tides - earth science · tides are not explicitly included in the ngss performance expectations....

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.

TidesareoneofthemostreliablephenomenaonEarth-theyoccuronaregularandpredictablecycle.Alongwithdeathandtaxes,tidesareacertaintyoflife.

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I.WhatareTides?

Tidesareapparentchangesinlocalsealevelthataretheresultoflong-periodwavesthatmovethroughtheoceans.PhotosoflowandhightideonthecoastoftheBayofFundyinCanada.

Thedifferenceinheightbetweenthehightideandthelowtideiscalledthetidalrange.

Whenthehighestpartofthetide(crest)reachesthecoast,hightideoccurs.Lowtideoccurswhenthelowestpartofthewave,oritstroughreachesthecoast.

TidalcurrentsareparticularlyimportantinregionsliketheS.F.Bay.

Atidalcurrentisthehorizontalmovementofwaterthataccompaniestherisingandfallingofthetide.Theincomingtidealongthecoastandintothebaysandestuariesiscalledafloodcurrent.Theoutgoingtidalcurrentiscalledanebbcurrent.

TwobulgesofwateroccuronoppositesidesoftheEarthduetothegravitationalinfluenceoftheMoon-onethatroughlyfacestheMoonandanotherontheoppositesideoftheEarth.

II.CausesofTides

SincetheMoonorbitstheEarthslowerthantheEarthrotatesonitsaxis,theEarthrotatesinandoutofthetidalbulges.

Mostcoastlinesexperiencetwohightidesandtwolowtidesabouteachday.

HightidesarecommonlyunequalinheightduetotheaxialangleoftheEarth’srotation.Forexample,thelocationontheEarthatAhasafairlylargehightide.Butwhenthispositionhasrotatedin12hourstotheothersideatA’,notethatthehightideisnotashigh.

NOTTOSCALE!

A

A’

NOAANOTTOSCALE!

ThemostcommonmodelforwhytheEarthhastwotidalbulgesisbecausegravityandinertiaareopposingforcesactingontheEarth’soceans,creatingtidalbulgesonoppositesitesoftheplanet.

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).

Tidalforcesarebasedonthegravitationalinteractionbetweentwoormorebodies.Withtidalforces,thedistancebetweentwoobjectsusuallyismorecriticalthantheirmasses.Thus,theMoon,beingcloserhasamuchstrongereffectontidesontheEarththandoestheSun.

The“back-of-the-envelope”calculationsshowthatthetide-generatingforceoftheMoonismorethantwicethatoftheSun.

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III.TidalVariations

Thevariationoftidesandtidalrangesisduetoavarietyoffactorsincluding:

1. AlignmentoftheMoonandSun2. DeclinationofMoon3. EccentricityoftheEarth’sorbit4. EccentricityofMoon’sorbit5. Regionalfactorssuchastheshapeofthecoastline

1.AlignmentoftheMoonandSun

RememberthatboththeSunandtheMoonaffecttidesonEarth.

WhentheEarth,SunandMoonarealigned,theaffectoftheSunandtheMoonareadditiveandthetidesareattheirhighestandlowest-theseareknownasspringtides.

Springtidesoccurduringfullandnewmoons.

NOAANottoscale.

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

WikimediaUserChris55

3.EccentricityoftheEarth’sorbitTheorbitoftheEartharoundtheSunisslightlyeccentric.PerihelionistheEarth’sclosestapproachtotheSunandis147.1millionkm.AphelionistheEarth’sfarthestdistancefromtheSunandis152.1millionkm.Thedifferencebetweenperihelionandaphelionis~5millionkm.

SincethetidalinfluenceoftheSunisafunctionofthedistance,thesmallchangethedistancebetweentheEarthandSunresultsinasmallchangeinthesolartidalcomponent(heightoftide).

Nottoscaleandorbitaleccentricityisexaggerated.

NASA

4.EccentricityofMoon’sorbit TheorbitoftheMoonaroundtheEarthisslightlyeccentric.PerigeeistheMoon’sclosestapproachtotheEarthandis363,396km.ApogeeistheMoon’sfarthestdistancefromtheEarthandis405,504km.Thedifferencebetweenperigeeandapogeeis~42,000km.

SincethetidalinfluenceoftheMoonislargelyafunctionofthedistance,thetidegeneratinginfluenceoftheMoonincreasesduringperigee.WhentheMoonisatperigee,itappears~14%largerwhenviewedfromEarthandiscommonlyknownasasupermoon.

