The Moon: study and explanation of lunar phases

Authors : Travail collectif(plus d'infos)
Summary :
Know that the Moon revolves around the Earth.
Publication : 1 January 2001
Objectives :
Notional objective: Identify Moon phases; predict the evolution of the Moon in the sky; know the lunar cycle (duration of a lunar revolution around the Earth). Methodological objectives: give one's point of view and argue it; observe the duration of a phase, submit hypotheses relating to this phenomenon, find an explanatory model of the lunar phases.
Duration :
7 sequences of about 90 minutes, plus some observation time outside of the classroom.
Material :

- Observation grid (square paper)
- Globe
- balls of different sizes,
- Slide projector,
- Lamps (flashlights and desktop lamps).

Note :
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The sequence is carried out in 4 stages: 
Stage 1: situation simulation (Sequence 1)

Stage 2: observation and first formulation of the problem (Sequence 2) /
Stage 3: research and investigation (Sequence 3)
Stage 4: explanation of the phenomenon (Sequences 4, 5, 6 and 7).


Stage 1: Situation Simulation

Sequence 1: Triggering Situation

Ask the children: "The Moon changes shape, what does that mean?"
From this question, pupils share their ideas and their representations. They all recognize the need to observe changes in the appearance of the Moon.

Stage 2: Observation and First Formulation of the Problem

Sequence 2: Observe and draw the moon every day, as it is seen.

This observation period lasted from October 1999 to March 2000.



Each pupil has an observation grid where he draws the Moon as he sees it, and also notes the date of observation.

Observations sheet

A first result, after a few weeks of observation the following observations and reflections were made:



    • The Moon changes shape.
    • It decreases, it grows.
    • Sometimes, it is full Moon (a whole round).
    • We can see the crescent moon, more or less wide, in quarter (the half). 
    • The crescent or quarter can be turned left or right.
    • We do not always see it at the same place in the sky.
    • We do not always see it at the same time in the day.
    • There are times when we do not see it at all (when there are no clouds). It was the Black Moon (on the calendar), new moon.
    • When it stops growing smaller, it is getting ready to grow again.
    • I saw the moon on weather forecaster; it had the same shape (as in the observation sheet).
    • We we move forward, the Moon moves forward with us.
    • We always see the same side (Kevin saw it on his computer).
    • No, because it turns around (right or left).
    • The Moon does not rotate on itself, so we always see the same face: how to check if we always see the same face?
    • There are stains: Is what we see always the same?


Pedagogical analysis

All these observations lead to  questions:

    • What causes the shape of the Moon to change?
    • Why does the moon change its shape?
    • How many times does the moon change its shape?
    • When does the moon change its shape?


Stage 3: When does the moon change its shape?

Sequence 3: it is now back in details of the observation by analysing records.


1- Pupils compare their observations for the same period.
2- They make the lunar calendar for the month.
3- They check their work by comparing it with a calendar of moon phases from the journal.

Pupils, in small groups (2 or 3), compare their sketches and have to reconstruct the sequence of the Moon phases.


Some pupils have the same sketch.

Some do not have sketches: they could not do the observation.
Some pupils do not have the same sketch.

Discussions arise.
Example: on the 17-11, Ophélie saw it in the evening, Laurie in the morning; it does not have the same shape. It is not in the same direction.
On Ophélie's observation grid the previous moon grows increasingly. The previous and the following moons are all in the same direction, except that of Laurie.


Proposed explanations

    • The Moon's shape may change according to time.
    • There may been a cloud.
    • May be Laurie did get a good look, she may have drawn it wrongly, facing the wrong direction.


It was neccessary to stop at that moment to argue scientifically, that is to look for a logical explanation for the observed changes, to resort to a a reliable observation and to rely on a reference observation. 

This is where the calendars from  "Astronomy magazine" were introduced.


Comments in the groups

    • Every day, the Moon changes, except during full Moon. Sometimes there is no Moon.
    • The marks are always the same. When you put the end of the month with the beginning of next month, it continues.
    • The sketches of the moon on 20 March  and 20 February are almost the same.
    • In each month, there are many days when we do not see the moon. On the first and the last day of every month, the Moon almost always has the same shape.
    • In each month, there are one or more days when the moon is full.
    • The moon grows bigger during  the second and third week of March, and in the third, fourth and fifth week, it decreases.
    • We see this every month.
    • We see the same Moon each month.
    • It is endless, it starts again. It is a cycle.


