Zhang Heng (78-139 AD), styled Pingzi, was a native of Xie in Nanyang (today's Shiqiao Town, Nanyang County, Henan Province). He was a great astronomer of China's Eastern Han Dynasty, making indelible contributions to the development of Chinese astronomy. In mathematics, geography, painting, and literature, Zhang Heng also demonstrated exceptional talent and broad knowledge.
As a child, Zhang Heng was bright by nature and humble in attitude, particularly fond of pondering questions. He was deeply interested in all things in nature. Flowers glistening with morning dew, the sun hanging high in the sky at noon, and the bright moon and twinkling stars in the night sky sparked endless associations in his mind. He would always follow his parents around, asking question after question.
Once, he went to the fields with his mother to dig for wild vegetables. When they set out, the sun had just risen in the east, red and beautiful to behold. Casually, he noticed how long his shadow was. He thought to himself, "How wonderful it would be if I could grow as tall and large as my shadow!" Unknowingly, it was already noon. His mother had filled her basket with vegetables. He followed behind her, hopping along. "Hey! Where did my shadow go?" he exclaimed in surprise. Looking down, he saw his shadow had shrunk into a small patch beneath his feet. Zhang Heng quickly asked his mother what was happening. She explained that at noon, when the sun reaches its highest point, shadows become shortest and shrink together; as evening approaches and the sun nears the horizon, the shadows would lengthen again.
Back home, Zhang Heng kept observing the length of his own shadow. He discovered it was indeed as his mother had said: by evening, his shadow had grown long again, just like in the morning. Feeling he had learned something new, he was overjoyed.
One summer evening, his parents took young Zhang Heng to the threshing ground to cool off. This was the most joyful time of day for people. Adults waved their fans while chatting freely; children chattered happily, playing hide-and-seek or house one moment, then another game the next. Only Zhang Heng stood quietly to the side, silently gazing up at the vast night sky, softly counting under his breath: "one, two..." Thinking he might be tired from the day’s outing, his mother said, "Heng’er, if you’re tired, go back inside and rest. Don’t just stand there, looking dazed." But Zhang Heng seemed not to hear, continuing to stare intently at the heavens.
Seeing he didn't respond, his parents left him alone. After a while, the adults grew drowsy and began going home one after another to sleep, yet he remained, still staring at the sky. Then, an older boy came over and patted his shoulder, saying, "Hey! Are you crazy? What are you staring at in the sky for? It's not like gold beans will fall from it!" Zhang Heng finally snapped out of his trance, rubbed his sore neck, and replied, "Who's hoping for gold beans to fall? I'm counting the stars." At this, everyone froze. "What? Counting stars? How strange! A fool who counts stars! So tell me, have you finished counting them?" asked the older boy. "I haven't finished yet, but I've already counted over a thousand."
An old man nearby chimed in: "Child, don't bother. The stars in the sky are countless—endless and unpredictable." But Zhang Heng interrupted the old man: "That's not true! That part of the sky only has over a thousand stars. As long as I keep counting, I'll definitely finish!" The old man was deeply moved by Zhang Heng's persistence and found himself momentarily speechless.
Zhang Heng's father quickly stepped in to smooth things over: "You mustn't speak to the elder like that." Realizing his mistake, Zhang Heng hurriedly apologized to the old man. But turning back, he still wanted to explain himself to his father. His father, already understanding his thoughts, said: "Heng’er, I know what you're thinking, but counting them one by one won't work. The stars in the sky are arranged according to patterns. You need to divide them into constellations according to these patterns. Only then can you understand and remember them clearly."
Young Zhang Heng nodded, followed his father's advice, and indeed recognized many more new stars.
As he grew older, Zhang Heng's thirst for knowledge became increasingly intense. At the time, natural disasters occurred frequently, and earthquakes caused especially severe devastation. They struck without warning and were immensely destructive once they hit. This prompted Zhang Heng to think deeply. He wondered if it was possible to invent an instrument that could accurately determine the direction of an earthquake after it happened, so relief supplies could be dispatched promptly. After setting this goal, he studied a vast amount of material on earthquakes and conducted numerous field surveys.
Sometimes, to obtain first-hand data, Zhang Heng personally visited places recently struck by earthquakes to measure the earth's tremors. Once, as he inserted his seismoscope into the ground and prepared to read the data, a violent aftershock hit. A nearby earthen wall suddenly collapsed. Fortunately, he was quick on his feet and leapt aside just in time, narrowly escaping being crushed to pulp. He brushed the dust off his clothes and continued checking the instrument's readings. The local villagers far away couldn't understand his actions at all, thinking he was simply mad. But Zhang Heng remained unmoved, persisting day and night in his research on the seismoscope. Soon, the seismoscope was successfully invented.