According to Chinese astronomy, the 24 Seasonal Segments (二十四节气 èrshísì jiéqì) were incorporated into the calendar during the Warring States Period. The calendar of twenty-four periods and climate matches particular astronomical events, signifies some natural phenomena, and is also synchronized with the seasons, which govern agricultural arrangements in ancient China and functions even now. It predicts the seasonal variation using the moon's motion to measure the time it will take for the earth to return to a certain position (the solstices and equinoxes) in relation to the sun. In other words, it takes into account the longest and the shortest days and the two days each year when the length of the day equals that of the night: the Summer and Winter Solstices and the Spring and Autumn Equinoxes. Thus, strictly speaking, the Chinese calendar is a lunisolar calendar.
The "Song of Solar Terms" is used to ease the memorization of 24 seasons: