section_path: Science and technology/Astronomy
section_name: Astronomy
paragraph_text: Astronomy
  
  Mathematics were essential in drafting the astronomical calendar, a lunisolar calendar that used the Sun and Moon as time-markers throughout the year. During the spring and autumn periods of the 5th century BC, the Chinese established the Sifen calendar (古四分歷), which measured the tropical year at 365.25 days. This was replaced in 104 BC with the Taichu calendar (太初曆) that measured the tropical year at 365 + 385 ⁄ 1539 (~ 365.25016) days and the lunar month at 29 + 43 ⁄ 81 days. However, Emperor Zhang later reinstated the Sifen calendar.
  
  Han dynasty astronomers made star catalogues and detailed records of comets that appeared in the night sky, including recording the appearance of the comet now known as Halley's Comet in 12 BC. They adopted a geocentric model of the universe, theorizing that it was a sphere surrounding the Earth in the centre. They assumed that the Sun, Moon, and planets were spherical and not disc-shaped. They also thought that the illumination of the Moon and planets was caused by sunlight, that lunar eclipses occurred when the Earth obstructed sunlight falling onto the Moon, and that a solar eclipse occurred when the Moon obstructed sunlight from reaching the Earth. Although others disagreed with his model, Wang Chong accurately described the water cycle of the evaporation of water into clouds.