Students think that phases of the moon can be explained by shadows cast on the moon, or by the rotation of the Earth
What's the idea?
Phases of the moon are explained by shadows thrown on a disk, or by movement of the Earth, rather than varying illumination by the Sun.
Why having this idea affects learning physics
The absence of multiple-step explanations about the illumination of carefully arranged spheres is unnecessary, which might make this appear seductive. But the moon is a non-luminous source, not a patch of light on a celestial sphere. The idea that the phases rely on the daily spinning of the Earth undermines the critical difference between spinning on an axis and orbiting a planet: both result in a change of view, but not necessarily the same change.
How the idea gets traction
The explanation in terms of shadows seems simple and ties into earlier work on shadows and lived-in world experiences with creating shapes by casting shadows. The explanation for the phases of the Moon is quite tricky, so don't be surprised when many pupils struggle to offer any ideas at all about what is happening. They are unlikely to be firmly committed to the explanation they offer.
Diagnosing and fixing
It is a complicated explanation, so develop the explanation carefully. To show how things can go wrong, start with a circle of bright light on a wall, perhaps from a torch beam. Place a disc in the beam to partially block the torch beam. You can adjust the disc so that you get patterns of illumination on the wall, remarkably like the moon's phases.
Use physical models so that students base their understanding on real 3D experiences of arrangements of spheres suitably illuminated.
Explaining the phases of the Moon (Supporting Physics Teaching 11-14)
But even before that, ensure that all students are aware of the phenomenon they are trying to explain.
Phases of the Moon—an activity (Supporting Physics Teaching 11-14)
Why does the Moon change? (Supporting Physics Teaching 11-14)
Wrong Track: The Moon changes shape because the clouds shade it and you only see the part which hasn't been shaded. The clouds might cover a bit of the Moon and the clouds move so it might get bigger or smaller, depending on how big the clouds are.
Two kinds of spinning around (Supporting Physics Teaching
spinning on an axis.
Phases of the Moon (Supporting Physics Teaching 11-14) The Moon's appearance changes in a regular way, moving through a sequence of phases in one lunar month.
The moon—our nearest neighbour (Supporting Physics Teaching 5-11) The reason for the shape of the Moon changing periodically, going through its different phases is tricky and widely misunderstood. It is caused by where the Moon is in relation to the Sun and the Earth and so how you see the Sunlight reflecting off the Moon.
Students explain phases based on
moonwill appear to people on the spinning sphere representing Earth
A whole chapter summarises children's views on the Earth in Space.