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.
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.
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