A shadow swinging with the sky
How Does a Sundial Tell Time?
A shadow swings steadily around as Earth turns, so its direction marks the hour.
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Step 1 of 5
The Sun's apparent motion is Earth turning.
The Sun is not moving across the sky. You are on a planet rotating once a day, and the Sun only appears to travel.
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Step 2 of 5
A steady rotation means a steady shadow.
Earth turns 15 degrees every hour, so the shadow swings by the same amount. Equal angles mark equal times.
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Step 3 of 5
The pointer must aim at the pole.
Tilt the gnomon to match your latitude and it lines up with Earth's axis. Only then does the shadow swing evenly all year.
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Step 4 of 5
Shadow length tells the season, not the hour.
In London a noon shadow in December is about seven times its June length. The Sun rides higher in summer and lower in winter, so noon shadows are short in summer and long in winter.
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Step 5 of 5
It disagrees with your watch.
Earth's tilted, slightly oval orbit makes solar days uneven, and time zones shift things further. A sundial can be a quarter of an hour out.
The correction between sundial and clock time is called the equation of time, and old sundials often carve it on the base.
The short version
A sundial reads Earth's rotation: the shadow swings 15 degrees an hour, and the pointer must aim at the pole to stay even.
Try it yourself
Push a stick into the ground and mark its shadow tip each hour. The marks are evenly spaced.