Go outside on a sunny day and something dark tags along wherever you walk. Run, and it runs. Stop, and it stops. Wave an arm and it waves the very same arm back. You can chase it, but you can never quite catch it — because it is your own shadow.
A shadow appears for one simple reason: light travels in straight lines, and you are standing in the way. Light streams out of the Sun in dead-straight beams. When those beams run into something solid — a person, a tree, a wall — they cannot bend round it and cannot pass through it. So the ground behind the object is left with no light reaching it, and that light-less patch stays dark. That dark, object-shaped patch is the shadow.
Something that stops light like this — that you cannot see through at all — is called opaque. You are opaque. A door is opaque. A cat is opaque. Anything opaque, stood in front of a light, casts a shadow.
Not everything blocks light. Try holding different things up to a lamp and watch what happens behind them. It turns out there are three kinds of stuff:
So the darkest, sharpest shadows come from opaque things; see-through transparent things make hardly any shadow; and the in-between translucent things make soft, ghostly ones.
Here is a lovely thing about shadows: a shadow is the same shape as the object that made it. Hold up a round ball and its shadow is a dark circle. Hold up a fork and you see a fork-shaped shadow, prongs and all. Hold up a cardboard cut-out of a cat and a cat slinks across the wall. The shadow doesn't show the object's colour, and it doesn't show the little bumps on the front — only the outline, the shape of the edge that the light runs into.
Because the shadow follows the outline, you can turn an object to make its shadow change. Turn a dinner plate face-on to the light and its shadow is a fat circle; turn it edge-on and the shadow becomes a thin line. The object is exactly the same — you've just shown the light a different outline to bump into.
People have played with this for thousands of years. In a darkened room, hold one hand up in front of a single lamp or a candle. Your hand is opaque, so it throws a nice sharp shadow on the wall — and by curling your fingers just so, that shadow stops looking like a hand and starts looking like a rabbit, or a barking dog, or a snapping crocodile. Wiggle a finger and the rabbit's ear twitches.
You are not drawing anything and you are not making darkness — you are simply choosing what outline to put in the way of the light. Whole theatre shows, called shadow puppetry, have been performed like this for over a thousand years, telling entire stories with nothing but cut-out shapes held in front of a bright screen.
A shadow is never fixed — move the light or move the object and the shadow changes its size, its direction and its sharpness. The most surprising rule is about size: the closer the object is to the light, the bigger its shadow. Right up close to the lamp it blocks a huge fan of light and throws an enormous shadow; slide it back near the wall and the shadow shrinks down to almost the object's own size.
In the box below there's a lamp on the left, an object in the middle and a wall on the right. Slide the object to and fro and watch the dark band on the wall grow and shrink. Nothing about the object has changed — only where it stands between the light and the wall.
Direction works the same way. Put the lamp on the object's left and the shadow stretches off to the right; carry the lamp round to the right and the shadow swings across to the left. The shadow always points straight away from the light, on the far side. (And a small, bright light far away gives lovely sharp-edged shadows, while a big, soft light nearby gives fuzzy-edged ones — that's why photographers fuss so much about their lamps.)
The biggest light in your life — the Sun — is always on the move across the sky, so your outdoor shadow spends the whole day slowly changing. Early in the morning and late in the evening the Sun sits low, near the horizon, and shines almost sideways at you. Light skimming along the ground has to graze right past your head, so the dark patch behind you stretches out long and thin — sometimes taller than a house!
At noon the Sun climbs high overhead and shines almost straight down, so your shadow squashes into a tiny puddle around your feet. Same you, same Sun — only the angle of the light changed through the day, and your shadow changed with it, growing long, then short, then long again. And because the Sun rises in the east and sets in the west, your shadow also swings slowly round from one side to the other as the hours pass.
Long before anyone had a watch, people learned to read the time straight off a shadow. A sundial is just a stick standing up in the middle of a flat plate marked with hour lines. As the Sun crosses the sky, the stick's shadow sweeps slowly round the plate like the hand of a clock. In the morning the shadow points one way; by the afternoon it has swung round to point another. You simply read off the hour line the shadow is resting on.
It costs nothing, has no battery and never breaks — but of course it only tells the time when the Sun is out. On a cloudy day, or at night, the shadow vanishes and the clock stops. People used sundials for thousands of years, and you can still find them in old gardens today.
Shadows are not only a thing on the ground — they stretch out through space too. Our Moon is a big opaque ball, and now and then it slides directly between the Earth and the Sun. When it does, it blocks the sunlight and casts a real shadow — a cone of darkness — that races across the surface of the Earth. If you are standing in that shadow, the sky goes dark in the middle of the day and the Sun seems to switch off for a few minutes. That is a solar eclipse.
It is exactly the same idea as the lamp and the ball in the box above — an opaque object blocking a light and leaving a dark shape behind it — just done on an enormous scale, with the Sun as the lamp, the Moon as the object, and the whole Earth as the wall.