Earth, Moon, Sun

Authors : David Wilgenbus(plus d'infos)
Summary :
From an old television advertisement for a telephone operator, participants question the possibility that sunrise and sunset occur simultaneously on Earth. They are thus placed in an investigation situation and explore the Sun-Earth system to understand the origin of the day/night and seasons alternation. A second problem situation offers a new point of view: by "standing" on the moon, participants try to predict the movement of the Earth, the presence or absence of phases. This helps them understand the origin of the Moon phases and eclipses, and the visible and the hidden sides of the Moon. Thirdly, participants characterise the different stages of the investigation .
Publication : 13 September 2012
Objectives :
How to carry out a scientific activity based on the investigative approach? The interest of astronomy is to enable both an experimental and modelling approach, which cannot be carried out in other areas
Material :

-  a room that be darkened;  a video projector or an overhead projector;  10 Styrofoam balls 10 cm in diameter (or 10 grapefruits); 10 balls 5 cm in diameter (or 10 small oranges); 5 flashlights; 1 globe (used for discussion sessions, "forbidden" to the groups at the beginning); - skewers; 5 canvasses; 2 identical desktop lamps; a few squares of chocolate; toothpicks; some cardboard; scissors, glue, markers, string; large sheets of paper for posters

Note :
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The astronomy workshop starts from a situation, inspired from a French Télécom advertisement, according to an idea of Gilles Cappe, a science resource teacher at Montivilliers, in the Academy of Rouen.

  
A man with a mobile phone facing the sea, watching the sunset. A woman with her mobile phone sitting on the Great Wall of China, looking at the horizon.
The man, when he sees the sun disappearing, asks the woman: "Do you have it?" "At this moment, the woman sees the sun rising and replies: "Yes, I have it"

NB: can also project the video (online here) :

 

Questions :

  1. Is it possible ?
  2. If yes, where is the man (left image) ?
  3. And if no, why ?

Material (some useful and some deliberately useless) is available, allowing the trainer to assess  whether the participants reflect on the usefulness of the material before carrying out an experiment.
At one workshop, many groups, for instance, took canvasses, without a prior idea about their use. During the workshop’s sharing phase, this was discussed.

Deliberately, no instruction was given on writing. During the sharing phase, those who had not taken any notes could not remember the process they underwent.  We thus thought about what could have been jotted down: questions, hypotheses, the manner in which they might be tested, the description of the experiment , the results and their interpretation.

The first question (Is it possible to have sunrise and sunset simultaneously in 2 places on Earth?) enables work on what  day and the night are (part of the globe illuminated or not by sunlight). Generally, teachers do not face any difficulty at this stage, but they are always unable to explain where the sun rises, and where it sets, which tells the direction in which the Earth rotates. They use their knowledge (it is anticlockwise / East to West, etc.), but are not convinced and unable to  to provide any  justification other than "I know it." Similarly, all of them have the Earth rotate, when we could also make the Sun revolve around the Earth. During the workshop we discussed about different ways of reproducing day and night, and seek to validate the following hypothesis:

    1. The Earth rotates;
    2. The Sun revolves around the Earth;
    3. The Earth revolves around the sun (the day and night last 6 months!);
    4. The Sun is eclipsed periodically (there is no sun rise or sunset)
    5. And many other fancier hypotheses, that can easiliy be refuted (examples: "Somebody periodically switches off the Sun" or "a planet or the Moon passes in front of the Sun", we should thus not see the rising  or setting of the sun but either an abrupt day / night transition, or eclipses of the sun each day!)

Hence, the two first hypotheses remain, and the trainer asks the trainees to think of a way to validate or invalidate them. After having played  around with their flashlights, made shadows, drawn the path of the Sun in the sky, we conclude that no experiment can lead to a conclusion! The trainer then introduces a simple pendulum (a bunch of keys at the end of a string) asked how the pendulum will oscillate if he swings it while being motionless, or while moving in translation or especially while spinning around. We note the different hypotheses and carry out the activity: surprise! the pendulum keeps oscillating in the same plane. We discuss Foucault and his pendulum, and we collectively attempt to interpret the path.  It is a proof that the Earth rotates.

 

Once this fact is acquired (the Earth rotates, we then want to know the direction of this rotation.  Still in groups, trainees imagine an experiment or an observation enabling them to form a conclusion . Only one group had the idea to represent the trajectory of the Sun in our sky: if we "see" the Sun move in this direction, then it means that the Earth rotates in the opposite direction.
For some, the passage from one frame of reference to another is obvious; to others, it is very difficult. Based on an idea by J.M. Ronaldo, (IUFM lecturer), the trainer proposes to them a "game" outside.


