《隋书·列传·卷三十四》
《周官》上记载用土圭之法测日影之长，夏至日影长一尺五寸。
《周官》以土圭之法正日影，日至之影尺有五寸。
The Zhouguan details the method of measuring the length of the Sun’s shadow with a sundial; on the day of summer solstice, the Sun’s shadow is one chi and five cun long.