Hemispherical Sundial

Tomb of King Sejong
Yeoju, Korea

The original of this sundial, called angbu ilgu in Korean, was created in 1437 AD by Jang Yeong-sil, a Korean astronomer and polymath who also built water clocks, armillary spheres, and many other scientific instruments as Chief Engineer at the court of Sejong the Great.

On this sundial, the time is read off in ten-minute increments as the shadow of the gnomon is projected upon the longitude lines of the grid (the longitude lines are horizontal in this photo). Thirteen latitude lines, vertical in this photo, measure the length of the gnomon's shadow. This determines the elevation of the sun and therefore the corresponding season. The outer circle displays the 24 hours.