The 24 Solar Terms
The 24 solar terms is a
gross name of the system that comprises of 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 |
|
Bright
and Clear |
|
Summer
Commences |
|
Corn
on Ear |
|
Moderate
Heat |
Sun's
longitude |
0
deg. |
15
deg. |
30
deg. |
45
deg. |
60
deg. |
75
deg. |
90
deg. |
105
deg. |
Major
solar term |
Great
Heat |
|
End
of Heat |
|
Autumnal
Equinox |
|
Frost |
|
Minor
solar term |
|
Autumn
Commences |
|
White
Dew |
|
Cold
Dew |
|
Winter
Commences |
Sun's
longitude |
120
deg. |
135
deg. |
150
deg. |
165
deg. |
180
deg. |
195
deg. |
210
deg. |
225
deg. |
Major
solar term |
Light
Snow |
|
Winter
Solstice |
|
Severe
Cold |
|
Spring
Showers |
|
Minor
solar term |
|
Heavy
Snow |
|
Moderate
Cold |
|
Spring
Commences |
|
Insects
Waken |
Sun's
longitude |
240
deg. |
255
deg. |
270
deg. |
285
deg. |
300
deg. |
315
deg. |
330
deg. |
345
deg. |
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.
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.
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