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1、个人收集整理勿做商业用途TEＸＴAThemerｉｄiansof ｌoｎgitudeａｒｅimagｉnary greａｔ cirｃｌeｓdrawｎ fｒｏmpolｅ topoleaｒouｎdtheearth.Byinternａtionaｌagreement,ｔｈｅmｅrｉdiaｎofｌｏｎgitudepａsｓingthrougｈGreeｎwich,Ｅnglａｎd,is numbｅred zero．Thｅearｔhｉsｄｉvideｄｉｎto3６0 dｅgｒeeｓ,andthｅ merｉdｉａns aｒenumbered eastanｄｗeｓｔｆｒｏmＧreeｎwｉｃh．The
2、reare1８0degreｅsoflongiｔｕdeeastofＧｒeenwiｃｈａｎd １80ｄegreesinthewesｔerly dirｅctioｎ.NewYoｒk hａsａ longitude ｏf ７4ｄegreeswest(74ºＷ) wｈich meａnstｈaｔ it liesoｎthｅ74ｔhmerｉdianwestoｆ Grｅｅnwｉch.Siｎcethesｕｎ appearstotraｖelａrouｎdtheeａrtｈ in2４hours,itwiｌｌmove ３６0/24or15degｒeｅｓ in onehｏur. Ｔｈｉsreａsoningc
3、ａｎ ｂeused ｂy ｎavigａtors ｔoｄetermiｎe their loｎgiｔude.Ｉmａginethatwe havesetsａiｌfrom Gｒｅeｎwｉｃh, Englanｄ,ａftｅrｈavingsｅtavery ａccｕｒatｅ ｃlock, ｏrchroｎometer,to ｔｈｅｌocal Greenwichtime.Ａsｗｅtrａvｅl weｓtwaｒd towardNewYork,weｎｏtice thａttheｓun isgoiｎg“ｓlowｅｒ” thaｎoｕrchronomｅｔer.Aｔ thetiｍeｔhａtｏurtiｍｅpi
4、eｃｅrｅads 12ｏ’cｌocｋ,tｈe suｎｈaｓnotｑｕitｅreachedthe ｚeniｔｈ.Aｓamaｔteroｆfａcｔ, wｈeｎourclockｒｅadsnoon,whaｔ iｔ reallymeａns is thatiｔ’snｏoninＧｒeenｗich,Ｅｎｇｌand．Ouｒclockｃonｔinueｓto teｌlｕsｔheｔime,nｏtat oｕr prｅｓｅntlocation，ｂuｔat Greenｗｉch.Leｔ us waituntｉlｔｈesｕniｓ dｉｒectｌyoverhead (nｏｏnaｔouｒlocation）and
5、thenreadthetｉmeonｏｕrclｏck.Ｓupｐoｓｅitrｅads1o’clocｋ.Tｈisｍeａnsthattheｒeiｓonｅ hour’sdiffereｎｃein tｉme betweｅｎoｕr ｌongitudｅ anｄ ｔｈatｏｆ Grｅenｗich. Ａswementionｅｄeaｒlieｒ, ｔhiｓcorrespｏnｄsｔoexactｌy 15degreesoflｏngituｄe,soｏurｌongitudｅmusｔbｅ １5ºW.Thｅworｌdisdiviｄedinto24ｔｉｍezones,andeacｈzｏneｃｏrresｐｏｎds
6、ｔｏ15degrｅesof loｎgｉtude.NｅwＹｏrkis ａppｒｏxｉmatelｙ5time zｏnｅswestoｆ Gｒｅｅnｗｉch,sｏｔhetｉme diｆｆｅrence mustbe about５hｏuｒs． Ｂymaｉntaining chroｎometｅrsｏnＧｒｅeｎwichtｉme,sｈiｐscaｎ deteｒｍinｅthｅir ｌｏngitudeｏｎ anysunｎｙdａybymｅrelynotingthedifferｅnｃe ｉn hoｕrsｂetwｅenGreｅnwicｈ ｔimｅ ａndlocaｌｓuntimｅａnd ｍultｉpl
7、ｙｉngｔｈisdiｆｆeｒｅｎcｅ by１5dｅgｒｅes.Ｏfcourse,lonｇitudegiｖesonlyhalf oftheinfｏrｍationneededto detｅrｍineｏｕrprecisｅ loｃａtｉon.Wｅｍuｓtalsokｎowoｕrlatitude,which tｅllsushｏw ｆarweａre nｏrtｈor southoｆ theEｑuａtor. ＴheEquatoｒｉsthezero linｅforｔhemeasuremｅnｔoflatiｔuｄe．Cirｃlesaredrawnpａ
