The Tower of Guo Shoujing is an impressive astronomical instrument. It allowed observers to measure the height of the sun, moon and stars above the horizon to a high accuracy. For its date 1276 ACE it is the most advanced instrument of the time.
A Gnomon in its simplest form is a vertical stick that is used to measure shadow. This can then take note of the shortest shadow (mid-summer) and the longest shadow (mid-winter). The advanced instrument at Dengfeng used a crosshair and water (as an accurate level) so that the motions of the heavens could be measured to five decimal places. A feat not achieved in Europe for another five hundred years.
Dengfeng Observatory is located within the Zhougong Miao temple some 85kms south-east of Luoyang, Henan Province.