STEM Zone: Predicting lunar phases

STEM Zone: Predicting lunar phases

Teach Moon phases with this easy lunar activity 

By Dr. Daniel Barth

One of the crucial questions that teachers and home school parents face is: ‘How do I provide high quality science instruction?’ This question really breaks down into two parts. First, can we provide quality STEM activities that children (and parents!) can understand? Second, can we afford to do this?

I believe that the answer to both questions is an unequivocal ‘Yes!’ Let’s take a look at a typical assignment a teacher or home school parent might conduct either in general science, or in astronomy and space science curriculum: Understanding the lunar phases.

The response of many educators, both face-to-face and remote delivery is to assign a video for students to watch. There are a wide variety of such videos available, some are excellent, many are poor; however discussing the quality of videos misses the point – all are grossly inferior to engaging the child in a hands-on activity and exploration. No matter how we deliver them, we cannot simply throw facts at a child and expect that they will develop a sound, conceptual understanding of anything.

Let’s start with some very simple activities that will engage the student in the study of the night sky. This first activity requires just pencil and paper, and is best conducted during the waxing moon phase, the 2-week period when the Moon grows brighter and more fully illuminated each night. The waxing moon is also visible each night just after sunset, meaning that students don’t have to stay up late.

This generally starts with the parent or teacher showing the child what the Moon looks like tonight. The student copies the phase of the Moon by drawing the terminator, the line that separates light and shadow. For our purposes, always draw the terminator starting at the top or ‘north pole’ of the Moon, and finishing up at the bottom, or ‘south pole of the Moon.

Drawing the Moon in its waxing crescent phase as it appears just 2-3 days after the new moon illustrates several things to the student. It is often easy in the crescent phase to see the entire disk of the Moon. This shows us that the shape of the Moon itself does not change (the Moon is always a sphere), but the amount of the Moon illuminated by sunlight does change.

With careful observation we can see that the terminator always stretches 180 degrees – from the Moon’s north pole to its south pole – no matter how the Moon appears tilted with respect to our horizon.

The next step is to ask the student to draw in the top row of boxes what they think the Moon will look like over the next 7-10 days. I ask my students to do this in ink, so that the prediction cannot be changed. Standards-based education has given students a tremendous fear of “being wrong”. This is an excellent opportunity to teach students about science culture. We do not do science to prove we are right, rather we do science to become more right. Every student should be praised for their prediction – it is the effort (and the risk!) that must be rewarded here, not a “correct answer”!

Over the next week or so, the students can go outside every two to three days and look at the Moon, sketching the phase that they see in the boxes in the lower row. This process provides several valuable insights to everyone involved. Science is not an instant process; it cannot always be boiled down into a 1-hour activity suitable for a single class period. Students will have to contend with clouds and weather, too; science sometimes proceeds from incomplete data! Although the sky is our laboratory, we do not control it. Our young scientists must plan their observing time and go out when the Moon is visible; the phenomena will not wait for us and there is no redo button!

It is fascinating when the week of observing is over to compare what we thought would happen to what we actually observed. Once again, it is critical at this point for the teacher or parent to emphasize what we have learned about the Moon and its phases instead of looking for easy ‘right vs. wrong’ answers. Remind the child that in science, virtually all first predictions are incorrect! Science is an adventure in learning more precisely how the world works!

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Dr. Daniel Barth left a career as a research scientist to teach; he has spent more than 30 years teaching astronomy, physics and chemistry at the high school and college level. A successful science fiction writer, Barth is the author of Maurice on the Moon, Doomed Colony of Mars and other works. He is currently Assistant Professor of STEM Education at the University of Arkansas in Fayetteville, and author of the Astronomy for Educators program. For questions or comments, he can be reached at 

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 Click here to download a PDF of this activity.

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