(Taken from L'astronomie à l'école, construire des compétences et des savoirs au level 3, J. M. Rolando, Delagrave)

A trainee plays the role of the sun, and the others make a circle (representing for instance the terrestrial equator), facing out.

  1. We first reproduce what we "see": the sun revolves around the earth (which does not rotate).  Each time the trainee-Sun passes in front of a trainee-Earth, he says his name aloud.
  2. We then try to reproduce what we "know": the Earth rotates and the Sun is still.  The aim is that, given that the Earth and the Sun travels the same path as in 1, we wil hear the succession of names in the same order. The "Earth" notices that, he is forced to turn in the opposite direction to the one the Sun had chosen in 1.

This game is very efficient to make the sense of the rotation of the Earth: no need to remember, just think about the trajectory of the Sun in the sky, and say that in reality it is the Earth that rotates in the opposite direction.

During the discussion session that follows this game, we discuss about the interest, for children, to use their body to "study" sciences. See, hear, feel with one's body is sometimes often more effective than a thought experiments.

From there the trainer proceeds with the following questions:

    • Is the length of a day the same everywhere, at a given time?
    • Is the length of a day  the same all the time, at a given place?
    • Why is it colder in winter than in summer?

The first question requires introducing the tilt of the the Earth’s axis of rotation.  In general, trainees consider this as acquired at the beginning of the session (although the previous questions do not need this famous tilt of the Earth’s axis of rotation). While almost all say "the Earth is tilted" few really understand what is tilted (the Earth’s the axis of rotation), especially in relation to what (the plane of Earth's orbit or ecliptic).
While manipulating their spheres and their flashlights, they come to the conclusion that we cannot simply explain the difference between the duration of a day around the planet without introducing this element in the "pattern". We consider that as a sufficient justification and accept that the assertion 'The Earth is tilted" (we now know what this approximate conclusion means).

 

The second question "Is the duration of a day is it the same all the time, at a given place? " has to be asked now, not before. Indeed, the variation in the length day at a given location, during the year, is certainly due to the fact that the Earth’s axis of rotation is tilted, but also that this inclination is constant. That is to say that the direction pointed by the axis does not change during the year, during the motion of the Earth around the sun.

 

   
SUMMER                                                  WINTER

Finally, the last question "Why is it colder in winter than in summer" is the conclusion of this stage. Trainees generally answer that the Earth is farther from the Sun in winter. A group discussion easily invalidates this hypothesis, other trainees note that, in this case, winter should take place at the same time all over the world, which is not the case: when it is winter in the Northern Hemisphere,it is summer in the southern hemisphere, and vice versa.
This wide spread belief that the Earth is farther from the Sun in winter is probably due to a misrepresentation of what the Earth's orbit is. Many represent it as a very elongated ellipse (we say very "eccentric"), when in reality, the eccentricity of the ellipse is very low. The Earth's orbit is almost a circle! To be convinced of this, one can compare the Earth-Sun distance at different times of the year or better still draw a circle of about 1 meter in diameter, with a marker or chalk on the blackboard. The difference between the ellipse (that the Earth's orbit would be) and the "perfect" circle is contained in the thickness of the line!

A group discussion is then initiated, during which we summarise what has been learned: the Earth revolves around the sun and its axis of rotation is inclined to the ecliptic. In addition, this tilt is constant. After a few minutes of reflection in small groups, most trainees give another explanation; "Rays are tilted."

Again, it is to discuss this conclusion and to reach a less ambiguous statement, such as: the sun's rays reach the Earth at an angle to the surface. This angle depends on the location, time, and season! Once this stage completed, the hardest part is done because participants have an intuitive understanding of what happens: "Energy is diluted, the same ray illuminates a larger area, then it heats less"  etc.

 

What is the relationship between the tilt of the rays of light on a surface and the temperature?
A simple experiment helps everyone agree.   We cut two pieces of chocolate, we expose each to the light of a small desk lamp (close enough to the bulb: ten centimetres maximum). The two pieces are placed horizontally on a table. In one case, the lamp is placed vertically (it lights chocolate from the top), while in the other case, it is tilted on the table (it lights chocolate from the side).  After waiting about fifteen minutes, we made ​​the following test:

    • A trainee places a finger on the square of chocolate illuminated from the side: the chocolate is still hard.
    • Then, he does the same with the one illuminated from the top: it sticks to the finger. It melted, evidence that the temperature was higher in this case.

For the experiment to succeed, we need the following:

    • Chocolate pieces that are the same size size (and made of  the same chocolate);
    • Identical flashlights and bulbs;
    • The same distance between the bulbs and chocolate squares in the two cases.

