How would a compass work on Mars

Information sheet 18: Cardinal points and compass

Publication: 26.3.2009
Origin: La main à la pâte, Paris

From the curriculum

Level 3:

Heaven and Earth

  • Cardinal points and compass

Difficulties due to common language use or the language of science

The word "north" describes both areas (in the north) and the direction of the compass (see information sheet 20: "The rotation of the earth around itself" on the subject of the rotation of the earth from west to east).

The physical phenomenon of magnetism should not be confused with the occult notion of a person's "magnetism".

The four cardinal points are indicated on a weather vane (or on a map). One might think that there are poles in the east and west as well as in the north and south, but that is not the case.

Difficulties due to students' preconceptions

Students sometimes say that the compass needle is attracted to the north because of the cold, the temperature, or the wind.

The students believe that with a compass you can know where you are and, if you get lost, you can use it to find your way. This is not the case (the compass alone is not enough, see the following paragraph).

Up to level 3, students have difficulty using anything other than their "egocentric reference points" (front, back, left, right). They often try to memorize the cardinal points in relation to their own body position ("North is in front of me or above me", "West is on the left" etc.). They also tend to absolutize the cardinal points without realizing that the position of one place can only be given in relation to that of another. For example, they like to say "Location X is north" instead of saying "Location X is north of location Y". They also have difficulty understanding that location X can be, for example, in the north of location Y and at the same time in the west of a third and in the south of a fourth location.

A few cliffs where observation and experiment could fail

If you want to determine north, you must make sure that the compass is far away from any magnetic material that could interact with it and that you hold it horizontally so that the needle can rotate freely.

On a map you can use the compass to determine the place where you are if you transfer your changes to the map from a known position. In this way you can reach a given point on the map.

The pupils should be made aware that although one can orientate oneself to the cardinal points with the help of the compass, it is not possible to orientate oneself in interplanetary space. However, you could also use the compass to determine cardinal points on other stars, such as the planets or the moon, which also rotate around an axis (see "More on the subject" ...).

knowledge

The magnetic needle on a compass (when it is far away from magnetic material) points approximately in a north-south direction, with north generally indicated by the colored end of the needle. The basic feature of the compass is that the needle maintains its direction when you turn the case.

With the compass you can also find your way in fog. On the other hand, the display can be disturbed when a thunderstorm is approaching. If you get lost, the compass prevents you from walking around in circles, you can keep a direction chosen on the map.

There are two poles (the north and south poles) on the surface of the earth and the axis of rotation of the earth goes through these two poles. There is no east pole and no west pole.

more on the subject

You can also say "Bussole" instead of "Compass". [The French term "boussole" originally means "small box". "Compas" (French) does not designate the compass, but the circle with which circles are drawn. But compasso (Italian) means compass and this is probably where the German name, A.d.Ü. comes from]. The magnetic compass was first used in China (around 1050).

Whenever a celestial body has an axis of rotation, a north and a south can be defined on it, as well as an east and a west, and maps can be created, similar to the one on earth: maps of the moon, Venus, and Mars.

Whether a compass works on these planets depends on the existence of a magnetic field (which is absent on the moon and is absent or very weak on Venus and Mars).

If at one point on earth north and south have been determined with the compass, one can also determine east and west and all other directions, such as southwest and northeast, and one can construct a compass rose and follow it.

By drawing latitudes and longitudes on a sphere (world map) one overcomes the "egocentric reference system" (front-back-top-bottom) and objectifies the cardinal points.

If one goes straight north, one follows a longitude (meridian). When you arrive at the North Pole, there is no more north and if you want to go south, you have the choice between an infinite number of paths, each path corresponding to a meridian. In the northern hemisphere, for example, you can also use the North Star for orientation. It stands in the sky roughly where the extension of the earth's axis points.

The compass may arouse curiosity to want to know more about magnetism. But caution is advised with all further explanations and one is well advised to be satisfied with a few statements in the knowledge of the complexity of the topic.

The compass needle is a magnet with a north and a south pole ("pole" is an extension of the term in analogy to the poles of the earth). Two magnets can attract or repel each other, depending on the position of the poles to each other. Therefore, one must keep a compass away from any magnet or magnetic material. Non-magnetic materials made of iron are magnetized when they come near a magnet (example: needles, nails). Some coins, for example the 1 euro coin, are attracted to a magnet because they contain nickel. A coil of conductive wire, in which a current flows, behaves like a magnet.

Use of knowledge in other contexts, related terms

The subject of cardinal points is related to the exploration of the apparent movement of the sun (see information sheet 19: "Apparent movement of the sun") and the formation of shadows (see information sheet 17: "Light and shadow").

If we look towards the sun at (solar) noon in the northern hemisphere, west is on our right and east on our left: the shadow of a gnomon points due north at this time (the gnomon, ancestor of the sundial, is a simple one Rod that sticks vertically in the ground). In the southern hemisphere the situation is exactly the opposite. By working with a compass and points of the compass, students learn to orientate themselves, take directions and read maps. The work can be done in conjunction with geography and sports.

Last update: April 20, 2015

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