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普通高中课程标准实验教科书 英语选修9 Unit 2 Sailing the ocea

时间:2009-10-13 06:40来源:互联网 提供网友:liyydaana10   字体: [ ]
We may well wonder how seamen1 explored the oceans before latitude2 and longitude3 made it possible to plot a ship's position on a map. The voyages of travellers before the 17th century show that they were not at the mercy of the sea even though they did not have modern navigational aids. So how did they navigate4 so well? Read these pages from an encyclopedia5.
Page 1:
Using nature to help Keeping alongside the coastline
This seems to have been the first and most useful form of exploration which carried the minimum amount of risk.
Using celestial6 bodies
North Star
At the North Pole the North Star is at its highest position in the sky, but at the equator it is along the horizon. So accomplished7 navigators were able to use it to plot their positions.
On a clear day especially during the summer the sailors could use the sun overhead at midday to navigate by. They can use the height of the sun to work out their latitude.
Sea captains observed the clouds over islands. There is a special cloud formation which indicates there is land close by.
Using wildlife
Sailors often saw seaweed in the sea and could tell by the colour and smell how long it had been them. If it was fresh and smelled strongly,then the ship was close to land.
Sea birds could be used to show the way to land when it was nowhere to be seen. In the evening nesting birds return to land and their nests. So seamen could follow the birds to land even if they were offshore8 and in the open sea.
Using the weather
Fog 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 were close to land.
Wise seamen used the winds to direct their sailing. They could accelerate the speed, but they could also be dangerous. So the Vikings would observe the winds before and during their outward or return journeys.
Using the sea
Certain tides and currents could be used by skillful sailors to carry ships to their destination.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 help
Finding longitude
There was no secure method of measuring longitude until the 17th century when the British solved this theoretical problem. Nobody knew  that the earth moved westwards 15 degrees 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 the side of the ship. The rope was tied to a log which was then thrown into the sea. As the ship advanced through the water the knots were counted as they passed through a seaman9's hands. The number of knots that were counted during a fixed10 period of time gave the speed of the ship in nautical11 miles per hour.
Later, when seamen began to use the compass in the 12th century they could calculate longitude 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 that the ship needs to go. In this way the ship could set a straight course even in the middle of the ocean.
Finding latitude
The Bearing Circle
It was the first instrument to measure the sun's position. A seaman would measure the sun's shadow and compare it with the height of the sun at midday. Then he could tell if he was sailing on his correct rather than a random12 course.
                  A Bearing Circle
The Astrolabe
The astrolabe, quadrant and sextant are all connected. They are developments of one another. The earliest, the astrolabe, was a special all-in-one tool for telling the position of the ship in relation to the sun and various stars which covered the whole sky. This gave the seamen the local time and allowed them to find their latitude at sea. However, it was awkward to use as one of the points of reference was the moving ship itself.
The Quadrant
This was a more precise and simplified version of the astrolabe. It measured how high stars were above the horizon using a quarter circle rather than the full circle of the astrolabe.It was easier to handle because it was more portable. Its shortcoming was that it still used the moving ship as one of the fixed points of reference. As the ship rose and plunged13 in the waves, it was extremely difficult to be accurate with any reading.
 The sextant
The sextant was the updated version of the astrolabe and quadrant which reduced the tendency to make mistakes. It proved to be the most accurate and reliable of these early navigational instruments. It works by measuring the angle between two fixed objects outside the ship using two mirrors. This made the calculations more precise and easier to do


1 seamen 43a29039ad1366660fa923c1d3550922     
  • Experienced seamen will advise you about sailing in this weather. 有经验的海员会告诉你在这种天气下的航行情况。
  • In the storm, many seamen wished they were on shore. 在暴风雨中,许多海员想,要是他们在陆地上就好了。
2 latitude i23xV     
  • The latitude of the island is 20 degrees south.该岛的纬度是南纬20度。
  • The two cities are at approximately the same latitude.这两个城市差不多位于同一纬度上。
3 longitude o0ZxR     
  • The city is at longitude 21°east.这个城市位于东经21度。
  • He noted the latitude and longitude,then made a mark on the admiralty chart.他记下纬度和经度,然后在航海图上做了个标记。
4 navigate 4Gyxu     
  • He was the first man to navigate the Atlantic by air.他是第一个飞越大西洋的人。
  • Such boats can navigate on the Nile.这种船可以在尼罗河上航行。
5 encyclopedia ZpgxD     
  • The encyclopedia fell to the floor with a thud.那本百科全书砰的一声掉到地上。
  • Geoff is a walking encyclopedia.He knows about everything.杰夫是个活百科全书,他什么都懂。
6 celestial 4rUz8     
  • The rosy light yet beamed like a celestial dawn.玫瑰色的红光依然象天上的朝霞一样绚丽。
  • Gravity governs the motions of celestial bodies.万有引力控制着天体的运动。
7 accomplished UzwztZ     
  • Thanks to your help,we accomplished the task ahead of schedule.亏得你们帮忙,我们才提前完成了任务。
  • Removal of excess heat is accomplished by means of a radiator.通过散热器完成多余热量的排出。
8 offshore FIux8     
  • A big program of oil exploration has begun offshore.一个大规模的石油勘探计划正在近海展开。
  • A gentle current carried them slowly offshore.和缓的潮流慢慢地把他们带离了海岸。
9 seaman vDGzA     
  • That young man is a experienced seaman.那个年轻人是一个经验丰富的水手。
  • The Greek seaman went to the hospital five times.这位希腊海员到该医院去过五次。
10 fixed JsKzzj     
  • Have you two fixed on a date for the wedding yet?你们俩选定婚期了吗?
  • Once the aim is fixed,we should not change it arbitrarily.目标一旦确定,我们就不应该随意改变。
11 nautical q5azx     
  • A nautical mile is 1,852 meters.一海里等于1852米。
  • It is 206 nautical miles from our present location.距离我们现在的位置有206海里。
12 random HT9xd     
  • The list is arranged in a random order.名单排列不分先后。
  • On random inspection the meat was found to be bad.经抽查,发现肉变质了。
13 plunged 06a599a54b33c9d941718dccc7739582     
v.颠簸( plunge的过去式和过去分词 );暴跌;骤降;突降
  • The train derailed and plunged into the river. 火车脱轨栽进了河里。
  • She lost her balance and plunged 100 feet to her death. 她没有站稳,从100英尺的高处跌下摔死了。
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