Introduction:

Once they left land behind, early navigators of Cook’s time relied on navigational instruments from many cultures.

One of the most important insturments, the astrolabe was a metal disk developed by Greek astronomers 2,000 years ago. It was held vertically and utilized a movable pointer or sight that turned on a pivot that was aimed at the sun. It measured the angle between a celestial body and the horizon, i.e. the sun or the North Star. The pointer on the astrolabe would indicate the sailor’s degree of latitude.

What is latitude and why were early navigators able to travel far distances using instruments that determined it? Latitude describes the position of a point on the earth’s surface in relation to the equator. Latitude lines run parallel to each other from the equator north and south. It is one of two points that describe position, the other being longitude, which is determined by time.
Early navigators also used the compass. The compass-invented by the Chinese to determine direction and widely used by the Arabs-had come into use in Europe only about a century before Columbus. Early compasses consisted of a magnetized piece of metal on a straw. The straw floated in a container of water.

The quadrant, like its modern descendant the sextant, was derived from an Arabic instrument, the kamal. It was used to determine latitude by measuring the altitude of the North Star or the sun above the horizon.

Captain Cook had the most sophisticated celestial navigational equipment of his day on board the Endeavour. To determine how far his ship had traveled, he often used ‘dead reckoning’, essentially guessing the distance by estimating the ship’s speed and how far sideways the ocean currents and winds had blown it each day. To determine Endeavour’s speed, a sailor would drop a cork overboard off the ship’s bow (front), then see how long it took the cork to reach the stern (end). The formula for figuring dead reckoning is: course steered (heading) + time + speed + adjustment (for currents and drift) = distance. (By the way, the term ‘dead’ reckoning’ comes from the term ‘deducted’ reckoning, which, when abbreviated, was spelled ‘de-d’)

These navigational aids allowed Captain James Cook to sail extensively throughout the south Pacific and even as far north as Canada!

Vocabulary:

• Celestial
• Navigation
• Calculation
• Reckoning

Materials:

• Tagboard
• string cut into 12” lengths
• Protractors
• 3/4 “ metal washers
• Pencils
• Journals

Preparation:

• Explain the position of the North Star in relation to the Big and Little Dipper. Point out that it is the only star in a fixed position in the northern latitudes, thereby making it a point of reference for navigators.
• Demonstrate the use of a protractor. Identify its use in measurement of half circles, i.e. 180 degrees.
• Collect the materials listed above to create an astrolabe. Include reference and resource materials that provide visual descriptions.

Procedure:

• Cut a semicircle out of tagboard, the base being 8” in length, the height 4”.
• With the 8” base at the lower edge of the tagboard, the semicircle facing toward the top, use a protractor to mark the degrees. Beginning with 0 degrees at the midpoint of the half circle, label by 10 degree intervals down either side of the semicircle, ending with 90 degrees at either end of the 8” base. With the semicircle facing up, the 0 degrees marking should be at the top.
• Tie the 12-inch string to the middle of the drinking straw. Attach it so it does not slip out of position along the straw.
• Tape the straw firmly in place above the 8” base of the semicircle.
• Tie a metal washer to the opposite end of the string.
• Locate the North Star, then ‘fix’ your position by sighting the North Star through the end of the drinking straw.
• After the weighted string quits swinging, hold the string into place against the tagboard.
• The position of the string at the lower edge of the circle will indicate your approximate latitude by the degrees indicated.

Analyze and Conclude:

• Describe how to locate the North Star. What is its importance in early navigation skills?
• Identify lines of latitude and their position to the equator, north and south.
• Construct a simple navigational instrument, labeling with reasonable accuracy. Demonstrate the ability to locate present latitude with this instrument.
• Record in your journal, descriptions indicating your success in the project, listing difficulties encountered, observations, questions, and relevant information pertaining to the use of navigational instruments.

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