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高中英语 Sailing the oceansReading课件 新人教选修9.ppt

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ReadingThefourbasicinstrumentsfornavigation:SinanInchAstrolabeSextantandnauticalmap.ItwasinventedbyZhangHengintheHandynasty.Itismadeupofabronzefoundation(青铜地盘)andmagneticspoon(磁勺)whichismadeofnaturalmagnetpositionedrightinthecentralofthefoundation.Thehandleofthespoonpointstothesouthwhiletheothersidepointstothenorth.WhentheSinanisinthestillthehandleofthespoonpointstothesouthbecauseofinfluenceoftheterrestrialmagnetism(地磁).Ithas17starslistedaswellasamapoftheskyoverhead(for37degrees)andtheecliptic(黄道).ThetimeofdayornightisontheoutsideedgewithnoonatthetopoftheAstrolabeandmidnightatthebottom.Itcomeswithacarryingchainapouchandabookonitsuse.Thereversesidehasasightformeasuringtheelevationsoftheheavenlybodies.sextantThesextantisanavigationalinstrumentformeasuringtheanglebetweenthehorizonandsomeobjectinthesky.Firstasailorlooksatthehorizonthroughaneyepiece.Atthesametimehecanseelightfromthesunorastarreflectedoffasmallmirrorontopoftheinstrumentontoasecondmirrorandintotheeyepiece.Thenavigatorcanthenseetwoimagesthehorizonandthesunsidebyside.Hecanthenmeasuretheanglebetweenthemonascaleatthebottomoftheinstrument.Thescalegoesfrom0to120degrees.Anearlierinstrumentcalledaquadrantmeasuredinangleinthesamewaybutitcouldonlymeasureanglesfrom0to90degrees.nauticalchartNauticalchartsaremapsofthedepthoftheseaandthecurrentsoftheoceans.Theyprovidemodernsailorswithroutesthroughtheseasratherlikeroadsthroughthecountryside.Theinformationbegantobecollectedinthenineteenthcenturyandhascontinuedtobeupdatedeversince.Thesechartswerenotavai