Nottoscaleandorbitaleccentricityisexaggerated.

NASA

NOAA

TheeffectoftheeccentricorbitsoftheEarthandMoonhaveaverysmallinfluenceontides.Atmost,theeffectmaybeonlyafewinches.Althoughthiseffectmaybesmall,itcanbeadditivewithotherfactorsthataffecttides.

SincethetidalrangeisdrivenbytherelativepositionsoftheEarth,Moon,andSunandtheirellipticalorbits,kingtidesoccurwhentheEarth,MoonandSunarealignedatperigeeandperihelion.

Nottoscaleandorbitaleccentricitiesareexaggerated.

KGOTV

Theterm“kingtide”isnotascientificterm;itiscommonlyusedtodescribeextralargehightides.

Thisimageshowsamodeloftheseasurfaceheightamplitudeinmetersduetoatides.

NASA/TOPEX

5.Regionalfactors

Wefindthatthetidalrangemayvaryconsiderablealongdifferentcoastlines.Underwaterbathymetry(shapeofseafloor)andtheshapeofthecoastlinecandramaticallyaffectthelocaltidalrange.AstheEarthrotateswithinthetidalbulges,thecontinentsdisruptthemigrationofthebulgeintheoceans.

IV.LunarDayandFrequencyofTidesMostcoastalareasexperiencetwohightidesandtwolowtidesinacyclethattakes24hoursand50minutes-alunarday.AlunardayistheamountoftimetakesforaspecificpointontheEarthtorotatefromanexactpointundertheMoontothesamepointundertheMoon-24hoursfortherotationoftheEarthandanadditional50minutesbecausetheMoonhasmovedinitsorbitaroundtheEarth.BecausetheEarthrotatesthroughtwotidal“bulges”everylunarday,coastalareasexperiencetwohighandtwolowtidesevery24hoursand50minutes.

NavalPostgraduateSchool

Anareahasasemidiurnaltidalcycleifitexperiencestwohighandtwolowtidesofapproximatelyequalsizeeverylunarday.Anareahasamixedsemidiurnaltidalcycleifitexperiencestwohighandtwolowtidesofdifferentsizeeverylunarday.

Anareahasadiurnaltidalcycleifitexperiencesonehighandonelowtideeverylunarday.

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NOAA

IftheEarthhadnocontinentsandwasuniformlycoveredbyocean,theentireplanetwouldexperiencetwoequallyproportionedhighandlowtideseverylunarday.OnEarth,thecontinentsblockthemigrationofthetidalbulgesastheEarthrotates.Theinterferenceofthecontinentsresultincomplextidalpatternsindifferentregionsdependingupontheshapeofthecoastlineandseafloorbathymetry(shapeoftheseafloor).

V.MonitoringTides

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Predictingtideshasalwaysbeenimportantforshipping,marineindustriesandcommercialfishing.Forexample,navigatingshipsthroughshallowwaterportsrequiresknowledgeofthetimeandheightofthetidesaswellasthespeedanddirectionofthetidalcurrents.In2002,fourlargeindustrialcraneswereshippedtoOaklandfromChina.TheshipcarryingthecranesneededtogoundertheBayBridgeandclearedthebottomofthebridgebyonly6feetduringlowtide.

Atidegaugeisadeviceformeasuringthechangeinsealevelrelativetoabaselinesuchasmeansealevel.Sensorscontinuouslyrecordtheheightofthewaterlevel.Waterentersthebottomofapipeandsensorsmeasureitsheightinside.Inrecentyears,tidegaugesareautomatedwithelectronicsensorsandreal-timetidedataareavailableonline.

Dataareavailablefrommorethan1,750tidegaugesworldwide.Sometidegaugeshavebeenincontinuousoperationforcenturies.ThemapshowsthedistributionoftidegaugesfortheGlobalSeaLevelObservingSystem(GLOSS)network.

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ThetidegaugeatCrissyFieldhasbeenincontinuousoperationsince1845.

Tidegaugeshavebeenanimportantsourceofdatafordocumentingtheincreaseinmeansealevelduetoclimatechange.ThedatashowmonthlymeanwaterlevelsfromtheSanFranciscoCrissyFieldtidegaugethathasbeenoperatingsince1854.Globalsealevelrise~2.0mm/year.

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