Ask a class why the Moon changes shape and someone will almost always say that the Earth gets in the way and casts a shadow on it. It is one of the most common science misconceptions, among adults too. It sounds reasonable, it is wrong, and it is surprisingly hard to dislodge with a diagram alone.
This lesson gives students the tools to test the shadow idea themselves and replace it with the real explanation.
The real reason in one sentence
The Sun always lights exactly half of the Moon. As the Moon orbits the Earth, we see different amounts of that lit half.
Nothing is covering the Moon. At new moon the lit half faces away from us. At full moon it faces us completely. Everything in between is a mix of lit half and dark half seen at an angle.
See it from above
The key step is to show two views at once: the Moon going round the Earth seen from far above the North Pole, and the Moon as it looks from the ground. The moon phases lab puts these side by side, and you can drag the Moon around its orbit or play a whole month.
Ask students to stand on the Earth in the picture and look at each Moon in turn. At the position nearest the Sun they see only the dark side. A quarter of the way round they see half lit and half dark. On the far side they see the whole lit face.
What it looks like from Earth
Three ways to test the shadow idea
Do not just tell students the shadow idea is wrong. Give them evidence and let them reject it.
1. Where does the shadow point?
The Earth's shadow always points directly away from the Sun. In the top view, that means it can only reach the Moon when the Moon is on the far side, at full moon. But crescents happen when the Moon is on the same side as the Sun, where no shadow of the Earth can possibly reach.
2. The shape is wrong
The shadow of the Earth is round and much bigger than the Moon, so a shadow biting into the Moon always leaves a dent in the lit part. A gibbous Moon has a lit part that bulges outwards on both sides. No round shadow can make that shape, but a ball lit from one side and seen at an angle makes it every month.
3. We do see the shadow sometimes
When the Moon really does pass through the shadow of the Earth, we call it a lunar eclipse. It only happens at full moon, takes a few hours rather than two weeks, and the Moon often turns a dull red. That is what the shadow of the Earth looks like, and it is nothing like a crescent.
Why do eclipses not happen every month? The orbit of the Moon is tilted by about 5 degrees, so the Moon usually passes just above or below the shadow. The lab keeps the orbit flat to make the phases easy to see, so it is a good question to ask once students understand the basic picture.
When can you see each phase?
Students are often surprised that the Moon is visible in the daytime. Turn on the times of day in the lab and the timetable follows from the same picture: the Moon is highest in the sky about 50 minutes later each day.
| Phase | Days since new moon | Rises about | Highest in the sky about | Sets about |
|---|---|---|---|---|
| New moon | 0.0 | 06:00 | 12:00 | 18:00 |
| Waxing crescent | 3.7 | 09:00 | 15:00 | 21:00 |
| First quarter | 7.4 | 12:00 | 18:00 | 00:00 |
| Waxing gibbous | 11.1 | 15:00 | 21:00 | 03:00 |
| Full moon | 14.8 | 18:00 | 00:00 | 06:00 |
| Waning gibbous | 18.5 | 21:00 | 03:00 | 09:00 |
| Last quarter | 22.1 | 00:00 | 06:00 | 12:00 |
| Waning crescent | 25.8 | 03:00 | 09:00 | 15:00 |
Approximate local times, ignoring latitude and the tilt of the orbit, as in the lab.
So the First quarter rises around 12:00 and is already high in the afternoon sky before the Sun sets. The Full moon rises around 18:00, just as the Sun goes down, which is why it is up all night. A good homework task: look for the Moon after school a few days after new moon and sketch what you see.
Northern and southern hemispheres
If you teach in Australia, New Zealand, South Africa or South America, the Moon looks flipped. A waxing Moon is lit on the left instead of the right. The lab has a hemisphere switch, so students can see that the phase is the same everywhere on Earth: only the way we stand and look at it changes.
A lesson sequence
- Ask the class to draw how they think the phases happen. Keep the drawings: they are the before picture.
- Show the top view and the view from Earth together. Play one month slowly and stop at each named phase.
- Turn on the marker for the half facing Earth and ask students to predict each phase before revealing it.
- Run the three shadow tests above as a class discussion.
- Ask students to redraw their explanation and compare it with the first drawing.
Questions to check understanding
- If the Moon is a first quarter tonight, what phase will it be in about two weeks?
- Why can a crescent Moon never be seen at midnight?
- Someone on the other side of the Earth looks at the Moon tonight. Do they see a different phase?
- At new moon, is the far side of the Moon in darkness?