werklike gedrag kan nie akkuraat voorgestel word deur 'n skema van eenvormige sirkelbewegings nie. Om die waargenome bewegings van die planete te laat ooreenstem, moes Ptolemeus die uitstel sirkels sentreer, nie op die aarde nie, maar op 'n afstand van die aarde af. Daarbenewens het hy eenvormige sirkelbeweging om nog 'n ander as ingestel, genaamd die gelykstaande punt. Al hierdie dinge het sy plan aansienlik bemoeilik.

Dit is 'n huldeblyk aan die genie van Ptolemeus as wiskundige dat hy so 'n ingewikkelde stelsel kon ontwikkel om die waarnemings van planete suksesvol te verreken. It may be that Ptolemy did not intend for his cosmological model to describe reality, but merely to serve as a mathematical representation that allowed him to predict the positions of the planets at any time. Whatever his thinking, his model, with some modifications, was eventually accepted as authoritative in the Muslim world and (later) in Christian Europe.


The Inner Planets:

The four inner planets are called terrestrial planets because their surfaces are solid (and, as the name implies, somewhat similar to Earth — although the term can be misleading because each of the four has vastly different environments). They’re made up mostly of heavy metals such as iron and nickel, and have either no moons or few moons. Below are brief descriptions of each of these planets based on this information from NASA.

Mercury: Mercury is the smallest planet in our Solar System and also the closest. It rotates slowly (59 Earth days) relative to the time it takes to rotate around the sun (88 days). The planet has no moons, but has a tenuous atmosphere (exosphere) containing oxygen, sodium, hydrogen, helium and potassium. The NASA MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) spacecraft is currently orbiting the planet.

The terrestrial planets of our Solar System at approximately relative sizes. From left, Mercury, Venus, Earth and Mars. Credit: Lunar and Planetary Institute

Venus: Venus was once considered a twin planet to Earth, until astronomers discovered its surface is at a lead-melting temperature of 900 degrees Fahrenheit (480 degrees Celsius). The planet is also a slow rotator, with a 243-day long Venusian day and an orbit around the sun at 225 days. Its atmosphere is thick and contains carbon dioxide and nitrogen. The planet has no rings or moons and is currently being visited by the European Space Agency’s Venus Express spacecraft.

Earth: Earth is the only planet with life as we know it, but astronomers have found some nearly Earth-sized planets outside of our solar system in what could be habitable regions of their respective stars. It contains an atmosphere of nitrogen and oxygen, and has one moon and no rings. Many spacecraft circle our planet to provide telecommunications, weather information and other services.

Mars: Mars is a planet under intense study because it shows signs of liquid water flowing on its surface in the ancient past. Today, however, its atmosphere is a wispy mix of carbon dioxide, nitrogen and argon. It has two tiny moons (Phobos and Deimos) and no rings. A Mars day is slightly longer than 24 Earth hours and it takes the planet about 687 Earth days to circle the Sun. There’s a small fleet of orbiters and rovers at Mars right now, including the large NASA Curiosity rover that landed in 2012.

The outer planets of our Solar System at approximately relative sizes. From left, Jupiter, Saturn, Uranus and Neptune. Credit: Lunar and Planetary Institute


The Science: Orbital Mechanics

Kepler&rsquos Laws of Planetary Motion

While Copernicus rightly observed that the planets revolve around the Sun, it was Kepler who correctly defined their orbits. At the age of 27, Kepler became the assistant of a wealthy astronomer, Tycho Brahe, who asked him to define the orbit of Mars. Brahe had collected a lifetime of astronomical observations, which, on his death, passed into Kepler&rsquos hands. (Brahe, who had his own Earth-centered model of the Universe, withheld the bulk of his observations from Kepler at least in part because he did not want Kepler to use them to prove Copernican theory correct.) Using these observations, Kepler found that the orbits of the planets followed three laws.

Like many philosophers of his era, Kepler had a mystical belief that the circle was the Universe&rsquos perfect shape, and that as a manifestation of Divine order, the planets&rsquo orbits must be circular. For many years, he struggled to make Brahe&rsquos observations of the motions of Mars match up with a circular orbit.

Eventually, however, Kepler noticed that an imaginary line drawn from a planet to the Sun swept out an equal area of space in equal times, regardless of where the planet was in its orbit. If you draw a triangle out from the Sun to a planet&rsquos position at one point in time and its position at a fixed time later&mdashsay, 5 hours, or 2 days&mdashthe area of that triangle is always the same, anywhere in the orbit. For all these triangles to have the same area, the planet must move more quickly when it is near the Sun, but more slowly when it is farthest from the Sun.

