The Chinese calendar was in need of reform since it inadequately measured the solar year at 365 ¼ days, giving an error of 10 min and 14 sec a year or roughly a full day every 128 years. Although the Ming had adopted Guo Shoujing's "Shoushi" calendar of 1281, which was just as accurate as the Gregorian Calendar, the Ming Directorate of Astronomy failed to periodically readjust it; this was perhaps due to their lack of expertise since their offices had become hereditary in the Ming and the Statutes of the Ming prohibited private involvement in astronomy. A sixth-generation descendant of the Hongxi Emperor, the "Prince" Zhu Zaiyu (1536–1611), submitted a proposal to fix the calendar in 1595, but the ultra-conservative astronomical commission rejected it. This was the same Zhu Zaiyu who discovered the system of tuning known as equal temperament, a discovery made simultaneously by Simon Stevin (1548–1620) in Europe. In addition to publishing his works on music, he was able to publish his findings on the calendar in 1597. A year earlier, the memorial of Xing Yunlu suggesting a calendar improvement was rejected by the Supervisor of the Astronomical Bureau due to the law banning private practice of astronomy; Xing would later serve with Xu Guangqi in reforming the calendar (Chinese: ) in 1629 according to Western standards.