Da Ming Calendar
Zu Chongzhi (429-500), also known as Wen Yuan, was a native of today's Laishui County of North China's Hebei Province, and a great mathematician, astronomer, and engineer of the Northern and Southern Dynasties (420-589).
He became famous for his outstanding contributions to mathematics, as he was the first person in the world to bring the calculation of the pi (the ratio of the circumference of a circle to its diameter) to the seventh decimal place, between 3.1415926 and 3.1415927. His achievement was more than a thousand years earlier than that of his European counterparts. In fact, he was also a highly accomplished astronomer, whose Da Ming Calendar was China's most advanced calendar of his era.
After many observations and studies, he concluded that a year lasted exactly 365.24281481 days, which was only 46 seconds different from the modern estimate. The midwinter was the starting point in making the calendar; hence it was of significant importance to pinpoint the position of the sun on that day.
However, the astronomers before Zu all believed the position was fixed, which caused errors in the calendar-making task from the very beginning. To solve the problem, Zu introduced the concept of procession (of the sun) in making the Da Ming Calendar, greatly enhancing the accuracy of calendar computation.