Gnomon Sundial

Yuan Dynasty (replica), Ancient Observatory, Beijing

This type of sundial was oriented north-south; it measures the length of the shadow that the sun casts at noon. Its apparent prototype is the Gaocheng Observatory in Henan Province. The maximum length of its shadow occurs at the winter solstice, when the sun is lowest in the sky; the minimum length of the shadow occurs at the summer solstice, when the sun is highest in the sky. For more details, see the next page. The half-year interval between these two events is divided by a scale, and the 24 solar terms are indicated at the circular endpoint (next page).

Observing the solstices (Midsummer Eve and Midwinter Eve) was of primary importance for agricultural communities all over the world, from the Americas to Northern Europe and East Asia, and from Neolithic to near-modern times, since those dates formed the endpoints of their agricultural and ritual calendars; see, for example, Summer Solstice Traditions for some interesting European traditions connected with Midsummer's Eve.