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The Observatory is located in Gaocheng Town, Dengfeng City, Henan Province. It is by far the most ancient building for astronomical observation.
The Observatory, originally built in the 13th year (1276) of the Zhiyuan reign under Emperor Shizu in the Yuan Dynasty (1127-1279), leans against Jishan Mountain in the south, faces Songshan Mountain in the north and is near the joint point of the Yinghe River and the Wudu River. In the 13th year of the Zhiyuan reign, Emperor Shizu called for the well-known astronomers, Guo Shoujing and Wang Xun, to preside the calendar revolution and built 27 observatories all over the country with this observatory as the center.
The Observatory was built of stones and bricks. It has two parts: the body and Shigui (also called the ruler to measure the sky). It is 9.46 meters high by itself, and 12.62 meters high if the cabinet on the top is included. Its plane is similar to a square. The border width of the base is 16 meters, while that of the platform on the top is more than 8 meters. In the north, stairs in symmetry lead to the top of the Observatory. The central part of the north wall was cut into straight grooves that connect Shigui Shigui extending to the far north is 31.19 meters long and 0.53 meters wide. It is made up of 36 square stones with two parallel waterways on it. The location of Shigui is in accordance with the direction we take today to measure the meridian. During measurement, a beam is put across the grooves. Jingfu (an instrument with many holes) on the waterways is used to measure the shade, whose precision is within 2 millimeters.
In this way, Guo Shoujing reckoned that a tropical year included 365 days, 5 hours, 49 minutes and 20 seconds. Compared with the measurement of modern times, there is only a disparity of 26 minutes, while the book named Shou Shi Li (Season-Granting Calendar) of Guo Shoujing is 300 years earlier than the Gregorian calendar common in use today. This book embodies the high level of China’s ancient astronomy and calendar science research.
South to the Observatory lies the Zhougong Temple, in front of which there is a Shigui chart made by Zhang Yixing, the astronomer of the Tang Dynasty, in the 11th year (723) of the Kaiyuan yuan. This place is recorded to be the center of the earth in Zhou Li (Rites of Zhou). It is said that Ceyingtai (observatory measuring the shade) built by Zhougong in the Western Zhou Dynasty (11th century 771BC) was located here.
(Source: chinaculture.org)
