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Teacher's Notes for Navigating by the North Star


This navigation lesson plan looks at how sailors used the North Star to navigate on the open ocean. Students make a sextant out of classroom materials and measure the angle of elevation of the North Star to determine their own latitude. Here is some background information for teachers:

Background: Some History of Navigation

The North Star and the Sun provided the most reliable method of determining latitude and direction provided the weather was fair and you were traveling in the Northern Hemisphere. The Sun was also reliable in the Southern Hemisphere, but the North Star wasn't because it sank below the horizon.

The importance of determining latitude and longitude was paramount for navigators in the age of exploration right up until the 20th century. Scientific societies and governments offered great prizes for improvements in these areas. Many of the greatest scientific minds devoted energy to solving the technical problems of navigation. Indeed, many theoretical breakthroughs came as a result of work done on navigation. The English Parliament, for example, offered a large sum of money to anyone who could come up with a method of successfully measuring longitude (the Longitude Act of 1714). It took close to 100 years before anyone could claim this prize.  In 1598, King Phillip III of Spain offered a large pension in ducats to the 'discoverer of longitude'.

This unit emphasizes three important aspects of navigation: direction, latitude, and longitude. Direction and latitude are relatively easy to determine. Practical measurements of longitude, on the other hand, stumped the greatest minds for centuries. The reason for the difference is that latitude is fixed by nature (the location of the poles), whereas longitude is arbitrary.

A compass was one method of determining direction. The Greeks had known about magnetic properties. As well, the Chinese knew that iron stroked by a lodestone would line itself up in a north-south property. But it may have been over 1000 years before an actual compass was made. By around the 10th century the compass had come to Europe by way of Arab traders. By the 12th century compasses were in use in the Mediterranean. 

Early compasses consisted of an iron pin that was magnetized by a lodestone and then either suspended by a thread or placed through a straw and floated in water. They were unreliable for two reasons: they pointed to magnetic north and not true north and they tended to move around a lot as the ship was tossed about in the waves. Indeed, a great deal of energy was devoted to finding ways to stabilize compasses. Finally, in 1908, the gyroscopic compass was developed, which not only stabilized the compass but also pointed to true north. But in the early days of the Age of Exploration, the North Star was a more reliable method for determining direction.

Like compass direction, latitude was also easy to measure. There were various methods to determine latitude available to mariners. They could use the length of the day at any time of year or they could use guide stars. Of the guide stars, the easiest to use was the North Star. It is so easy to use in fact, that students should have no problem determining their latitude using the crude sextant that they make in this class. The latitude on the map is the angle of elevation of the North Star relative to the horizon.

The real problem of navigation was longitude. To measure longitude you needed an accurate clock. But the clocks of the past weren’t up to the job. Pendulum clocks swayed wildly at sea. Spring clocks wound down and as they wound down they gave imprecise readings of longitude. The spring was also affected by changes in temperature and changes in gravity at different latitudes. There were many attempts to determine longitude some serious and some based on quackery. Of the serious attempts, the problem was often the practical application of the theory in a harsh environment at sea. Here follows some of the attempts to determine longitude:

(1)   The Moons of Jupiter: Galileo, who discovered the moons of Jupiter, noted that the moons of Jupiter were eclipsed by Jupiter according to a regular, predictable pattern over 1000 times a year. Sailors could determine their local time, by the rise of certain stars and they could determine their time relative to home by watching for Jupiter eclipse events if they knew what time those eclipse events were supposed to be seen at home. This worked well on land. And, indeed, it became the standard for surveyors measuring longitude across continents.

(2)   Lunar Measurement: Similar to the moons of Jupiter, other astronomers tried to calculate when certain stars would become eclipsed by the moon as the reference to measure longitude (Again giving them a reference time to compare local time by). They also tried to calculate the distance of the Moon from the Earth as a means of determining longitude.

(3)   Magnetic Variation: Edmund Halley tried to calculate longitude based on the difference between true north as determined by the North Star and magnetic north as determined by a compass. But magnetic needles rarely pointed precisely to magnetic north and the magnetic field strength waxed and waned at different regions of the sea, which made this method too unreliable to be practical.

(4)   Wounded Dog Theory: This quack theory was based on a ‘Powder of Sympathy’, which was powder that could ‘heal’ from a distance if spread on an article of clothing from the injured person. In this method, a dog was cut and put aboard a ship. A piece of bandage from the dog was given to someone left at home. Everyday at an agreed upon time, the powder would be applied to the dog’s bandage and this would cause the dog to howl wherever it was. The sailors on the ship would listen for the dogs howling and ‘know’ the time back home, giving them the second time reference point. 

Recommended Reading: Longitude: The True Story of a Lone Genius Who Solved the Greatest Scientific Problem of His Time. By Dava Sobel

Unit: Navigation by the North Star

Students should learn the following key ideas:

(1)   Why navigation was important.

(2)   What the important aspects of navigation are.

(3)   How to find the North Star, Big Dipper, and Cassiopeia.

(4)   How Stars move across the night sky.

(5)   How a sextant works and how to make a crude sextant.

(6)   How to measure latitude using a sextant and the North Star. 

Using the sextant

I have constructed the sextant as described in the student handout. I was pleasantly surprised to get a very accurate reading. A real sextant doesn’t use a weight, but instead uses a series of mirrors to superimpose the horizon on top of the object that needs to be measured. Using a weight would be difficult on the ocean where the waves are rocking the boat. Nevertheless, the principal is the same: Measure the angle of elevation of the North Star and you can figure out your latitude.

However, I find it difficult to ‘site; the stars at first and nearly gave up. But once I got the hang of it, I experienced no further difficulties. I took several readings over several nights and I was within 1 degree of my latitude every time.



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