A shadow is longer in October than it was in July. Stand on the same spot in the schoolyard at the same time of day, and it stretches much further across the tarmac now than it did during the summer holidays. Your child hasn't grown that much. So what has changed? Shadows for kids are one of the simplest experiments there is. All you need is a sunny day, a piece of chalk and a clock. Trace the shadow once a week this autumn, and the tarmac turns into a calendar.
What is a shadow?
A shadow is a place where light can't reach because something is in the way. Light travels in straight lines, so your body blocks part of the sunlight and leaves a dark patch in exactly your shape on the ground. How long that patch is depends on how high the sun is. A high sun drops its light almost straight down and the shadow huddles around your feet. A low sun sends light in from the side, and the shadow stretches far across the ground.
The shadow grows, but you are not the one changing. The sun has moved.
Earth is tilted. In autumn our part of the planet slowly turns away from the sun, so even at midday the sun no longer climbs as high. In Oslo, your midday shadow in July is shorter than you are. In early October it is about twice your height. Around Christmas it is more than eight times as long. In Tromsø the sun doesn't rise above the horizon at all from late November to mid-January, so there is no sun shadow to draw. Autumn is full of free experiments like this one; we've gathered more in Why Is Autumn the Best Laboratory a Child Can Have?
Try it at home: the shadow calendar
Suitable for: 4–12 years. ⚠️ An adult picks a spot with no car traffic, and nobody looks straight at the sun.
You need chalk, a sunny day, a clock, a flat sunny patch of tarmac, a measuring tape, and paper and pencil.
- Pick a spot that gets midday sun and draw a cross where your child will stand.
- Your child stands on the cross with their back to the sun while you trace around their feet.
- Trace the whole shadow while they stand completely still.
- Write the date and time by the shadow's head.
- Measure from heels to head and write the number down.
- Come back to the same cross, at the same time, once a week.
The chalk scrapes on the tarmac and leaves white dust on your fingers. Next week, last week's line is still there, a little fainter, and the new shadow runs past it. When rain washes the chalk away, the numbers on the paper survive. On Sunday 25 October the clocks go back an hour, so from then on measure an hour earlier by the clock. The shadow also moves during a single day, which is exactly what you need to make a sundial.
What happens if a grown-up stands on a cross next to your child and has their shadow traced every week too? Whose shadow grows more?
Questions to wonder about
- How long do you think your shadow will be on Christmas Eve?
- Why does your shadow have your shape but never quite your proportions?
- A child on the other side of the world traces their shadow at the same moment as you. Is their shadow growing this autumn, or shrinking?
Every child is made of good atoms
One day in December your child stands on the cross again, and the shadow runs right across the yard to the fence. The child is about as tall as in July. It is the sun that now skims low over the hill. In spring it all goes the other way, and the shadow creeps slowly back towards their feet. Every child is made of good atoms, and every child casts a shadow that shows where we are in the year.
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