搜索
TheGreatWallofChinaisaseriesofstoneandearthenfortificationsinnorthernChina,builtoriginallytoprotectthenorthernbordersoftheChineseEmpireagainstintrusionsbyvariousnomadicgroups.Severalwallshavebeenbuiltsincethe5thcenturyBCthatarereferredtocollectivelyastheGreatWall,whichhasbeenrebuiltandmaintainedfromthe5thcenturyBCthroughthe16thcentury.Oneofthemostfamousisthewallbuiltbetween220–206BCbythefirstEmperorofChina,QinShiHuang.Littleofthatwallremains;themajorityoftheexistingwallwasbuiltduringtheMingDynasty.TheGreatWallstretchesfromShanhaiguanintheeast,toLopNurinthewest,alonganarcthatroughlydelineatesthesouthernedgeofInnerMongolia.ThemostcomprehensivearchaeologicalsurveyhasrecentlyconcludedthattheentireGreatWall,withallofitsbranches,stretchesfor8,851.8km.Thisismadeupof6,259.6kmsectionsofactualwall,359.7kmoftrenchesand2,232.5kmofnaturaldefensivebarrierssuchashillsandrivers.TheChinesewerealreadyfamiliarwiththetechniquesofwall-buildingbythetimeoftheSpringandAutumnPeriod.DuringtheWarringStatesPeriodfromthe5thcenturyBCto221BC,thestatesofQin,Wei,Zhao,QiandYanallconstructedextensivefortificationstodefendtheirownborders.QinShiHuangconqueredallopposingstatesandunifiedChinain221BC,establishingtheQinDynasty.ToprotecttheempireagainstintrusionsbytheXiongnupeoplefromthenorth,heorderedthebuildingofanewwalltoconnecttheremainingfortificationsalongtheempire'snewnorthernfrontier.Mostoftheancientwallshaveerodedawayoverthecenturies,andveryfewsectionsremaintoday.Later,theHan,SuiandJindynastiesallrepaired,rebuilt,orexpandedsectionsoftheGreatWallatgreatcosttodefendthemselvesagainstnortherninvaders.TowardstheendoftheMingDynasty,theGreatWallhelpeddefendtheempireagainsttheManchuinvasions.However,whenthegatesatShanhaiguanwereopenedbyWuSangui,theManchusquicklyseizedBeijingandestablishedtheQingDynasty.UnderQingrule,China'sbordersextendedbeyondthewallsandMongoliawasannexedintotheempire,soconstructionandrepairsontheGreatWallwerediscontinued.从春秋时代开始，我国古代人民便已熟悉城墙建造技术。在从公元前5世纪至公元前221年的战国时期，秦、魏、赵、齐和燕各国建立广泛的防御工事来保卫自己的边界。公元前221年，秦始皇征服了所有反对国家而统一了个国，建立秦朝。为了避免匈奴人的入侵，他下沿着整个国家的北部边界建立新的城墙，以连接残留的防御工事。古城墙大部分已侵蚀了数百年，只有极少数路段保留至今。后来的长城由汉，隋和晋朝以极大的代价来修理，改建或是扩建长城以捍卫自己免于北方的侵略。在15世纪40-60年代，明朝修建了所谓的“辽东长城”。明朝末年，长城抵御了满清帝国入侵。然而，当山海关隘口被吴三桂打开后，满族人迅速占领了北京，建立了清朝。在清朝统治下，中国的边界一直延伸到长城之外和蒙古，所以建造和维修长城的工事被停止。1.NorthPassofJuyongguanpass,knownastheBadaling.Madeofstoneandbricksfromthehills,thisportionoftheGreatWallis7.8meters(25.6ft)highand5meters(16.4ft)wide.“北通”居庸关，也就是八达岭。由石头和砖块砌成，这部分长城高7.8米（25.6英尺）宽5米（16.4英尺）。2.“WestPass”ofJiayuguan.ThisfortisnearthewesternedgesoftheGreatWall.“西通”嘉峪关。这个要塞靠近长城的西部边缘。3.EastPassofShanhaiguan.ThisfortisneartheeasternedgesoftheGreatWall.“东通”山海关。这个要塞靠近长城的东部边缘。TheGreatWallisthemightiestworkofman,theonlyonethatwouldbevisibletothehumaneyefromthemoon.--RichardHalliburton长城是人类最伟大的工程，是唯一可以从月球上直接用肉眼看到的东西。——理查德·哈里伯顿。TheGreatWallisamaximum9.1m(30ft)wide,andisaboutthesamecolorasthesoilsurroundingit.Basedontheopticsofresolvingpower,onlyanobjectofreasonablecontrasttoitssurroundingswhichis70mi(110km)ormoreindiameterwouldbevisibletotheunaidedeyefromthemoon.TheapparentwidthoftheGreatWallfromthemoonisthesameasthatofahumanhairviewedfrom2milesaway.Toseethewallfromthemoonwouldrequirespatialresolution17,000timesbetterthannormalvision.Unsurprisingly,nolunarastronauthaseverclaimedtohaveseentheGreatWallfromthemoon.长城最宽是9.1米（30英尺），颜色和周围土壤的颜色相似。基于光学分辨能力，人们从月球上直接用肉眼可以看到直径70英里（110公里）以上的物体。从月球表面看长城的宽度就好比从2英里外看人类的头发一样。要从月球上看到长城的话将需要优于正常视力17,000倍的空间分辨率。因此，月球的宇航员从未声称已经从月球上看到长城也就不奇怪。AmorecontroversialquestioniswhethertheWallisvisiblefromlowearthorbit(analtitudeofaslittleas100miles(160km)).NASAclaimsthatitisbarelyvisible,andonlyundernearlyperfectconditions;itisnomoreconspicuousthanmanyotherman-madeobjects比较有争议的问题是，长城在低地球轨道上(海拔仅为100英里（160公里）是否可见。美国国家航空航天局声称几乎是看不到的，只有在近乎完美的条件下才可以；它并不比其他许多人造的物体更显著。