This discovery (which became Kepler&rsquos second law of orbital motion) led to the realization of what became Kepler&rsquos first law: that the planets move in an ellipse (a squashed circle) with the Sun at one focus point, offset from the center.

Kepler&rsquos third law shows that there is a precise mathematical relationship between a planet&rsquos distance from the Sun and the amount of time it takes revolve around the Sun. It was this law that inspired Newton, who came up with three laws of his own to explain why the planets move as they do.

Newton&rsquos Laws of Motion

If Kepler&rsquos laws define the motion of the planets, Newton&rsquos laws define motion. Thinking on Kepler&rsquos laws, Newton realized that all motion, whether it was the orbit of the Moon around the Earth or an apple falling from a tree, followed the same basic principles. &ldquoTo the same natural effects,&rdquo he wrote, &ldquowe must, as far as possible, assign the same causes.&rdquo Previous Aristotelian thinking, physicist Stephen Hawking has written, assigned different causes to different types of motion. By unifying all motion, Newton shifted the scientific perspective to a search for large, unifying patterns in nature. Newton outlined his laws in Philosophiae Naturalis Principia Mathematica (&ldquoMathematical Principles of Natural Philosophy,&rdquo) published in 1687.

Law I. Every body perseveres in its state of rest, or of uniform motion in a right line, unless it is compelled to change that state by forces impressed theron.

In essence, a moving object won&rsquot change speed or direction, nor will a still object start moving, unless some outside force acts on it. The law is regularly summed up in one word: inertia.

Law II. The alteration of motion is ever proportional to the motive force impressed and is made in the direction of the right line in which that force is impressed.

Newton&rsquos second law is most recognizable in its mathematical form, the iconic equation: F=ma. The strength of the force (F) is defined by how much it changes the motion (acceleration, a) of an object with some mass (m).

Law III. To every action there is always opposed an equal reaction: or the mutual actions of two bodies upon each other are always equal, and directed to contrary parts.

As Newton himself described: &ldquoIf you press a stone with your finger, the finger is also pressed by the stone.&rdquo

Gravity

Within the pages of Principia, Newton also presented his law of universal gravitation as a case study of his laws of motion. All matter exerts a force, which he called gravity, that pulls all other matter towards its center. The strength of the force depends on the mass of the object: the Sun has more gravity than Earth, which in turn has more gravity than an apple. Also, the force weakens with distance. Objects far from the Sun won&rsquot be influenced by its gravity.

Newton&rsquos laws of motion and gravity explained Earth&rsquos annual journey around the Sun. Earth would move straight forward through the universe, but the Sun exerts a constant pull on our planet. This force bends Earth&rsquos path toward the Sun, pulling the planet into an elliptical (almost circular) orbit. His theories also made it possible to explain and predict the tides. The rise and fall of ocean water levels are created by the gravitational pull of the Moon as it orbits Earth.

Einstein and Relativity

The ideas outlined in Newton&rsquos laws of motion and universal gravitation stood unchallenged for nearly 220 years until Albert Einstein presented his theory of special relativity in 1905. Newton&rsquos theory depended on the assumption that mass, time, and distance are constant regardless of where you measure them.

The theory of relativity treats time, space, and mass as fluid things, defined by an observer&rsquos frame of reference. All of us moving through the universe on the Earth are in a single frame of reference, but an astronaut in a fast-moving spaceship would be in a different reference frame.

Within a single frame of reference, the laws of classical physics, including Newton&rsquos laws, hold true. But Newton&rsquos laws can&rsquot explain the differences in motion, mass, distance, and time that result when objects are observed from two very different frames of reference. To describe motion in these situations, scientists must rely on Einstein&rsquos theory of relativity.

At slow speeds and at large scales, however, the differences in time, length, and mass predicted by relativity are small enough that they appear to be constant, and Newton&rsquos laws still work. In general, few things are moving at speeds fast enough for us to notice relativity. For large, slow-moving satellites, Newton&rsquos laws still define orbits. We can still use them to launch Earth-observing satellites and predict their motion. We can use them to reach the Moon, Mars, and other places beyond Earth. For this reason, many scientists see Einstein&rsquos laws of general and special relativity not as a replacement of Newton&rsquos laws of motion and universal gravitation, but as the full culmination of his idea.


