教材文本Unit2Reading

1、Unit 2 Sailing the OceansPre-readingLook at the pictures. Then answer the questions.1Howdoyouthinkseamenfoundtheirwaybeforemodernaccuratemethodsofnavigation were invented?2 Which do you think was easier to workout: latitude or longitude?3Canyouidentifytheseearlynavigational instruments seamen usedan

2、d explain how they are used?1 _2 _3_4 _4 Which ones do you think are still used today?Now read about what navigation was like before modern instruments were used.ReadingSAILING THE OCEANSWe may wellwonderhowseamenexploredthe oceansbeforelatitudeand longitudemade it possible to plot a ship s position

3、 on a .mapThe voyages of travellers before the 17th century show that they were not at the mercy of the sea even though they did nothave modern navigational aids. So how did they navigate so well? Read these pages from an encyclopedia.Page l :Using nature to helpKeeping alongside the coastlineThis s

4、eems to have been the first and most useful form of exploration which carried theminimumamount of risk.Using celestial bodies North StarAt the North Pole the North Star is at its highest position in the sky, but at the equator it is along the horizon. So accomplished navigators were able to use it t

5、o plot their positions. SunOn a clear day especially during the summer the sailors could use the sunoverheadatmidday to navigate by. They can use the height of the sun to work out their latitude. CloudsSea captains observed the clouds over islands. There is a special cloud formation whichindicates t

6、here is land close by.Using wildlife SeaweedSailors often saw seaweed in the sea and could tell by the colour and smell how long ithad been there. If it was fresh and smelled strongly, then the ship was close to land. BirdsSea birds could be used to show the way to land when it wasnowhereto be seen.

7、 In theevening nesting birds return to land and their nests. So seamen could follow the birds to land even if they were offshore and in the open sea. Using the weather FogFog gathers at sea as well as over streams or rivers. Seamen used it to help identify the position of a stream or river when they

8、 were close to land. WindsWise seamen used the winds to direct their sailing. They couldacceleratethe speed, butthey could also be dangerous. So the Vikings would observe the winds before and duringtheir outwardor return journeys.Using the seaCertaintidesandcurrentscouldbeusedbyskillfulsailorstocarr

9、yshipsto theirdestination.These skills helped sailors explore the seas and discover new lands. They increased their ability to navigate new seas when they used instruments.Page 2 :Using navigational instruments to helpFinding longitudeThere was no secure method of measuring longitude until the 17th

10、century when the British solved this theoretical problem. Nobody knew that the earth moved westwards 15degrees every hour, but sailors did know an approximate method of calculating longitude using speed and time. An early method of measuring speed involved throwing a knotted rope tied to a log over

11、the side of the ship. The rope was tied to a log which was thenthrown into the sea. As the ship advanced through the water the knots were counted asthey passed through a seaman s hands.The number of knots that were counted during a fixed period of time gave the speed of the ship in nautical miles pe

12、r hour.Later, when seamen began to use the compass in the 12th century they could calculatelongitude using complicated mathematical tables. The compass has a special magnetic pointer which always indicates the North Pole, so it is used to help find the direction thatthe ship needs to go. In this way

13、 the ship could set a straight course even in the middle of the ocean.Finding latitudeThe Bearing -CircleIt was the first instrument to measure the sun s position.Aseaman would measure thesun s shadow and compare it with the height of the sun at midday was sailing on his correct rather than a random

14、 course. Then he could tell if heA Bearing Circle The AstrolabeThe astrolabe, quadrant andsextant are all connected. They are developments of oneanother. The earliest, the astrolabe, was a special all-in-one tool for telling the position ofthe ship in relation to the sun and various stars which cove

15、red the whole sky. This gavethe seamen the local time and allowed them to find their latitude at sea. However, it wasawkwardto use as one of the points of reference was the moving ship itself. The QuadrantThis was a morepreciseand simplifiedversion of the astrolabe. It measured how highstarswereabov

16、ethe horizonusinga quartercircleratherthanthe full circleastrolabe. It was easier to handle because it was moreportable. Its shortcomingthat it still used the moving ship as one of the fixed points of reference. As the ship roseand plunged in the waves, it was extremely difficult to be accurate with any reading.ofthewas The SextantThe sextant was theupdatedversion of the astrolabe and quadrant which reduc