Shadows on the Moon: why craters show up best at half Moon
Pupils measure how a shadow changes as a torch is lowered, then shine light across model craters to discover why the Moon's craters stand out along the line between day and night and almost vanish at full Moon.
- Explain that a shadow forms where light from a source is blocked by an opaque object.
- Measure shadow lengths in centimetres and find the pattern: the lower the light, the longer the shadow.
- Use the pattern to explain why the Moon's craters are easiest to see near the line between its bright and dark parts, and know never to look at the Sun.
National Curriculum links
- Year 3, Light: recognise that shadows are formed when the light from a light source is blocked by an opaque object.
- Year 3, Light: find patterns in the way that the size of shadows change.
- Year 3, Light: recognise that light from the sun can be dangerous and that there are ways to protect their eyes; notice that light is reflected from surfaces.
- Lower Key Stage 2, working scientifically: taking accurate measurements using standard units; recording findings in tables; using results to draw simple conclusions and make predictions.
Key vocabulary
- light source
- opaque
- shadow
- reflect
- crater
- rim
- terminator
- phase
Resources
- For each group: a torch, a 30 cm ruler, a stack of books to raise the torch, and a small upright object about 5 cm tall, such as a short pencil stuck in modelling clay.
- For each group, a tray of modelling dough with three or four craters pressed in with marbles or balls, so each has a raised rim.
- A room that can be darkened, with clear floor space.
- If possible, your own pictures of the Moon at half Moon and at full Moon.
- The worksheet, one per pupil.
Background for the teacher
The Moon makes no light of its own; we see sunlight reflected from its surface. Its craters are bowls with raised rims, made by asteroids and comets crashing into it over billions of years. They have lasted because the Moon has almost no air, no running water and no moving plates to wear or bury them, unlike the craters on Earth.
The line between the sunlit and dark parts of the Moon is called the terminator. Standing on the Moon near that line, you would see the Sun low on the horizon, so every crater rim and mountain casts a long shadow and the craters stand out sharply. At full Moon the sunlight falls almost straight onto the side facing us, shadows are short, and the craters are surprisingly hard to make out, although bright streaks of splashed-out rock show up well. That is why a half Moon seen through binoculars is far more impressive than a full Moon.
The classroom model shows the same pattern. With an object 5 cm tall and the torch 40 cm away, the shadow is roughly 40 cm long with the torch 10 cm above the table, about 13 cm with it at 20 cm and about 8 cm at 30 cm. Real torches give fuzzy edges, so pupils' measurements will vary, but the pattern is clear.
Lesson plan
Starter 10 minutes
Darken the room. Shine a torch straight down onto a sheet of crumpled kitchen foil, then from low at the side. Ask: when could you see the bumps best? Why?
Remind pupils that a shadow forms where an opaque object blocks the light, and that we must never look at the Sun, even with sunglasses, because it can damage our eyes.
Main 35 minutes
Shadow investigation: groups place the small upright object 40 cm from the torch. They raise the torch on books to 10, 20 and 30 cm above the table and measure the shadow each time. They record results in the table and describe the pattern.
Crater model: groups shine the torch onto the crater tray, first from straight above, then from low at one side. They draw what they see each time on the worksheet, shading the shadows.
Show the half Moon and full Moon pictures. Ask groups to match each picture to one of their torch positions.
Plenary 15 minutes
Pupils explain to a partner why the craters look clearer at half Moon than at full Moon, using the words light source, shadow and opaque. Take two or three explanations and agree the key idea: near the terminator the Sun is low in the Moon's sky, so the shadows are long.
Finish by checking the safety message again: binoculars are great for the Moon, but never point them anywhere near the Sun.
Differentiation
Use two torch heights only and say whether the shadow got longer or shorter; an adult helps to read the ruler.
Measure at three heights, record in centimetres and describe the pattern in a sentence.
Predict the shadow length at a height you did not test, then check it, and find out what happens when the torch is moved closer to the object instead of higher.
Safety and risk assessment
Check these points against your employer's risk assessment and your school's policies before the lesson.
| Hazard | How to control it |
|---|---|
| Eye damage from sunlight | Never look directly at the Sun, with or without sunglasses, and never through binoculars or a telescope. |
| Trips in a darkened room | Clear bags and chairs from walkways before darkening the room; pupils stay at their tables. |
| Torch shone into eyes | Keep torches pointing down at the table. |
| Modelling dough allergies or eating | Check for allergies, use shop-bought dough, nobody eats it, and wash hands afterwards. |
Assessment questions with answers
- What is a shadow?Answer: A dark area where light from a light source is blocked by an opaque object.
- What happened to the shadow when the torch was held lower?Answer: It got longer.
- Does the Moon make its own light?Answer: No. It reflects light from the Sun.
- Why are the Moon's craters easiest to see near the line between its bright and dark parts?Answer: The Sun is low in the Moon's sky there, so crater rims cast long shadows that make the craters stand out.
- Why must you never look at the Sun through binoculars?Answer: Sunlight is strong enough to damage your eyes, and binoculars gather even more of it, which can cause instant and permanent damage.
Homework
On a sunny day, with an adult, stand in the same place in the morning, at midday and in the afternoon and ask them to measure your shadow. Do not look at the Sun. When was your shadow longest? On a clear evening near half Moon, look at the Moon and draw the line between its light and dark parts.
Answer guidance: Shadows are longest in the morning and late afternoon, when the Sun is low, and shortest around the middle of the day. This is the same reason crater shadows are long near the terminator.
Use with these pages
- Craters: when big rocks hit
- Crater rim
- Ray crater
- Impact craters
- Space rocks make holes
- Space missions to meteorites' homes
These eight lessons are a free starter pack, written specially for this site. The full set of 25 lesson plans, with worksheets, end-of-unit tests and mark schemes, is in the Corvanelle book Teaching Meteorites. The lessons in the book are different lessons, so the starter pack and the book work together rather than repeating each other.