The Han Chinese used astronomical studies in order to construct and revise their calendar. Different from the Gregorian calendar but similar to the Hellenic calendar, the Chinese calendar is lunisolar, meaning it uses the movements of the Sun and Moon as time-markers. In the 5th century BCE during the Spring and Autumn Period, the Chinese established the Sifen calendar, which measured a year at 3651⁄4 days. Emperor Wu later replaced this with the new Taichu calendar in 104 BCE, which improved the measurements of the tropical year to 365385⁄1539 days. In addition, the lunar month was found to be 2943⁄81 days. After the Taichu calendar became inaccurate over two centuries, Emperor Zhang of Han halted its use and revived use of the Sifen calendar. Later, astronomer Guo Shoujing would set the tropical year at 365.2425 days for his Shoushi calendar (授時曆), the same value used in the Gregorian calendar. The calendar was used for regulating agricultural practices throughout the seasons, and also marked several important dates in the Sexagenary cycle, consisting of by celestial stems and Earthly Branches, each of the latter associated with an animal of the Chinese zodiac.