Conversation so far:
what did Zhang Heng have to do with science and technology?
Zhang Heng's mechanical inventions influenced later Chinese inventors.
what did it influence them to do?
Zhang's water-powered armillary sphere as the inspiration for his 11th-century clock tower.
what was his biggest accomplishment?
Based on the theories of his predecessors, Zhang Heng systematically developed the celestial sphere theory.
how did the world react to this theory?
Only a single step separates the celestial globe from the earth-centered theory, but Chinese astronomers never took that step.

Question: what was his greatest accomplishment?
Self-contained rewrite: What was Zhang Heng greatest accomplishment?