The Tower That Measured a Year to the Minute

In 1276, the astronomer Guo Shoujing was given an unusual assignment: fix China's calendar, which had drifted out of step with the seasons. His tool was the oldest instrument in astronomy, a gnomon, a vertical rod whose shadow marks the sun's path through the year. Guo's version, built at an observatory near what is now Dengfeng, was enormous: a brick platform nearly ten metres tall, its crossbar standing five times higher than the traditional gnomon, throwing its shadow down a stone scale running due north for more than thirty metres.

The height solved one problem and created another. A taller gnomon casts a longer, more precise shadow, but also a blurrier one, because the sun is a disc in the sky, not a point of light. Guo's answer was the jingfu, a small sheet of copper pierced with a pinhole. Held over the scale, it projected a tiny image of the sun, no bigger than a grain of rice, with the crossbar's shadow cutting across it as a fine line. When that line split the sun's image exactly in half, Guo knew he was reading the shadow of the sun's true centre, not its blurred edge.

The measurements were exacting enough to matter. Guo's Shoushi calendar, finished in 1281, set the length of the year at 365.2425 days. Europe would not arrive at the same figure until the Gregorian reform of 1582, three hundred years later.

The platform and its scale still stand at Dengfeng, restored over the centuries but built to Guo's original design. Every watch with a calendar function still runs on the year length his shadow-tower measured.