Ancient geometry: cosmological and metaphysical

The Pythagoreans used geometrical figures to illustrate their slogan that all is number—thus their “triangular numbers” ( n(n−1) /2 ), “square numbers” (n 2 ), and “altar numbers” (n 3 ), some of which are shown in the figure . This principle found a sophisticated application in Plato’s creation story, the Timaeus, which presents the smallest particles, or “elements,” of matter as regular geometrical figures. Since the ancients recognized four or five elements at most, Plato sought a small set of uniquely defined geometrical objects to serve as elementary constituents. He found them in the only three-dimensional structures whose faces are equal regular polygons that meet one another at equal solid angles: the tetrahedron, or pyramid (with 4 triangular faces) the cube (with 6 square faces) the octahedron (with 8 equilateral triangular faces) the dodecahedron (with 12 pentagonal faces) and the icosahedron (with 20 equilateral triangular faces). (Sien animation .)

The cosmology of the Timaeus had a consequence of the first importance for the development of mathematical astronomy. It guided Johannes Kepler (1571–1630) to his discovery of the laws of planetary motion. Kepler deployed the five regular Platonic solids not as indicators of the nature and number of the elements but as a model of the structure of the heavens. In 1596 he published Prodromus Dissertationum Mathematicarum Continens Mysterium Cosmographicum (“Cosmographic Mystery”), in which each of the known six planets revolved around the Sun on spheres separated by the five Platonic solids, as shown in the photograph. Although Tycho Brahe (1546–1601), the world’s greatest observational astronomer before the invention of the telescope, rejected the Copernican model of the solar system, he invited Kepler to assist him at his new observatory outside of Prague. In trying to resolve discrepancies between his original theory and Brahe’s observations, Kepler made the capital discovery that the planets move in ellipses around the Sun as a focus.


The History of How We Discovered All the Planets in the Solar System

Satellites such as the Kepler have been working overtime to uncover hundreds of new planets in our galaxy. But how did we first discover the planets in our local volume of space? Here are the stories of how astronomers living hundreds of years ago discovered each planet in our solar system.

Photo of the 18th Century Jantar Mantar observatory in Jaipur, India, by Norman Koren .

The innermost planet in our solar system, Mercury orbits our sun between just under 70 million and 46 million kilometers. Ancient astronomers knew of the planet's speed around the sun: Assyrian astronomers associated it with gods such as Nabu, the scribe and messenger to the gods, while the Greeks named the body Mercury, the messenger of the gods. The association is apt: the planet has a short year of 88 days in all.

In 1631, astronomer Pierre Gassendi first observed Mercury making a transit across the sun, and just a couple of years later, another astronomer, Giovanni Zupi discovered phases, indicating that the planet orbited the sun. Other astronomers followed, making incremental discoveries along the way: Italian Astronomer Giovanni Schiaparelli observed the planet, and concluded that Mercury was tidally locked with the sun.

More discoveries came during the modern era of space exploration: much more about the planet has been found recently. Soviet scientists first used radar to study the planet in the early 1960s, while scientists at Puerto Rico's Arecibo Observatory Radio Telescope discovered that the planet in fact rotated once every 59 days, rather than 88 as previously thought. In 1974, Mariner 10 first visits the planet, making several passes, mapping the surface, and in 2008, the MESSENGER satellite marked a return to the planet, where it's currently in orbit.

The second planet in the solar system, Venus is the brightest of the planets as observed Earth. As a result, it's been studied since ancient times, with the first records coming from the Babylonians, who named the planet Ishtar. The Romans viewed Venus as the goddess of beauty, while the Mayans believed that the planet was the brother of the Sun. In 1610, Galileo Galilei observed phases of Venus, confirming that the planet did indeed orbit the sun. Due to the planet's thick atmosphere, observation of the surface wasn't possible until the 1960s, but many believed that Venus harbored life, due to the planet's similar size to Earth.

In 1958, radar imagery found that the planet's surface was hot – inhospitably so. Mankind was about to get a closer look. The first attempt, the Soviet Union's Venera 1, launched in 1961, failed, but Mariner 2, launched by the United States, succeeded in a flyby confirming the planet's temperature and that it lacked a magnetic field. A new Soviet mission, Venera 4, successfully reached Venus and sent back information about its atmosphere before it burned up during entry. Several additional probe followed: Mariner 5, Venera 5 and 6, before Venera 7 successfully landed, becoming the first manmade object to land on another planet while Venera 8, landed two years later. Both were destroyed by the planet's heat and pressure, but the Soviet Union continued to send probes: 9 through 12 took pictures and gathered information on the planet's geology. NASA also continued to send probes: Pioneer 12 orbited the planet for 14 years, mapping the surface, while Pioneer 13 sent several probes down to the surface.

Earth has been continually observed by humanity as long as we've been around. But, while we knew we stood on solid ground, the true nature of our home took a little while longer to figure out. For many centuries, humanity believed that Earth wasn't an object such as those that they observed above them: everything was thought to revolve around us. As early as Aristotle, philosophers had determined that Earth was spherical by observing its shadow against the Moon.

