Warming up、Pre-reading、Reading、Comprehending教学设计(教案)

选修9 Unit 2

Sailing the oceans

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We may well wonder how seamen explored the oceans before latitude and longitude 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 navigate so well? Read these pages from an encyclopedia.

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Using nature to help Keeping alongside the coastline

This seems to have

been the first and most useful form of exploration which carried theminimum

amount of risk.

Using celestial bodies

North Star

At the North Pole the North Star is at its highest position in the sky, but at the equator it is alongthe horizon. So accomplished navigators were able to use it to plot their positions.

Sun

On a clear day especially during the summer the sailors could use the sun overhead at middayto navigate by. They can use the height of the sun to work out their latitude.

Clouds

Sea captains observed the clouds over islands. There is a special cloud formation whichindicates there is land close by.

Using wildlife

Seaweed

Sailors often saw seaweed in the sea and could tell by the colour and smell how long it hadbeen them. If it was fresh and smelled strongly,then the ship was close to land.

Birds

Sea birds could be used to show the way to land when it was nowhere to be seen. In theevening nesting birds return to land and their nests. So seamen could follow the birds to landeven if they were offshore and in the open sea.

Using the weather

Fog

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.

Winds

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 theiroutward 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.

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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 logover the side of the ship. The rope was tied to a log which was then thrown into the sea. Asthe ship advanced through the water the knots were counted as they 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 per hour.

Later, when seamen began to use the compass in the 12th century they could calculate longitudeusing complicated mathematical tables. The compass has a special magnetic pointer whichalways indicates the North Pole, so it is used to help find the

direction that the ship needs togo. 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'sshadow and compare it with the height of the sun at midday. Then he could tell if he wassailing on his correct rather than a random course.

A Bearing Circle

The Astrolabe

The astrolabe, quadrant and sextant are all connected. They are developments of oneanother. The earliest, the astrolabe, was a special all-in-one tool for telling the position of theship in relation to the sun and various stars which covered the whole sky. This gave theseamen the local time and allowed them to find their latitude at sea. However, it was awkwardto 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 plunged in thewaves, 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.

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THE GREATEST NAVIGATIONAL JOURNEY:A LESSON IN SURVIVAL

I am proud to have sailed with Captain Bligh on his journey of over 40 days through about 4,000miles in an open boat across the Pacific Ocean in 1789. Our outward voyage in the "Bounty" to

Tahiti

had been filled with the kind of incidents that I thought would be my stories when I returned home. But how wrong I was! On our departure from Tahiti, some of the crew took over the ship.They deposited the captain into a small boat to let him find his own way home. But who else was to go with him? Those of us on board the "Bounty" were caught in a dilemma. Was it better to risk certain death by sitting close together on a small, crowded open boat with very little food and water? Or should one stay on the "Bounty" with the crew and face certain death from the British Navy if caught? The drawback of staying on the ship seemed to grow as I thought about how wrong it was to treat Captain Bligh in this way. So I joined him in the small boat. As dusk fell, we seemed to face an uncertain future. We had no charts and the only instruments the captain was allowed to take with him were a compass and a quadrant.