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Why does the Gnomon on a Sundial Point South?

A sundial in Sussex Square, Brighton
A sundial in Sussex Square Gardens, Brighton

The pointy bit of a sundial is called a ‘gnomon’. It is the part that casts the shadow and in the northern hemisphere it points south.

But why?

When we think of the word ‘midday’, it is tempting to think that it means 12 o’ clock. But that is a sign of how the clocks have come to warp our understanding of nature.

Traditionally, midday is the ‘middle of the day’ and this is always halfway between sunrise and sunset. It is also when the sun is highest in the sky.

But why?

The sun is highest in the sky when it passes our ‘meridian’ – that is the imaginary line that runs from the North Pole to the South Pole, between our legs. Before that moment it is east of us and rising, after that moment it is west of us and sinking.

Put another way, if the sun is highest in the sky for someone in Greenwich, London, then it will already have started sinking for someone ten miles east of them and will still be rising for somebody ten miles west of them. True midday is a very local phenomenon. It moves over the Earth from east to west.

Sundials tell the time relative to that true midday moment – that is why the gnomon is in the middle of the dial and why it points south. It is lined up with the meridian, it is a very small part of that line running from the North Pole to the South Pole. .

Sundials are no more complicated than a shadow stick. The only real difference is that someone has gone to the trouble of making something longer-lasting and marking off the hours.

A sundial with gnomon pointing south
A typical gnomon, showing a North-South alignment

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