Mikołaj Kopernik – known as Nicholas Copernicus posited a Heliocentric view of the solar system as early as 1514. Titled De revolutionibus orbium coelestium (On the Revolutions of the Heavenly Spheres), the book was first published in 1543, challenged the conventional wisdom of the day. The theory was controversial, but was followed up by Johannes Kepler with his three volume work, Epitome astronomiae Copernicanae (Epitome of Copernican Astronomy). Kepler had devised three laws of planetary motion: "Planets move around the Sun in ellipses, with the Sun at one focus", "the line connecting the Sun to a planet sweeps equal areas in equal times" and "the square of the orbital period of a planet is proportional to the cube (3rd power) of the mean distance from the Sun in (or in other words—of the "semi-major axis" of the ellipse, half the sum of smallest and greatest distance from the Sun)". These laws helped define the motion of the planets and allowed us the first real idea that our solar system was vastly different than previously thought. Kepler's theories weren't popular at first, but eventually caught on in Europe. At the same time that Copernicus was publishing his views, Ferdinand Magellan's expedition was able to circumnavigate the globe in 1519.

However, it wasn't until October 24th, 1946 when we first got a look at our home world when the first picture of Earth was taken from a modified V-2 Rocket fired from the White Sands Missile Range in New Mexico.

The blood-red fourth planet of our solar system has long been associated with the Roman God of war, bearing the same name: Mars. Where many people thought that Venus might very well enjoy an earth-like atmosphere, there were similar thoughts about Mars. Most notoriously, astronomer Giovanni Schiaparelli examined the planet through a telescope in 1877, describing a number of features, which he described as Canali. The mistranslated word was understood to mean that he had discovered Canals on Mars, and by extension, people assumed that they were artificial. Twenty years later, another astronomer, Camille Flammarion, further identified the features as artificial, and the general public generally assumed that the planet could support life. Undoubtedly, the public's perception led to the rise of a number of Mars-oriented SF novels, such as The War of the Worlds, by H.G. Wells, and Edgar Rice Burrough's John Carter series.

Advances in telescopic technology that came later allowed for new observations of the planet. Astronomers were able to measure the temperature of the planet, determine its atmospheric content and it's mass. Throughout the 1960s, the Soviet Union attempted to send eight probes towards Mars, each resulting in failure, although several additional orbiters that followed in the 1970s were successful in orbiting the planet. NASA would have poor luck with the Mariner 3 mission, but Mariner 4, launched in 1964, successfully flew by the planet, taking readings as it did so, revealing a dead world. Mariner 9 would later orbit the planet, further adding to our knowledge of the planet. Where those missions were the scouts, the Viking missions represented the initial invasion: On July 20th, 1976, the probe touched down on the planet for an unprecedented mission that would last for until 1982. Viking 2 followed shortly thereafter, landing in September 1976, remaining in operation until 1980.

Despite the mission's success, it wasn't until 1997 that the Mars Pathfinder mission successfully landed on Mars, the first mobile rover to be landed on a planetary body. A follow up mission, the Mars Climate Orbiter, failed due to human error, and several additional Mars Probes failed as well. It wasn't until 2004 when NASA launched the Mars Exploration Rover Mission with the Spirit and Opportunity rovers that a successful landing was made. The rovers outdid everyone's expectations, and it wasn't until 2011 that their missions were closed down. In 2012, NASA successfully landed the Curiosity Rover, which landed on August 6th, 2012, where it has begun to perform its duties.

The largest planet in our solar system, Jupiter has long been watched since ancient times. It helped guide the Chinese 12 year cycle, and the planet was named for the king of the Roman gods. It also provided a big target for early astronomers. Galileo was the first to observe Jupiter's four major moons, now known as the Galilean Moons: Io, Europa, Ganymede and Callisto, named for Zeus's lovers. Astronomer Robert Hooke first discovered a major storm system on the gas planet, and it was confirmed by Giovanni Cassini in 1665, believed to be the first sightings of Jupiter's Great Red Spot, which was later formally recorded in 1831. Without an underlying land mass, the storms of Jupiter are free to rage on, and the feature has remained on the planet since. Astronomers Giovanni Borelli and Cassini, using orbital tables and mathematics, discovered something odd: Jupiter, when in opposition to Earth, appeared to be seventeen minutes behind their calculations, lending the indications that light was not an instantaneous phenomenon.

