An astronomical observatory near Dengfeng in Henan province, China, built in 1276 during the early Yuan dynasty on the order of Kublai Khan and designed by astronomers Guo Shoujing and Wang Xun. It was the first of 27 observatories built across the empire to support a calendar reform, and using its tall gnomon and horizontal scale, Yuan astronomers measured the tropical year at 365 days, 5 hours, 49 minutes and 20 seconds, a precision matching the Gregorian calendar three centuries before that calendar existed. UNESCO designated the site a World Heritage Site in 2010.
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