The 24 solar terms: the sun's calendar behind Chinese tradition

Ask when the Chinese year begins and most people say "Lunar New Year". Ask a farmer, a doctor of traditional medicine or a BaZi practitioner and you get a different answer: the Start of Spring (立春), a point in the sun's path that falls on or about 4 February whatever the moon is doing. That second answer comes from the twenty-four solar terms (二十四節氣, èrshísì jiéqì), a solar calendar that has run alongside the lunar months for more than two thousand years and quietly organises a great deal of Chinese tradition. UNESCO added it to its list of intangible cultural heritage in 2016.

Twenty-four points on the sun's path

Seen from Earth, the sun travels a full circle through the sky each year — the ecliptic. Divide that circle into twenty-four arcs of 15° and you have the solar terms: each begins at the instant the sun's longitude reaches a multiple of 15°. The system is therefore purely astronomical. The terms fall on nearly the same Gregorian dates each year (within a day), because the Gregorian calendar is itself solar.

The four cardinal points you already know: the two equinoxes (春分, 秋分, sun at 0° and 180°) and the two solstices (夏至, 冬至, 90° and 270°). The other twenty fill in the quarters. Their names are a poem of the North China Plain's year — rain, insects waking, grain filling, dew turning to frost.

The list

Dates are approximate (±1 day); sun longitudes exact.

• Spring — 立春 Start of Spring (4 Feb, 315°) · 雨水 Rain Water (19 Feb, 330°) · 驚蟄 Awakening of Insects (5 Mar, 345°) · 春分 Spring Equinox (20 Mar, 0°) · 清明 Pure Brightness (5 Apr, 15°) · 穀雨 Grain Rain (20 Apr, 30°)
• Summer — 立夏 Start of Summer (5 May, 45°) · 小滿 Grain Buds (21 May, 60°) · 芒種 Grain in Ear (5 Jun, 75°) · 夏至 Summer Solstice (21 Jun, 90°) · 小暑 Minor Heat (7 Jul, 105°) · 大暑 Major Heat (22 Jul, 120°)
• Autumn — 立秋 Start of Autumn (7 Aug, 135°) · 處暑 End of Heat (23 Aug, 150°) · 白露 White Dew (7 Sep, 165°) · 秋分 Autumn Equinox (23 Sep, 180°) · 寒露 Cold Dew (8 Oct, 195°) · 霜降 Frost's Descent (23 Oct, 210°)
• Winter — 立冬 Start of Winter (7 Nov, 225°) · 小雪 Minor Snow (22 Nov, 240°) · 大雪 Major Snow (7 Dec, 255°) · 冬至 Winter Solstice (21 Dec, 270°) · 小寒 Minor Cold (5 Jan, 285°) · 大寒 Major Cold (20 Jan, 300°)

Notice that each season starts at a 立 ("establish") term and has the equinox or solstice in its middle. In the Chinese reckoning midsummer is the solstice, not the beginning of summer — which is why the hottest terms, Minor and Major Heat, come after it.

Nodes and midpoints

The terms alternate between two kinds. The odd ones (Start of Spring, Awakening of Insects, Pure Brightness…) are nodes (節, jie): they open the twelve solar months. The even ones (Rain Water, Spring Equinox, Grain Rain…) are midpoints (中氣, zhongqi): they sit in the middle of those months and, in the lunisolar calendar, decide which lunar month gets a number and when a leap month is inserted. The two interlock: a lunar month with no midpoint in it becomes the leap month.

Why the metaphysics runs on the sun

BaZi. The year pillar changes at the Start of Spring and each month pillar changes at a node term, not at a lunar new moon. Someone born on 2 February 2027 belongs to the 丙午 year of 2026 in a BaZi chart even though the Lunar New Year had already passed. The precise instant matters — a birth an hour before the Start of Spring and an hour after it get different year pillars — which is why our calculator computes the terms astronomically (to the second) and corrects clock time to true solar time. See why the zodiac animal begins at the Start of Spring.

Traditional medicine and daily life. Seasonal dietary advice, the "dog days" (三伏) counted from the Summer Solstice, the 九九 cold-counting songs from the Winter Solstice, tomb-sweeping at Pure Brightness, eating dumplings at the solstice — all of it keys to the solar terms.

Farming. The names are agricultural instructions: sow around Grain Rain, harvest wheat at Grain in Ear, expect the first frost at Frost's Descent. Proverbs attached to each term formed a practical almanac long before printed calendars.

Reading the terms today

The dates above are for the Northern Hemisphere; the terms were named for the climate around the Yellow River, so "Minor Snow" in Guangzhou or Sydney is a label, not a forecast. What stays true everywhere is the astronomy — the terms are a precise, elegant way of saying where the Earth is in its orbit, which is exactly why a calendar built on them has outlasted every dynasty that used it.