As observations advanced in the 1900s, other discoveries were made: While using a radio telescope to study the Crab Nebula between 1954 and 1955, astronomer Bernard Berke was hampered by interference from one part of the sky, and eventually found that Jupiter was emitting the waves as part of the planet's radiation. In 1973, the Pioneer missions became the first probes to fly past the planet, taking a number of close-up pictures. In 1977, two space probe missions were launched from Earth: Voyagers 1 and 2, designed to explore the outer planets of the Solar system. They first reached Jupiter two years later: Voyager 1 arrived in March 1979, and Voyager 2 arrived on July 1979. Both uncovered a wealth of new information about the planet and its moons before they left, uncovering a small ring system and a number of additional moons. Other robotic missions have since followed: the Ulysses mission arrived in 1992, the Galileo probes orbited the planet in 1995, Cassini flew past in 2000, and New Horizons passed by in 2007. In 1994, scientists observed an astonishing event: a planetary impact, when the Shoemaker-Levy comet crashed into the Jupiter's southern horizon, leaving enormous impact scars in the planet's atmosphere. Currently, there are efforts underway to examine Jupiter's moons, thought to be the next best candidates for life.

Our system's sixth planet from the Sun is possibly one of the most striking, and is the last classically recognized planet: the Romans named the planet for their God of Agriculture. However, it wasn't until Galileo turned his attention to the planet in 1610 before the planet's dominant feature was uncovered when. While he studied the planet's features, he believed heɽ uncovered several orbiting moons. However, it wasn't until 1655, when Christiaan Huygens, with a more powerful telescope, discovered that the feature was actually a ring that encircled the entire planet. Shortly thereafter, he uncovered the Saturn's first moon, Titan. During his own observations, Giovanni Cassini, in 1671, uncovered four additional moons: Iapetus, Rhea, Tethys and Dione and a gap in the planet's rings, leading him to believe that the ringswere made up of smaller particles. In 1789, German astronomer William Herschel noted two additional moons: Mimas and Enceladus, and over the next hundred years, two other satellites were found: Hyperion, in 1848, and Phoebe, in 1899.

As NASA began to explore the outer planets, Saturn would first be visited by the Pioneer 11 mission in September 1979, taking a number of pictures. The twin Voyager probes would come next, in 1980 and 1981, taking high resolution pictures. The planet became a divergent point for the pair: Voyager 1 used Saturn to arc out of the Solar System, while Voyager 2 was directed to Uranus. The planet wouldn't be visited again until 2004 with the Cassini mission, which orbited the planet and studied its moons, where it remains today.

The seventh planet, Uranus, is hard to detect without the aid of a telescope, and thus, the planet doesn't have the same long history as its other neighbors. Watching the skies in December 1690, astronomer John Flamsteed first noted the planet, but identified it as a star, which he named 34 Tauri. It wasn't until March 13th, 1781 that Herschel first believed that the star that he was studying was a comet. It wasn't until he began to study the object's orbit when he found that it was nearly circular, leading him to believe that it was in fact a planet. Herschel named the planet Georgium Sidus, in honor of King George III, but the eventually the planet was named Uranus, after Chronos. It's discovery was sensational, the furthest known object in the solar system. In the 19th century, astronomers noted something odd about the planet's orbit: it didn't quite follow mathematical theories, and it deviated from its course. It was clearly being influenced by something further out in the Solar system.

The planet's most unusual feature is its orientation: rather than rotating like the other planets in the system, Uranus rotates on its side, with its rings and moons orbiting in bulls-eye pattern. The underlying reasons for this are unknown, and have at times been attributed to a planetary collision. However, in 2009, members of the Paris Observatory theorized that a moon in the planetary disk, formed while the planet was in its infant stages, could have made the planet wobble. In 1986, the Voyager 2 probe passed by Uranus, examining the planet's atmosphere and discovering a number of additional moons and the planet's ring system. It was the first and only probe to reach the planet, and at this point, no further missions are planned.


The Planets in our Solar System:

Having covered the basics of definition and classification, let’s get talking about those celestial bodies in our Solar System that are still classified as planets (sorry Pluto!). Here is a brief look at the eight planets in our Solar System. Included are quick facts and links so you can find out more about each planet.

Mercury:
Mercury is the closest planet to our Sun, at just 58 million km (36 million miles) or 0.39 Astronomical Unit (AU) out. But despite its reputation for being sun-baked and molten, it is nie the hottest planet in our Solar System (scroll down to find out who that dubious honor goes go!)

