[from the planetarium program Who Discovered America?]
1. A Box Compass Projector
A set of transparencies that you can use with an overhead projector to show how Columbus adjusted his compass using the North Star. It is not a real compass, since the needle is not magnetized.
Use a file folder or cardboard to make a frame large enough to cover the surface of the overhead projector, even when turned in a circle. Cut out the center of the frame and tape the box transparency to it.
Cut out the needle circle and the rose circle from the transparency.
Using a pushpin or thumb tack, pierce the box, needle, and rose transparencies on the “x” at the center of each object.
Using a small nail (or larger thumbtack), enlarge the holes in the rose transparency and the box transparency so that they will be loose on the center pivot.
Make the center pivot by inserting a thumb tack through center hole from the bottom of the box transparency, and securing it with a small piece of transparent tape.
Optional Transparent Compass
You may wish to demonstrate a modern compass to show the deflection of the needle caused by a small magnet. Compass needles respond to the small magnet just as they do to the giant magnet—the Earth. You can use a regular compass and pass it around the room with a magnet for students to experiment with. Or, you can obtain a transparent compass to show on the overhead projector. Modern transparent magnetic compasses for classroom demonstration on the overhead projector are available from a number of sources. Three vendors are:
Mansfield, OH 44901-8101
800-225-FREY or 419-589-1522
Skokie, IL 60076-1026
800-SARGENT or 312-677-0624
Science Kit Boreal Laboratories
The North Star is good for telling navigators which way north is on clear nights. But to find your directions in the daytime or on cloudy nights, you need another device that was invented in China over a thousand years before Columbus made his famous voyages.
Do you know what that device was? [Compass.] Have you ever used a compass? [Take responses.]
About 1,530 years ago, a Chinese Buddhist priest named Hwui Shan and his company of missionaries sailed northeast to the Aleutian Islands and Alaska. He then followed the Pacific Coast of the Americas, calling it Fusang, which means “Fabulous.”
Hwui Shan may have traveled as far south as Mexico. Details of his forty-year journey are found in the Chinese Imperial Archives. In his report to the Emperor, Hwui Shan described in detail the lands and people he saw. Many of the descriptions closely match what archaeologists believe the Americas were like at that time.
The compass has been used by the Chinese for navigation since its invention many centuries ago.
Let’s see how the compass works. Let’s imagine that we are in ancient China, navigating with a compass. On this overhead projector, I am going to make a demonstration compass so we can see how it works.
Sunset. Stars. Move latitude to about 36° N. Turn the compass projector on.
This is the box that holds the compass. It is attached to the ship. The triangular part indicates the prow, or front of the ship. It shows which way the ship is pointing.
Demonstrate movement of box transparency (move by hand). Then add the Compass Needle layer. Demonstrate movement of compass needle (move it slightly by hand). Then adjust it so that it points about 15° to 20° away from the North Star.
This is a compass needle on the pivot at the center of the compass box. Ship’s pilots all carried two or three lodestones, or natural magnets, used to magnetize a needle. The needle was placed on a pivot, so that it would stay flat as the ship rocked. If the ship turns, the compass needle still points in the same direction.
Demonstrate by holding the needle in place so that it points in the same direction while you pivot the box layer.
Does the needle point to the North Star? [No.]
That’s because the compass needle does not necessarily point true north. It points in a direction towards the North Pole, called “magnetic north.” The difference between magnetic north and true north is not the same everywhere on the Earth. So, sailors needed to adjust their compasses to true north by the stars.
Place Compass Rose layer on the needle in some random orientation.
Here is a card, called a compass rose, that indicates the directions. We can adjust the compass rose so that it points true north, and then attach it to the top of the needle. This way, the compass rose always points to true north, even as the ship turns.
How do we find true north? [North Star. Have someone point it out. If necessary review how to find the Big Dipper and North Star.]
The most accurate way to find north, east, south, and west is by using the North Star. Now we’ve found the North Star. Since the North Star is directly over the North Pole of the Earth, if we turn toward the North Star, we are facing directly north.
Can you find east, south, and west, now that we know where true north is located? [Invite responses.]
Turn on N,E,S,W and point to them, or hang up NESW signs on your planetarium dome.
We need to adjust the rose so it points to true north. May I have a volunteer please?
Turn the compass back on. Show volunteer how to adjust rose so it points to the North Star. Hold the Box and Needle still while the student adjusts the Rose. The navigator has the best angle, so he or she is in charge. At other places in the planetarium, the needle may not appear to be quite aligned with the North Star. Another visitor can come up to check the navigator’s alignment.
If the ship turns, the compass rose still points toward north. This pointer on the box shows us the direction the ship is sailing.
Turn box layer with one hand. Keep needle and rose from moving with your other hand.
Let’s start sailing eastward from China.
Is our ship headed eastward? [Take group suggestions for correct adjustment of box.]
Ocean sounds. Turn daylight up and down a couple of times to indicate days going by. Secretly throw the compass needle off by 10° or more.
The difference between magnetic north and true north changes a little bit as we continue to sail hundreds of miles to the east. Every time the stars are out, we can check the compass to see if the rose is still pointing towards the North Star. We must adjust it, if it is off.
Have a student volunteer adjust the compass. Continue sailing for a few more days; secretly throw off the compass; and have other students come up to adjust the compass as desired.
Turn off compass.
Even with modern magnetic compasses, you need to know the angle between magnetic north and true north to use a compass successfully. It is called the angle of deviation and is shown on navigational maps. The angle of deviation is different for various locations on the Earth.
Show a modern transparent compass on overhead projector. Use a magnet to show how it attracts the compass needle. Remove the permanent magnet and turn the compass to show how the needle always points towards magnetic north—aligning itself with the Earth’s magnetic field.