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P. 16
April 2017
24 Solar Terms4 Solar Terms
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Mr Shun Chi-ming, Director of the Hong Kong Observatory
Link between the 24 Solar Terms and Astronomyink between the 24 Solar Terms and Astronomy
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The “24 solar terms” form an integral part of the Chinese calendar. It is a time knowledge system progressively
built upon the long-term observation of periodic movement of the Sun, seasonal march and phenology, and
evolves in time to meet the needs of social development. Originated from the Huanghe (Yellow River) basin in
ancient times, the use of 24 Solar Terms has since been widely adopted by other places in China as well as its
neighbouring countries.
Earth orbits around the Sun along an elliptical path. Its axis of rotation is not perpendicular to the plane of the
orbit but is tilted at about 23 degrees. An observer on Earth, looking at the apparent movement of the Sun
against remote stars, can de ne a path called the “ecliptic”. The solar terms, evenly spread conceptually along
the ecliptic, mark the moments when the Sun reaches these 24 pre-de ned positions and form a cycle that
corresponds exactly to one year and more precisely one “sidereal year”.