Mercury, as imaged by the MESSENGER spacecraft, revealing parts of the never seen by human eyes. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

Mercury is also the smallest planet in our Solar System, and is also smaller than its largest moon (Ganymede, which orbits Jupiter). And being equivalent in size to 0.38 Earths, it is just slightly larger than the Earth’s own Moon. But this may have something to do with its incredible density, being composed primarily of rock and iron ore. Here are the planetary facts:

  • Diameter: 4,879 km (3,032 miles)
  • Mass: 3.3011 x 10 23 kg ( 0.055 Earths)
  • Length of Year (Orbit): 87.97 Earth days
  • Length of Day: 59 Earth days.
  • Mercury is a rocky planet, one of the four “terrestrial planets” in our Solar System. Mercury has a solid, cratered surface, and looks much like Earth’s moon.
  • If you weigh 45 kg (100 pounds) on Earth, you would weigh 17 kg (38 pounds) on Mercury.
  • Mercury does not have any moons.
  • Temperatures on Mercury range between -173 to 427 degrees Celcius (-279 to 801 degrees Fahrenheit)
  • Just two spacecraft have visited Mercury: Mariner 10 in 1974-75 and MESSENGER, which flew past Mercury three times before going into orbit around Mercury in 2011 and ended its mission by impacting the surface of Mercury on April 30, 2015. MESSENGER has changed our understanding of this planet, and scientists are still studying the data.
  • Find more details about Mercury at this article on Universe Today, and this page from NASA.

Venus:
Venus is the second closest planet to our Sun, orbiting at an average distance of 108 million km (67 million miles) or 0.72 AU. Venus is often called Earth’s “sister planet,” as it is just a little smaller than Earth. Venus is 81.5% as massive as Earth, and has 90% of its surface area and 86.6% of its volume. The surface gravity, which is 8.87 m/s², is equivalent to 0.904 g – roughly 90% of the Earth standard.

A radar view of Venus taken by the Magellan spacecraft, with some gaps filled in by the Pioneer Venus orbiter. Credit: NASA/JPL

And due to its thick atmosphere and proximity to the Sun, it is the Solar Systems hottest planet, with temperatures reaching up to a scorching 735 K (462 °C). To put that in perspective, that’s over four and a half times the amount of heat needed to evaporate water, and about twice as much needed to turn tin into molten metal ( 231.9 °C )!

  • Diameter: 7,521 miles (12,104 km)
  • Mass: 4.867 x 10 24 kg (0.815 Earth mass)
  • Length of Year (Orbit): 225 days
  • Length of day: 243 Earth days
  • Surface temperature: 462 degrees C (864 degrees F)
  • Venus’ thick and toxic atmosphere is made up mostly of carbon dioxide (CO2) and nitrogen (N2), with clouds of sulfuric acid (H2SO4) droplets.
  • Venus has no moons.
  • Venus spins backwards (retrograde rotation), compared to the other planets. This means that the sun rises in the west and sets in the east on Venus.
  • If you weigh 45 kg (100 pounds) on Earth, you would weigh 41 kg (91 pounds) on Venus.
  • Venus is also known and the “morning star” or “evening star” because it is often brighter than any other object in the sky and is usually seen either at dawn or at dusk. Since it is so bright, it has often been mistaken for a UFO!
  • More than 40 spacecraft have explored Venus. The Magellan mission in the early 1990s mapped 98 percent of the planet’s surface. Find out more about all the missions here.
  • Find out more about Venus on this article from Universe Today, and this page from NASA.

Earth:
Our home, and the only planet in our Solar System (that we know of) that actively supports life. Our planet is the third from the our Sun, orbiting it at an average distance of 150 million km (93 million miles) from the Sun, or one AU. Given the fact that Earth is where we originated, and has all the necessary prerequisites for supporting life, it should come as no surprise that it is the metric on which all others planets are judged.

Earth, pictured by the crew of the Apollo 17 mission. Credit: NASA

Whether it is gravity (g), distance (measured in AUs), diameter, mass, density or volume, the units are either expressed in terms of Earth’s own values (with Earth having a value of 1) or in terms of equivalencies – i.e. 0.89 times the size of Earth. Here’s a rundown of the kinds of

  • Diameter: 12,760 km (7,926 miles)
  • Mass: 5.97 x 10 24 kg
  • Length of Year (Orbit): 365 days
  • Length of day: 24 hours (more precisely, 23 hours, 56 minutes and 4 seconds.)
  • Surface temperature: Average is about 14 C, (57 F), with ranges from -88 to 58 (min/max) C (-126 to 136 F).
  • Earth is another terrestrial planet with an ever-changing surface, and 70 percent of the Earth’s surface is covered in oceans.
  • Earth has one moon.
  • Earth’s atmosphere is 78% nitrogen, 21% oxygen, and 1% various other gases.
  • Earth is the only world known to harbor life.
  • Find out more about Earth at a series of articles found here on Universe Today, and on this webpage from NASA.