 

This experience can of course be done with a thermometer, but the chocolate adds a touch of whim that relaxes the atmosphere after hours of hard work. And then, when the experiment is over, we can eat what remains.

Our scenario has helped to address the alternation of day and night, the various movements of the Earth, and the phenomenon of seasons, while working on a vocabulary often poorly understood: hemisphere, rotation, ecliptic, solstice, equinox, etc..

We can continue the training (there are still many items of the astronomy syllabus to study!) with a new problem situation such as this one: the teacher shows them a photo taken by a probe (Clementine, in this case), where we see a clear Earth from the Moon.  "Imagine what would happen if, instead of a photograph, I  showed you a film What would move, what would change?  And what would not change?".

 

After a collective discussion, we divided the problem into sub-questions, depending to what is likely to change in the scene:

    • Will we see day and night in succession on the Moon?
      • Responses are divided, no consensus
    • Will the sky be black?
      • Almost everybody answers "yes", because there is no atmosphere. It is the correct answer, but this question cannot be treated within the framework of the implementation of the situation. We should address the concept of diffusion (doable with simple material, a transparent container, water, milk, a source of intense light) which would get us away from astronomy, theme of this training.
    • Will we see the Earth rotating on itself?
      • Almost everybody answers "no, because, from Earth, we see only one side of the Moon.  Seen from the Moon, the Earth always shows the same face ". This answer is wrong . 
    • Will we see a succession of phases of the Earth (Full Earth, crescent, etc.)?
      • Responses are divided, no consensus
    • Will we see the Earth moving in the sky, and if so, how?
      • Answers are unanimous: "Yes, the Earth will move in the lunar sky, in the same way that we see the Moon moving in our sky ". This unanimous answer is wrong .

 

On its own, this little problem can address almost all the astronomy syllabus for primary school: movement of the Earth and Moon, moon phases, eclipses, visible face / hidden face, day / night, seasons . The more interesting question is probably the last one: "Will we see the Earth  moving in the sky, and if so, how? "

Trainees, in groups of 4, should justify their answer using the same material as before. They construct a pattern of the Earth-Moon system, false in general:

    • Either they rotate the moon around the Earth without addressing the fact that the Moon should always present the same face to the Earth
    • or, to take into account this phenomenon, they introduce a strong constraint: they "weld" the Moon and the Earth with skewer:

 

This pattern is of course absurd because it implies that the Moon revolves around the Earth at the same speed as the Earth rotates on itself.  If this was the case, there should be 1 day = 1 month!

 

Following a phase of trial and error, they arrive at a correct pattern: the Earth rotates on itself "quickly" (24 hours), and the Moon rotates around the Earth "slowly" (28 days) at the same time rotating on itself, so as to always present the same face to the Earth. For that, it is often necessary to place markers on the moon: a toothpick; a drawing of a gentleman on the visible face.
At this stage some will realize, that seen from the Moon, the Earth does not seem to move, those who are on the hidden face do not see it and will never see it, and those who are on the visible face always see it and always in the same place. For others, the change of perspective is really difficult and a role play should be used again to make them perceive the scene. A trainee plays the role of the Earth and another that of the Moon. The trainee-Moon should revolve around the Earth, showing it only one of its faces (face, side, back...).  He describes where the Earth "in his sky":  just in front of him (it corresponds, on the Moon, to an earth to the zenith) because he choosen to face the Earth. If he chose to be in profile, he will then always see the Earth on the horizon, etc. Whatever the position of the observer on the Moon, he never sees the Earth moving in the sky. He always sees it in the same place (or never sees it if he is on the hidden face) and this place (zenith, horizon...) depends on the location where he is.
 
We can go back to our list of questions and answer those that we had put aside, notably those for which we did not have a consensus such as "Will we see a succession of phases of Earth, seen from the Moon?".

 

The last discussion phase enables the most advanced groups to present their work to others. We once more emphasize on the need to act in order to understand: experiment, model, observe and not just imagine. All hypotheses given by the participants, and some with great conviction, have been shown to be false. Sometimes, it took just a simple activity with a ball and a lamp, sometimes it was necessary, in addition, to take to the stage "with one's body"

 

The workshop is concluded by asking the participants to design and make a poster that retraces the pedagogical approach they went through, how problems were introduced, what were the positions and roles of the trainer, ditto for trainees, how was the "class" organised, what were the different moments of the session, what were the difficulties, etc. Here is an example of poster produced.

It is interesting not to stop there but to compare and discuss these posters, especially if participants are required to do the same in other themes (life science, technology). This is very useful in adopting the investigative approach.

Training activities: