The 24 solar terms is a gross name of the system that comprises 12 major solar terms and 12 minor solar terms interlaced with each other. Starting from "vernal equinox", the 12 major solar terms are "vernal equinox", "corn rain", "corn forms", "summer solstice", "great heat", "end of heat",
"autumnal equinox", "frost", "light snow", "winter solstice", "severe cold" and "spring showers". Each major solar term falls on one of the 12 lunar months designated by the 12 earthly branches (Figure 1).
The minor solar term after "vernal equinox" is "bright and clear", and then in turn "summer commences", "corn on ear", "moderate heat", "autumn commences", "white dew", "cold dew", "winter commences", "heavy snow", "moderate cold", "spring commences" and "insects waken".
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
Major solar term
Minor solar term
Vernal
Equinox
Figure 1 Tropical year, synodic months and the 24 solar terms.
Bright and
Clear
Corn Rain
Summer
Commences
Corn Forms
Corn on Ear
Summer
Solstice
Moderate
Heat
Sun's longitude
Major solar term
Minor solar term
Sun's longitude
0°
Great Heat
120°
15°
Autumn
Commences
135°
30°
End of Heat
150°
45°
White Dew
165°
60°
Autumnal
Equinox
180°
75°
Cold Dew
195°
90°
Frost
210°
105°
Winter
Commences
225°
Major solar term
Minor solar term
Sun's longitude
Light Snow
240°
Heavy
Snow
255°
Winter
Solstice
270°
Moderate
Cold
285°
Severe Cold
300°
Spring
Commences
315°
Spring
Showers
330°
Insects
Waken
345°
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.
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.
Figure 2 The 24 solar terms and the 12 zodiac constellations.
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.
The 24 solar terms is a gross name of the system that comprises 12 major solar terms and 12 minor solar terms interlaced with each other. Starting from "vernal equinox", the 12 major solar terms are "vernal equinox", "corn rain", "corn forms", "summer solstice", "great heat", "end of heat",
"autumnal equinox", "frost", "light snow", "winter solstice", "severe cold" and "spring showers". Each major solar term falls on one of the 12 lunar months designated by the 12 earthly branches (Figure 1).
The minor solar term after "vernal equinox" is "bright and clear", and then in turn "summer commences", "corn on ear", "moderate heat", "autumn commences", "white dew", "cold dew", "winter commences", "heavy snow", "moderate cold", "spring commences" and "insects waken".
Figure 1 Tropical year, synodic months and the 24 solar terms.
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
Major solar term
Vernal
Equinox
Corn Rain Corn Forms
Summer
Solstice
Minor solar term
Sun's longitude
0°
Bright and
Clear
15°
30°
Summer
Commences
45°
60°
Corn on Ear
75°
90°
Moderate
Heat
105°
Great Heat End of Heat
Autumnal
Equinox
Frost
Major solar term
Minor solar term
Sun's longitude
Major solar term
Minor solar term
Sun's longitude
120°
Light Snow
240°
Autumn
Commences
135°
Heavy
Snow
255°
150°
Winter
Solstice
270°
White Dew
165°
Moderate
Cold
285°
180°
Severe Cold
300°
Cold Dew
195°
Spring
Commences
315°
210°
Spring
Showers
330°
Winter
Commences
225°
Insects
Waken
345°
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.
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.
Figure 2 The 24 solar terms and the 12 zodiac constellations.
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.