Mars:
Mars is the fourth planet from the sun at a distance of about 228 million km (142 million miles) or 1.52 AU. It is also known as “the Red Planet” because of its reddish hue, which is due to the prevalence of iron oxide on its surface. In many ways, Mars is similar to Earth, which can be seen from its similar rotational period and tilt, which in turn produce seasonal cycles that are comparable to our own.

Global image of the planet Mars. Credit: NASA

The same holds true for surface features. Like Earth, Mars has many familiar surface features, which include volcanoes, valleys, deserts, and polar ice caps. But beyond these, Mars and Earth have little in common. The Martian atmosphere is too thin and the planet too far from our Sun to sustain warm temperatures, which average 210 K (-63 ºC) and fluctuate considerably.

  • Diameter: 6,787 km, (4,217 miles)
  • Mass: 6.4171 x 10 23 kg ( 0.107 Earths)
  • Length of Year (Orbit): 687 Earth days.
  • Length of day: 24 hours 37 minutes.
  • Surface temperature: Average is about -55 C (-67 F), with ranges of -153 to +20 °C (-225 to +70 °F)
  • Mars is the fourth terrestrial planet in our Solar System. Its rocky surface has been altered by volcanoes, impacts, and atmospheric effects such as dust storms.
  • Mars has a thin atmosphere made up mostly of carbon dioxide (CO2), nitrogen (N2) and argon (Ar).If you weigh 45 kg (100 pounds) on Earth, you would weigh 17 kg (38 pounds) on Mars.
  • Mars has two small moons, Phobos and Deimos.
  • Mars is known as the Red Planet because iron minerals in the Martian soil oxidize, or rust, causing the soil to look red.
  • More than 40 spacecraft have been launched to Mars. You can find out more about missions to Mars here.Find out more about Mars at this series of articles on Universe Today, and at this NASA webpage.

Jupiter:
Jupiter is the fifth planet from the Sun, at a distance of about 778 million km (484 million miles) or 5.2 AU. Jupiter is also the most massive planet in our Solar System, being 317 times the mass of Earth, and two and half times larger than all the other planets combined. It is a gas giant, meaning that it is primarily composed of hydrogen and helium, with swirling clouds and other trace gases.

Io and Jupiter as seen by New Horizons during its 2008 flyby. (Credit: NASA/Johns Hopkins University APL/SWRI).

Jupiter’s atmosphere is the most intense in the Solar System. In fact, the combination of incredibly high pressure and coriolis forces produces the most violent storms ever witnessed. Wind speeds of 100 m/s (360 km/h) are common and can reach as high as 620 km/h (385 mph). In addition, Jupiter experiences auroras that are both more intense than Earth’s, and which never stop.

  • Diameter: 428,400 km (88,730 miles)
  • Mass: 1.8986 × 10 27 kg ( 317.8 Earths)
  • Length of Year (Orbit): 11.9 Earth years
  • Length of day: 9.8 Earth hours
  • Temperature: -148 C, (-234 F)
  • Jupiter has 67 known moons, with an additional 17 moons awaiting confirmation of their discovery – for a total of 67 moons. Jupiter is almost like a mini solar system!
  • Jupiter has a faint ring system, discovered in 1979 by the Voyager 1 mission.
  • If you weigh 45 kg (100 pounds) on Earth, you would weigh 115 kg (253) pounds on Jupiter.
  • Jupiter’s Great Red Spot is a gigantic storm (bigger than Earth) that has been raging for hundreds of years. However, it appears to be shrinking in recent years.
  • Many missions have visited Jupiter and its system of moons, with the latest being the Juno mission will arrive at Jupiter in 2016. You can find out more about missions to Jupiter here.
  • Find out more about Jupiter at this series of articles on Universe Today and on this webpage from NASA.

Saturn:
Saturn is the sixth planet from the Sun at a distance of about 1.4 billion km (886 million miles) or 9.5 AU. Like Jupiter, it is a gas giant, with layers of gaseous material surrounding a solid core. Saturn is most famous and most easily recognized for its spectacular ring system, which is made of seven rings with several gaps and divisions between them.

  • Diameter: 120,500 km (74,900 miles)
  • Mass: 5.6836 x 10 26 k g ( 95.159 Earths )
  • Length of Year (Orbit): 29.5 Earth years
  • Length of day: 10.7 Earth hours
  • Temperature: -178 C (-288 F)
  • Saturn’s atmosphere is made up mostly of hydrogen (H2) and helium (He).
  • If you weigh 45 kg (100 pounds) on Earth, you would weigh about 48 kg (107 pounds) on Saturn
  • Saturn has 53 known moons with an additional 9 moons awaiting confirmation.
  • Five missions have gone to Saturn. Since 2004, Cassini has been exploring Saturn, its moons and rings. You can out more about missions to Saturn here.
  • Find out more about Saturn at this series of articles on Universe Today and at this webpage from NASA.

