A Sense of Direction
The Invisible Compass

Background
The outside world gives animals three primary cues from which they could develop their sense of direction. These cues come from the stars, the sun, and the Earth's magnetic field. Research suggests that different animals navigate using several of these cues in combination, rather than relying on a single method. This way, their ability to navigate operates whether it's night or day, clear or cloudy. In these activities your students can test their own sense of direction. They can then experiment with the methods animals use as natural compasses: the stars, sun, and the Earth's magnetic field.

Activities
Testing Your Own Sense of Direction
Before discussing any of the compasses below, tell students you are going to play a game that will test their own sense of direction and help them become familiar with compass bearings. Go into the gymnasium and arrange students in a large circle. Have them close their eyes. On the count of three, call out a direction. All students should point to that direction and freeze.

Next develop a game of elimination. In each successive round, tell students how closely they must estimate the given direction in order not to be eliminated. For the first rounds, use the cardinal directions so that students can become familiar with compass readings in degrees (i.e., north
= 0 degrees, east = 90 degrees, south = 180 degrees and west = 270 degrees). It may be helpful to post these directions and compass readings on the appropriate walls in the gym. Initially, eliminate students when they are off by 45 degrees. As the games continues and students improve,
you may want to decide as a class how close the guesses must be.

The Stars as a Compass
Few people realize that the night sky rotates in a complete circle every 24 hours. All the stars appear to move around the sky in a big circle except one-the North Star. This is because the North Star is positioned very near the North Pole, the axis of the Earth's rotation. Therefore, the North Star seems to stay in the same place all night. Explorers have used the North Star for navigational purposes for centuries. It might be a surprise to know that animals can use stars as a compass too.

A. Ask students how they might use the North Star as a compass. Show them how to locate the North Star in the night sky. To do this they must be able to find the Big Dipper. As shown in this diagram, two stars in
the bowl of the Big Dipper point to the North Star.

B. Have students make two sketches of the night sky. The sketches must be drawn on the same night, preferably one at dusk and the other at dawn. (If this is not possible, make certain that the sketches are drawn at least four hours apart.) Each sketch must include the position of the Big
Dipper and the North Star. Students should draw as if they are lying on their backs and looking up at the sky. Each sketch should indicate the directions north, east, south, and west. Students should also note the time the drawing was made.

C. The next day, help students visualize the full rotation of the stars by making paper models. Each student should draw a 24-hour clock on a circle. On a smaller disk, have them draw the North Star and Big Dipper. A paper fastener can be used to connect them. By rotating the model, students can compare the timings of their drawings to the position of the stars.
The Sun as a Compass
Humans are able to use the sun for a rough sense of time and direction. We know the sun is in the east in the morning, in the south at noon and in the west in the evening. In order to use the sun for navigational purposes, migratory animals must use far more precise and sophisticated methods.
Remember that while the North Star remains stationary, the sun is a moving target. It "moves" across the sky during the course of a day-so animals must have mechanisms in place to compensate for this.

A. In this activity, students measure the changing position of the sun in order to understand the difficulties involved when using it for navigation. Have students draw the sun's position in the sky at three distinct times during the day, i.e., at 8 am, noon, and 3 pm. Using a compass, have them measure the sun's bearing at each time.

B. To emphasize the complexity of relying on the sun for a sense of direction, repeat this process six to eight weeks later. Before going outside, have students review their previous measurements and predict the sun's location. Then, go outside and draw the sun's actual position and bearings.

Caution: So that students do not look directly at the sun, have them use their shadow and correct their bearings by 180º.

C. Encourage students to researchsuch topics as polarized light, sundials, human navigation and orienteering.

The Earth's Magnetic Field as a Compass
Perhaps the most astonishing discovery in navigation research is that animals may have a "sixth sense" that helps them navigate. This sense lets them detect the Earth's magnetic field.

A. Let your students observe a magnetic field in action. Using a classroom set of compasses and bar magnets, let them move a compass around the magnet. They should note the direction the compass needle points when it is held at different places beside the magnet. Ask students what is causing the needle to move. (See discussion questions below.) Explain that the needle is pointing toward the bar's magnetic field and that the Earth is surrounded by a similar
magnetic field.

B. Have your students consult an Earth Science textbook for a fuller explanation of the Earth's magnetic field. They can also investigate how animals may utilize the magnetic field in navigating. Have students watch for references to certain animals' use of the magnetic field as they track the migrations on-line.

Discussion
1. How does the position of the stars help in navigating?
2. Why is it more difficult to navigate by the sun?
3. What's happening to the compass needle as it moves beside the bar magnet? Why? Can you feel, see, smell, hear, taste anything in the air around the magnet?
4. If you could have a "sixth" sense, what would it be?