What did Gnomon / Sundial Standard look like?

A vertical pole erected on level ground, with a horizontal graduated scale laid out precisely north-south. At noon, sunlight casts the pole's shadow onto the gui scale.

How did Gnomon / Sundial Standard work?

Observe noon shadow length daily → record year-round shadow variation → the summer solstice has the shortest shadow, the winter solstice the longest → consecutive years of records precisely determine the tropical year length → subdivide the twenty-four solar terms.

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Gnomon / Sundial Standard

The most fundamental astronomical measurement tool: a vertical gnomon pole paired with a north-south horizontal graduated scale (gui). Determines the tropical year and twenty-four solar terms from noon shadow length.

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Scientific Principle

Solar altitude angle: gnomon height h, shadow length l → tan α = h/l. At winter solstice, α is smallest and shadow longest; at summer solstice, α is largest and shadow shortest.