Uranus:
Uranus is the seventh planet from the sun at a distance of about 2.9 billion km (1.8 billion miles) or 19.19 AU. Though it is classified as a “gas giant”, it is often referred to as an “ice giant” as well, owing to the presence of ammonia, methane, water and hydrocarbons in ice form. The presence of methane ice is also what gives it its bluish appearance.

Uranus is also the coldest planet in our Solar System, making the term “ice” seem very appropriate! What’s more, its system of moons experience a very odd seasonal cycle, owing to the fact that they orbit Neptune’s equator, and Neptune orbits with its north pole facing directly towards the Sun. This causes all of its moons to experience 42 year periods of day and night.

  • Diameter: 51,120 km (31,763 miles)
  • Mass:
  • Length of Year (Orbit): 84 Earth years
  • Length of day: 18 Earth hours
  • Temperature: -216 C (-357 F)
  • Most of the planet’s mass is made up of a hot dense fluid of “icy” materials – water (H2O), methane (CH4). and ammonia (NH3) – above a small rocky core.
  • Uranus has an atmosphere which is mostly made up of hydrogen (H2) and helium (He), with a small amount of methane (CH4). The methane gives Uranus a blue-green tint.
  • If you weigh 45 kg (100 pounds) on Earth, you would weigh 41 kg (91 pounds) on Uranus.
  • Uranus has 27 moons.
  • Uranus has faint rings the inner rings are narrow and dark and the outer rings are brightly colored.
  • Voyager 2 is the only spacecraft to have visited Uranus. Find out more about this mission here.
  • You can find out more about Uranus at this series of articles on Universe Today and this webpage from NASA.

Neptune:
Neptune is the eighth and farthest planet from the Sun, at a distance of about 4.5 billion km (2.8 billion miles) or 30.07 AU. Like Jupiter, Saturn and Uranus, it is technically a gas giant, though it is more properly classified as an “ice giant” with Uranus.

Neptune photographed by the Voyager 2 space probe. Credit: NASA/JPL

Due to its extreme distance from our Sun, Neptune cannot be seen with the naked eye, and only one mission has ever flown close enough to get detailed images of it. Nevertheless, what we know about it indicates that it is similar in many respects to Uranus, consisting of gases, ices, methane ice (which gives its color), and has a series of moons and faint rings.

  • Diameter: 49,530 km (30,775 miles)
  • Mass: 1.0243 x 10 26 kg ( 17 Earths)
  • Length of Year (Orbit): 165 Earth years
  • Length of day: 16 Earth hours
  • Temperature: -214 C (-353 F)
  • Neptune is mostly made of a very thick, very hot combination of water (H2O), ammonia (NH3), and methane (CH4) over a possible heavier, approximately Earth-sized, solid core.
  • Neptune’s atmosphere is made up mostly of hydrogen (H2), helium (He) and methane (CH4).
  • Neptune has 13 confirmed moons and 1 more awaiting official confirmation.
  • Neptune has six rings.
  • If you weigh 45 kg (100 pounds) on Earth, you would weigh 52 kg (114 pounds) on Neptune.
    Neptune was the first planet to be predicted to exist by using math.
  • Voyager 2 is the only spacecraft to have visited Neptune. You can find out more about this mission here.
  • Find out more about Neptune at this series of articles on Universe Today and this NASA webpage. We have written many articles about the planets for Universe Today. Here are some facts about planets, and here’s an article about the names of the planets.If you’d like more info on the Solar System planets, dwarf planets, asteroids and more, check out NASA’s Solar System exploration page, and here’s a link to NASA’s Solar System Simulator.We’ve also recorded a series of episodes of Astronomy Cast about every planet in the Solar System. Start here, Episode 49: Mercury.Venus is the second planet from the Sun, and it is the hottest planet in the Solar System due to its thick, toxic atmosphere which has been described as having a “runaway greenhouse effect” on the planet.

Now you know! And if you find yourself unable to remember all the planets in their proper order, just repeat the words, “My Very Educated Mother Just Served Us Noodles.” Of course, the Pie, Ham, Muffins and Eggs are optional, as are any additional courses that might be added in the coming years!

We have many great articles on the Solar System and the planets here at Universe Today. Here is a rundown of the Inner Planets, the Outer Planets, a description of Terrestrial Planets, the Dwarf Planets, and Why Pluto is no Longer a Planet?.