In the middle of the Eastern Han Dynasty (25-220), Zhang Heng, an outstanding astronomer, made bold innovations to the armillary sphere by designing and making the water-driven armillary sphere, which can completely demonstrate the theory of sphere heavens.

The major part was a spherical model, representing the celestial globe. Inside the globe there was a metal axis going through the center, with the sphere and the axis intersecting at two pints: the north and south celestial points. The direction of the globe represented the direction of the earth's axial rotation.

To enable the armillary sphere to rotate, Zhang employed the gear system, linking the celestial globe and the kettle clepsydra. Powered by the water leaked out of the kettle, the gear drove the celestial globe to rotate around the axis, one day per circuit, in synchronization with the celestial body (sun). In this way, the celestial changes were presented accurately. By observing the instrument, one could know when a star rose in the east and set in the west.

The world's first water-driven astronomic instrument with explicit historical records, the new armillary sphere employed an extremely complex gear system. It was a proud achievement for ancient Chinese people to work out such a complex instrument. Unfortunately, the drive system was not handed down, as it was made in the remote past.

Author: Jessie