Pedagogical analysis

The monthly calendar of moon phases (reference observation) allows a comparison of the statements made by pupils to the reality, and to have the complete succession of phases of the moon from day to day.
From the beginning of the discussion, it appeared that the children had implicitly understood that the phenomenon is reproduced regularly in an identical way, that we would find many phases.  After comparing all observations, the class has clearly expressed the notion of a cycle.


New questions arise, such as, why does the moon shrink from one side and grows on the other.
Then begins the next stage during which we will try to explain what we had just previously observed.


Stage 4: Research and Explanation of the Phenomenon

Question: Why do we see the moon change shape during the month?  Does it grow and shrink?

Sequence 4: Questioning and discussing to develop a hypotheses


In groups of two or three, pupils try to explain the phenomenon. Each group jots down its reflections and tries to sketch them.






Three main ideas appear in the groups, explaining the phenomenon with astronomical causes, as well as a few others, formulated individually and more "fanciful", which will not be considered.

    • When we are to the north of  the Earth, we see the full Moon.  If we are to the east we only see half the Moon.
    • The Moon is always to the north and does not move. The Earth rotates on itself and revolves around the Sun.
    • The Moon revolves around the Earth.
    • We do not always see the Moon in the same way, because the Earth rotates and we do not always see it at the same place.
    • We cannot answer the question as we do not know what the the grey part is.  What hides the moon?
    • It is the part of the Moon that the Sun hides.  No, because there would be no night. Other countries could not be illuminated.
    • It is the Earth that hides half the Moon.
    • It is the sun that illuminates the Moon.
    • The shadow of the Earth hides the Moon.  As the Earth rotates, the shadow is not always the same.
    • The Sun illuminates the Moon. It can only illuminate a portion of the Moon.
    • How does it manage to reduce or enlarge the lit up?
    • It is the white moon and the black sky that combine to make grey.
    • There would be no full Moon.
    • It may be like a magnet. It attracts things and, then it loses them ...
    • When astronauts went to the moon, it was full.

At the end of the sequence, three hypotheses are retained by children, after discussions within the class:

    • When we change the position of the Earth, we do not see the Moon in the same way.
    • The Earth’s shadow hides the Moon.
    • The Sun only illuminates a part of the Moon.


Word from the teacher

Some other proposals were abandoned because they did not resist when their arguments  were opposed ("the White Moon which mixes with the black sky, the moon which attracts then loses things").
The others were combined to formulate in a clearer way each of the major ideas expressed in the classroom.

Sequence 5: Pupils choose to work on one of the three hypotheses and form three groups.


Requested task

Imagine, using sketches, an experiment that can validate the chosen hypothesis.

The children make two sketches:

  • The first one tries to give an explanation of the phenomenon,
  • The second represents the experiment they want to implement.

Sequence 6: implementation of the experiment



    • Frequent reminders by the teacher that hypotheses the experiment has to verify.
    • The children are grouped in 3s or 4s.
    • Each group, from ideas proposed during sequence 5, chooses their material and carry out their plan:
      - A ball or globe to represent the Earth

 - A smaller ball to represent the Moon,

- A lamp to simulate the Sun.

  • Each group should attempt to reproduce the movement of the Moon revolving around the Earth, to confirm the chosen hypothesis and explain what hides part of the Moon.  The teacher took for granted that it is the Moon revolves around the Earth and not the opposite, as some children thought.


    • One of the groups points out that if one of the lamps illuminates the ball, one half of it is illuminated and visible while the other half is in the shadow. The ball has not changed shape, it is the light cast on it that can change our perception of it.
    • Here is a part of the answer to the question asked: the form does not change, it neither grows nor shrinks, and its volume does not change.
    • We only see a part, the one that is illuminated.
    • Some children, through their sketches show that they have understood that it is the position of the Moon with respect to the Sun and the Earth which explains the new moon and the full Moon.
    • The pupils "recognize" that the hypothesis to retain is: the Sun only illuminates part of the Moon. However, it does not explain the other phases of the Moon yet.
    • To justify the change of  the Moon's position with respect to the Earth, pupils suggest the idea of ​​a movement.  This sequence ended with a partial validation of one of the hypotheses, but without providing a full explanation of the phenomenon of moon phases.


Sequence 7

First step

A plan is proposed by the teacher to explain the phenomenon.

A ball is suspended in the middle of the class which is darkened. Children are placed in a circle around the ball which is illuminated by the beam of a slide projector.

Each child draws the ball as he sees it, distinguishing the illuminated part from the dark.  On another sketch, he uses a cross to indicate his position relative to the projector and the ball.

Collective Summary

We have drawings of the different moon phases.
It is as if we were revolving around the ball without moving the projector.
Each phase depends on the position of the observer relative to the ball.
It is necessary at this stage of the sequence that the teacher says that it is "the moon moves around the earth."

Second step: on a sunny day

Specific objective: become aware that the phases correspond to the position of the ball (Moon) with relative to that of the observer (Earth) and that of the Sun.

Operational objective: match the drawing (Moon phases) with the illuminated part of the ball.

Exercise 1: The teacher stands in front of pupils, holding a ball above his head. The pupil with the drawing corresponding to the illuminated side of the ball should move next to him, facing other pupils. The other pupils check that it corresponds. The teacher then makes sure that he is in a different position with respect to the Sun.

Exercise 2: simulation
Provide: four balls, drawings made in the first step, a circle drawn with chalk on the floor.

a - On the circle, four children carry a ball above their heads.  The observer placed at the centre of the circle must come and place at the foot of the four ball carriers the drawing corresponding to the phase he sees on the ball.

b - Problem: how to find the four phases with a single ball? It is the ball carrier who rotates around the observer.

c - Question: in which direction must he turn? We use the lunar calendar to find the order new moon, first quarter, full moon, last quarter.

An immediate evaluation is proposed to children, to check what they have learned at the end of this experiment.  They were asked to write a brief account, giving their response to questions asked before experiments.

Some children understand the phenomenon, even if they have difficulties to explain it in writing:
"The Moon changes shape because the Moon revolves around the Earth.  The Moon changes sides because the Moon revolves around the Earth."

"The Moon changes because the sun does not shine all the time. The Moon is illuminated at the beginning in full, then it is half, then almost half, then it is the new Moon, then  it starts again."

"The black part is a part of the moon that is not illuminated"

"The black part is the part not illuminated"

"The sun does not illuminate the Moon".

Others return to their original ideas, even if the manipulations and observations made during the experiments showed that they were wrong:
"We did the experiment: we placed a projector with a ball, the ball represented the Moon, and the projector represented the Sun. We saw how the black part and the illuminated part were.

We saw that the black part was the Earth's shadow that was hiding the sun".


We also decided to do another evaluation of the knowledge and experimental plans acquired, several weeks after the start of the next school year.


See also, on the same subject...

The professional report of Mr. Grégoire VALLIN, for his thesis on L’enseignement de l’astronomie à l’école primaire ; quelle démarche d’investigation mettre en place pour développer des compétences particulières chez les élèves ? (The teaching of astronomy in primary school: which investigation approach to implement to develop specific competences in pupils?)
A work headed by Mr. Guy CHAZEAUX, IUFM d’Auvergne, site de Chamalières.

This thesis was developed from the conception, the implementation and analysis of a cycle 3 astronomy sequence on the theme of Moon phases.
The problem is centred on the specificity of the of the investigation approach in astronomy.
This is a very well written thesis.  The theoretical part is based on many references, all highly relevant having regard to the problems developed.  The pedagogic implementation presents a sequence of class and offers a very detailed analysis.
Measures put in place to evaluate the plan helped each pupil to appreciate the critical activities that enabled them to make progress.
The author did a critical analysis of some approaches, relying in particular on the difficulties encountered by the pupils.
In addition, pupils' writings presented in the aAppendix exemplify the educational progress by providing practical information and reflecting the quality of this remarkable work in every way.