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I.WhatareTides?II. CausesofTidesIII.TidalVariationsIV. LunarDayandFrequencyofTidesV. MonitoringTides
Tides
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TidesarenotexplicitlyincludedintheNGSSPerformanceExpectations.
FromtheNGSSFramework(M.S.SpaceScience):“Thereisastrongemphasisonasystemsapproach,usingmodelsofthesolarsystemtoexplainastronomicalandotherobservationsofthecyclicpatternsofeclipses,tides,andseasons.”
FromtheNGSSCrosscuttingConcepts:Observedpatternsinnatureguideorganizationandclassificationandpromptquestionsaboutrelationshipsandcausesunderlyingthem.ForElementarySchool:• Similaritiesanddifferencesinpatternscanbeusedtosort,classify,communicate
andanalyzesimpleratesofchangefornaturalphenomenaanddesignedproducts.• Patternsofchangecanbeusedtomakepredictions• Patternscanbeusedasevidencetosupportanexplanation.ForMiddleSchool:• Graphs,charts,andimagescanbeusedtoidentifypatternsindata.• Patternscanbeusedtoidentifycause-and-effectrelationships.
Thetopicoftideshaveanimportantconnectiontoglobalchangesincespringtidesandkingtidesarecausingcoastalfloodingassealevelhasbeenrising.
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TidesareoneofthemostreliablephenomenaonEarth-theyoccuronaregularandpredictablecycle.Alongwithdeathandtaxes,tidesareacertaintyoflife.
SamuelWantman/WikimediaCommons.LicensedunderCCBY-SA3.0viaCommons
I.WhatareTides?
Tidesareapparentchangesinlocalsealevelthataretheresultoflong-periodwavesthatmovethroughtheoceans.PhotosoflowandhightideonthecoastoftheBayofFundyinCanada.
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Thedifferenceinheightbetweenthehightideandthelowtideiscalledthetidalrange.
Whenthehighestpartofthetide(crest)reachesthecoast,hightideoccurs.Lowtideoccurswhenthelowestpartofthewave,oritstroughreachesthecoast.
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TidalcurrentsareparticularlyimportantinregionsliketheS.F.Bay.
Atidalcurrentisthehorizontalmovementofwaterthataccompaniestherisingandfallingofthetide.Theincomingtidealongthecoastandintothebaysandestuariesiscalledafloodcurrent.Theoutgoingtidalcurrentiscalledanebbcurrent.
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TwobulgesofwateroccuronoppositesidesoftheEarthduetothegravitationalinfluenceoftheMoon-onethatroughlyfacestheMoonandanotherontheoppositesideoftheEarth.
II.CausesofTides
SincetheMoonorbitstheEarthslowerthantheEarthrotatesonitsaxis,theEarthrotatesinandoutofthetidalbulges.
Mostcoastlinesexperiencetwohightidesandtwolowtidesabouteachday.
HightidesarecommonlyunequalinheightduetotheaxialangleoftheEarth’srotation.Forexample,thelocationontheEarthatAhasafairlylargehightide.Butwhenthispositionhasrotatedin12hourstotheothersideatA’,notethatthehightideisnotashigh.
NOTTOSCALE!
A
A’
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NOAANOTTOSCALE!
ThemostcommonmodelforwhytheEarthhastwotidalbulgesisbecausegravityandinertiaareopposingforcesactingontheEarth’soceans,creatingtidalbulgesonoppositesitesoftheplanet.
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NOTTOSCALE!
OntheoppositesideoftheEarth,inertialforcesdominate,creatingasecondbulge.OnesimplewayofthinkingaboutitisthattheEarth(B)ispulledawayfromthewateronthatfarsidebecausethegravitationalforceexertedbytheMoonatpointBislargerthanthatexertedatpointC.Inthismodel,gravityandinertiacreatetwobulgesofwater.OneformswheretheEarthandmoonareclosesttogether,andtheotherformswheretheyarefurthestapart.Thisisgreatlyoversimplifiedbutmayserveasagoodmentalmodelfortidalbulges.
OnthesideoftheEarthfacingtheMoon,thegravitationalinteractionoftheMoonresultsinabulge(A).
ABCMoon
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Thepreviousmodelfortheformationoftwotidalbulgesisnottechnicallycorrect.ItisanoversimplificationtostatethatthetidalbulgesareduetothesimplegravitationalattractionoftheMoon.However,anaccurategravitationmodeltodescribethetide-generatingforceisbeyondthescopeofmostK-12classrooms.
LunargravitycreatesadifferentialfieldontheEarth’ssurfaceknownasthetide-generatingforce(asindicatedbythevectorsinthefigure).Themajordifferencebetweenthetwomodelsfortheformationofthetwotidalbulgesiswhetherthetidalbulgesarepulledbysimplegravitationalattraction(leastaccurate)orsqueezedbythedifferentialgravitationalfield(moreaccurate).
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Tidalforcesarebasedonthegravitationalinteractionbetweentwoormorebodies.Withtidalforces,thedistancebetweentwoobjectsusuallyismorecriticalthantheirmasses.Thus,theMoon,beingcloserhasamuchstrongereffectontidesontheEarththandoestheSun.
The“back-of-the-envelope”calculationsshowthatthetide-generatingforceoftheMoonismorethantwicethatoftheSun.
NOAA
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III.TidalVariations
Thevariationoftidesandtidalrangesisduetoavarietyoffactorsincluding:
1. AlignmentoftheMoonandSun2. DeclinationofMoon3. EccentricityoftheEarth’sorbit4. EccentricityofMoon’sorbit5. Regionalfactorssuchastheshapeofthecoastline
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1.AlignmentoftheMoonandSun
RememberthatboththeSunandtheMoonaffecttidesonEarth.
WhentheEarth,SunandMoonarealigned,theaffectoftheSunandtheMoonareadditiveandthetidesareattheirhighestandlowest-theseareknownasspringtides.
Springtidesoccurduringfullandnewmoons.
NOAANottoscale.
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WhentheSunandMoonareatrightanglestooneanother-thecontributionfromtheSunpartiallycancelsouttheeffectoftheMoonandresultsinlowerhightidesandhigherlowtides-knowasneaptides.
NeaptidesoccurduringthefirstandthirdquartersoftheMoon.
Nottoscale.
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NOAA
2.DeclinationofMoonTheMoon’sdeclination(angleoftheMoon’sorbitwithrespecttotheEarth’sequator)variesovera18.6yearcyclefromamaximumof~28°toaminimumof~18°.TheanimationshowsthechangeintheangleoftheMoon’sorbitalplane.TheEarth’stidalbulgestrackthepositionoftheMoonandhaveaeffectontheheightoftides.
Nottoscale.
NavalPostgraduateSchool
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WikimediaUserChris55
3.EccentricityoftheEarth’sorbitTheorbitoftheEartharoundtheSunisslightlyeccentric.PerihelionistheEarth’sclosestapproachtotheSunandis147.1millionkm.AphelionistheEarth’sfarthestdistancefromtheSunandis152.1millionkm.Thedifferencebetweenperihelionandaphelionis~5millionkm.
SincethetidalinfluenceoftheSunisafunctionofthedistance,thesmallchangethedistancebetweentheEarthandSunresultsinasmallchangeinthesolartidalcomponent(heightoftide).
Nottoscaleandorbitaleccentricityisexaggerated.
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NASA
4.EccentricityofMoon’sorbit TheorbitoftheMoonaroundtheEarthisslightlyeccentric.PerigeeistheMoon’sclosestapproachtotheEarthandis363,396km.ApogeeistheMoon’sfarthestdistancefromtheEarthandis405,504km.Thedifferencebetweenperigeeandapogeeis~42,000km.
SincethetidalinfluenceoftheMoonislargelyafunctionofthedistance,thetidegeneratinginfluenceoftheMoonincreasesduringperigee.WhentheMoonisatperigee,itappears~14%largerwhenviewedfromEarthandiscommonlyknownasasupermoon.
Nottoscaleandorbitaleccentricityisexaggerated.
NASA
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NOAA
TheeffectoftheeccentricorbitsoftheEarthandMoonhaveaverysmallinfluenceontides.Atmost,theeffectmaybeonlyafewinches.Althoughthiseffectmaybesmall,itcanbeadditivewithotherfactorsthataffecttides.
SincethetidalrangeisdrivenbytherelativepositionsoftheEarth,Moon,andSunandtheirellipticalorbits,kingtidesoccurwhentheEarth,MoonandSunarealignedatperigeeandperihelion.
Nottoscaleandorbitaleccentricitiesareexaggerated.
KGOTV
Theterm“kingtide”isnotascientificterm;itiscommonlyusedtodescribeextralargehightides.
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Thisimageshowsamodeloftheseasurfaceheightamplitudeinmetersduetoatides.
NASA/TOPEX
5.Regionalfactors
Wefindthatthetidalrangemayvaryconsiderablealongdifferentcoastlines.Underwaterbathymetry(shapeofseafloor)andtheshapeofthecoastlinecandramaticallyaffectthelocaltidalrange.AstheEarthrotateswithinthetidalbulges,thecontinentsdisruptthemigrationofthebulgeintheoceans.
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IV.LunarDayandFrequencyofTidesMostcoastalareasexperiencetwohightidesandtwolowtidesinacyclethattakes24hoursand50minutes-alunarday.AlunardayistheamountoftimetakesforaspecificpointontheEarthtorotatefromanexactpointundertheMoontothesamepointundertheMoon-24hoursfortherotationoftheEarthandanadditional50minutesbecausetheMoonhasmovedinitsorbitaroundtheEarth.BecausetheEarthrotatesthroughtwotidal“bulges”everylunarday,coastalareasexperiencetwohighandtwolowtidesevery24hoursand50minutes.
NavalPostgraduateSchool
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Anareahasasemidiurnaltidalcycleifitexperiencestwohighandtwolowtidesofapproximatelyequalsizeeverylunarday.Anareahasamixedsemidiurnaltidalcycleifitexperiencestwohighandtwolowtidesofdifferentsizeeverylunarday.
Anareahasadiurnaltidalcycleifitexperiencesonehighandonelowtideeverylunarday.
NOAA
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NOAA
IftheEarthhadnocontinentsandwasuniformlycoveredbyocean,theentireplanetwouldexperiencetwoequallyproportionedhighandlowtideseverylunarday.OnEarth,thecontinentsblockthemigrationofthetidalbulgesastheEarthrotates.Theinterferenceofthecontinentsresultincomplextidalpatternsindifferentregionsdependingupontheshapeofthecoastlineandseafloorbathymetry(shapeoftheseafloor).
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V.MonitoringTides
NOAA
Predictingtideshasalwaysbeenimportantforshipping,marineindustriesandcommercialfishing.Forexample,navigatingshipsthroughshallowwaterportsrequiresknowledgeofthetimeandheightofthetidesaswellasthespeedanddirectionofthetidalcurrents.In2002,fourlargeindustrialcraneswereshippedtoOaklandfromChina.TheshipcarryingthecranesneededtogoundertheBayBridgeandclearedthebottomofthebridgebyonly6feetduringlowtide.
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Atidegaugeisadeviceformeasuringthechangeinsealevelrelativetoabaselinesuchasmeansealevel.Sensorscontinuouslyrecordtheheightofthewaterlevel.Waterentersthebottomofapipeandsensorsmeasureitsheightinside.Inrecentyears,tidegaugesareautomatedwithelectronicsensorsandreal-timetidedataareavailableonline.
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Dataareavailablefrommorethan1,750tidegaugesworldwide.Sometidegaugeshavebeenincontinuousoperationforcenturies.ThemapshowsthedistributionoftidegaugesfortheGlobalSeaLevelObservingSystem(GLOSS)network.
NOAA
ThetidegaugeatCrissyFieldhasbeenincontinuousoperationsince1845.
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Tidegaugeshavebeenanimportantsourceofdatafordocumentingtheincreaseinmeansealevelduetoclimatechange.ThedatashowmonthlymeanwaterlevelsfromtheSanFranciscoCrissyFieldtidegaugethathasbeenoperatingsince1854.Globalsealevelrise~2.0mm/year.
NOAA