Moon Phases & What is a Theory?    

General information

The 4th grade state science curriculum includes moon phases. Though while I have seen that they can all name the phases of the moon, actual understanding of *why* the moon looks the way it does is perhaps somewhat absent. Part of the reason may be the tough time that this age group has with abstract spatial reasoning (heck, I know adults that come up short at this...). As an experiment one year I tried to extend the moon phases to the phases of Venus and predicting Venus' phases; I would say only about 25% of the class got it. After that experience I realized I needed to go more back to basics on the moon observing; I had not included a formal observing assignment. After adding that I think there was better understanding, though I have not been able to verify that with testing these groups with the Venus phases activities.

In the course of adding the observing it became obvious that this activity was a good opportunity to try and help the students understand what a scientific theory is, something that (at least in the USA) the general population seems confused about. Catch 'em early and all that.

The general outline is:

  • Use the "what's your theory " (Eleusis) game to have the students construct a theory
  • Construct a hypothesis as to why the moon looks the way it does in the sky
  • Teach them how to measure the moon/sun (angular) distance using fist-widths
  • Perform the moon phases activity and see if the data disagrees with the hypothesis

Note that with 4th graders this is not a rigorous scientific treatment of scientific theory, it's meant just to get them thinking in that direction.


At this site at the University of Wisconsin is an observing the moon activity. However we'll extend this to be a full lunar cycle.

ASP's Project Astro Universe at Your Fingertips, section A. This is similar to Activity 3 and 4 in "Earth Moon and Stars" (GEMS, Lawrence Hall of Science, Univ. of California at Berkeley, ISBN 0-912511-18-4). A shortened version of activity 4 is here (get the GEMS book, it's better). The "Mount Nose" is in Activity 3, but here's one that's more or less the same.

"what's your theory" game is based on Eleusis:


See the moon phases activity for a material list; see the text below about suggestions.



General plan



First Visit

time: 60 minutes + outside for a 10 minutes

Power point

The last slide should be edited to reflect appropriate hints for the time of your visit.

Theories using Eleusis

Slides 11 & 12 are where the students develop a hypothesis and test it. The students design the experiment, and you play "the universe". In this case the experiment consists of three numbers, A, B, and C. The results of the experiment are either TRUE or FALSE. A, B, and C could present the amount of chemicals used to make a drug (the result being if the patient is cured), the composition of some new metal, or properties of subatomic particles. Whatever their imagination wants (though if it's representing a physical property you want to keep the allowed numbers positive).

Splitting the class up into teams of 3 or 4 students each seems to work well. I give each group a chance to "propose" an experiment (i.e. 3 numbers in this case) which I write on the whiteboard. Once each team's experiment is recorded I become the universe and tell them the result of their experiment, TRUE or FALSE. They then work on a hypothesis and a new set of data to feed to the experiment. This repeats until each group gets TRUE every time (or maybe 4 rounds, start guiding them after the 3rd round). We then compare what they've deduced about the secret rule. (Note that everyone is sharing data here).

Once the teams are sure of the rule, I guide them to create an experiment that will uncover the 2nd "hidden" part of the secret rule.

I design the "secret rule" with two parts. The first part is what they will work towards solving. The second part I pick to keep it pretty unlikely that they will discover it. Once they have solved the first part, i.e. they *think* they have proven that their theory is correct, I ask them to come up with a set of data that will create a FALSE result. This demonstrates that while their theory is 'correct' for the data they tried it is not necessarily complete, and that more experiments are needed - which leads to the follow on material about the Theory of Gravity vs. the Theory of Relativity, and why we still test the latter (and also as to why I was testing the Theory of Gravity at the start of each prior visit).

Modify slide 11 to fit your secret rule. Mine involve not only a (simple) mathematical relation between the quantities, but also the odd/evenness of the quantities. The 2nd part that they don't discover is that some numbers are disallowed.

Moon observation

Come up with some 'crazy' hypothesis for why the moon looks like it does (aliens with a bucket of paint, for example).

For details on observing the moon, see the references as well as slides 16-22. It is easiest to have them make a journal entry for each day that they can observe. Explain that they may not be able to make an observation every day, and that's OK. It might not hurt to send a note home to parents, some have been overly helpful and printed off moon phases on a computer for their child, which is not the point.

