From the Earth's perspective, the Sun moves through a year across the stars or celestial sphere along a path known as the ecliptic, which is measured in 360 degrees longitude.  The 24 solar terms divide the ecliptic into 24 equal segments, with 15 degrees of the Sun's longitude between the terms.  At "vernal equinox", the Sun's longitude is  0 degree; at "bright and clear", the Sun's longitude is 15 degrees; and so forth (Table 1).

         Table 1             The 24 solar terms and the Sun's longitudes

        At "vernal equinox" and "autumnal equinox", the periods of daylight and the night are equal in length.  The period of daylight is the longest at "summer solstice" and the shortest at "winter solstice" (northern hemisphere).  These were the earliest solar terms determined in ancient time.  Then it came the four solar terms "spring commences", "summer commences", "autumn commences" and "winter commences".  Other solar terms were named later according to the weather and agricultural activities prevalent at the respective times of the seasons.  The "24 solar terms" reflects to some extent the climate over central China in ancient time.

        Each lunar month in the Agricultural Calendar contains a major solar term.  A lunar month that does not include a major solar term is taken as the leap month of the preceding month.  In 19 tropical years there will be 228 major solar terms and 235 synodic months.  So 7 lunar months will not contain major solar terms and they are classified as leap months.

The 24 Solar Terms and the 12 Zodiac Constellations

        Similar to the 24 solar terms, the 12 zodiac constellations were determined in the ancient time by dividing the zodiac (a band around the ecliptic) into 12 equal sectors measuring from the "vernal equinox" (Figure 2).  Hence the start or end date of each zodiac sign in astrology always falls within about one day on a major solar term.

Remarks:        The determination of the 12 zodiac constellations according to the Sun's longitude was made more than 2000 years ago.  Due to precession of the Earth's rotation axis, the positions of the constellations as observed nowadays have already shifted to other longitudes.

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