Assuming exposure to angles, have them convert fist widths to degrees (an out-stretched fist at arm's length is 10 degrees)

When measuring it gets tough to go more than 90 degrees. They can just estimate that it's something between 90 and 180 (i.e. if the moon is rising when the sun is setting that it's close to 180 degrees). [There's an assumption here that you're not so far north or doing this at the solstices where the ecliptic isn't also the 'obvious' path]

I have them record the distance to the sun, and the shape of the moon.

The first visit is done on day/time where we can go outside and actually make a practice measurement. The teacher takes them out 2 or 3 more times as well as reminds them to look from home, playground, etc.

This handout will help the students remember what they need to do. pdf and freehand source.

Remind them that they are to come up with a hypothesis and they will need to make sure that the data does not falsify their hypothesis. So for example, I use a globe and make it go around the 'earth' (someone's head) in a square (i.e. observations would show it gets larger/smaller 4 times) and an elongated ellipse.

Moon observations - easier version

After doing this for a while it's pretty clear that the "data taking" skills of 4th graders aren't too developed. I never would have imagined there were so many wrong ways to record what the moon looked like...I'm not sure if it's a question on 4th grade developmental stage (i.e. recording a visual experience requires abstract geometrical reasoning and that is a skill still developing) or that using pencil and paper is on the wane. Perhaps having an art teacher give the instruction/guidance would help?

The following worksheet was created to make it a bit easier; the measuring is made optional, and all they have to do is circle the right choices. There's a "2 per page" and "4 per page" version, and here are the freehand sources 2per, 4per.

Second Visit

time: 90 minutes

Part 1

After observing a full lunar cycle we do the moon phases activity. The first part is to get everyone's data up on the white board, hopefully ending up with something that shows the moon phases and relative distance from the sun in a reasonable way. (I'm on the fence about the best way to draw it - I think with the sun in the center of the board and the moon's coming off each side - though putting data on a calendar first is more logical)

For each day where you can get 'good' data have a student draw a (large) picture of the moon on a piece of paper and hang it up on the wall of the room at the same relative angle that the actual observation was. Assuming the class sits facing the whiteboard and it's on the center of the wall, I put up a "sun" paper up and work may way around the room in the order the observations are made in (i.e. the middle of the side walls are first and lass quarter, and the middle of the back wall is where the full moon is placed).

Also review what a theory is, and have them (in groups) be thinking about their hypothesis that will explain why the moon goes through phases and changes its location relative to the sun.

Part 2

Once the classroom phase is done relocate to a large, totally dark area (the stage in an auditorium, for example). Classrooms typically are not dark enough.

For the moon phase activity I start with the Mount Nose activity. This actually takes a little effort to get them to understand Earth's rotation, remember which way East vs. West is, etc.

The guides I have read generally say to use a styrofoam ball. However I find that they don't give a clean terminator. Instead I purchased 2 dozen plastic baseball practice balls (the solid type!). However as they are hollow they don't stick to well on a pencil. I drill a hole in the bottom put some of the expanding foam into them and let it harden. A sharp pencil can then be jammed into it.

Don't hand out the balls and pencils until *after* doing the Mt. Nose exercise, the students spend more time dropping them than anything else.

I use a clear glass bulb (frosted ones don't create clean shadows) on a socket mounted on a pole so that the bulb is about 6' up. A screw in dimmer allows me to tweak the brightness. The ideal room is one that has no light at all and the walls are far from the bulb and painted black. Reality is lots of light leaking in a small room with white walls. Test out where you plan on doing the exercise before hand! I find a small bulb (like 25 Watt) is better (and bring a spare).

Once the kids have done a complete lunar cycle they should do it again, this time recording what their model shows them at each quarter. They can then compare this to the data that was observed and comment as to if the model supports the theory.





Since the first visit needs good weather, it may need to be split into two visits, one for the background on theories and the second for the practice measurement (of course if familiar with the exercise the teacher could lead that). However as the two activities do reinforce each other they should be attempted to be scheduled close to each other.


The kids seem to struggle to accurately record what they observe. Biggest problems:

  • Wrong date (or no date) recorded
  • Moon not drawn the way it actually looks (e.g. drawn as a crescent when it's gibbous)
  • Drawn with "illuminated" side not facing the sun
  • Drawn on the wrong side (left/ right) of sun

I think there has to be 'daily' checking of the observations and/or taking the class outside for a few days in a row and verifying they are following the directions. Of course here in New England a few clear days doesn't happen that often.