24 solar terms

They are solar, which is why they barely move

The 節氣 are the part of the Chinese calendar that has nothing to do with the Moon. They are 24 equal slices of the Sun’s apparent ecliptic longitude: 春分 is the moment the Sun crosses 0°, 夏至 is 90°, 秋分 is 180°, 冬至 is 270°, and the other twenty fill in every 15° between.

Because the Gregorian calendar is also solar, the two agree almost exactly. Over the whole 201 years in this table, each term moves across a window of only three days:

That is why 清明 can be a fixed public holiday in a way Lunar New Year never could: it is already a Gregorian date in all but name. 春節, by contrast, ranges over a full month — from 21 January to 20 February across the same 201 years.

中氣 and 節氣 are not the same thing

The twenty-four alternate between two kinds, and the distinction does real work:

• The twelve at a multiple of 30° — 雨水, 春分, 穀雨, 小滿, 夏至, 大暑, 處暑, 秋分, 霜降, 小雪, 冬至, 大寒 — are 中氣, the major terms.
• The twelve between them are 節氣 in the narrow sense, the minor terms. In everyday Chinese 節氣 also names the whole set of twenty-four, which is why the distinction gets lost.

The 中氣 are what fix the lunar months. Month 11 is the month containing 冬至. And a month containing no 中氣 at all is the candidate for a leap month — that is the entire mechanism by which a lunar calendar stays in step with the seasons.

The table above marks the twelve major terms.

The gaps are not equal

Consecutive terms are 14, 15 or 16 days apart, and which one depends on where the Earth is in its orbit. Near perihelion in early January the Earth moves fastest, the Sun appears to cover 15° in about 14.7 days, and the terms crowd together. Near aphelion in early July it is about 15.7 days and they spread out.

The difference is about a day, and it is the reason leap months cluster in summer: a lunar month can slip between two widely spaced 中氣 and catch neither. Across 1900–2100 there are fourteen 閏四月 and fourteen 閏五月, but only one 閏十一月 — in 2033.

雙春年, and what it does not mean

Because a lunar year runs about 354 days and the 立春-to-立春 interval is 365¼, some lunar years contain two 立春 and some contain none. Across 1900–2100, seventy-three lunar years are 雙春 (two) — very nearly the same count as the seventy-four leap years, because a year with thirteen months has room for two.

Both are held to be auspicious or inauspicious for weddings depending on who you ask, which is a statement about custom rather than about the calendar. Arithmetically it is unavoidable book-keeping: the two cycles are different lengths, so the leftover has to land somewhere.

Where the dates come from

Computed at build time from Meeus’s solar coordinate algorithms using the VSOP87 series, then reduced to civil days at UTC+8 — the terms are instants, and an instant falls on different dates in different time zones. A term at 00:20 Beijing time is the previous day in Bangkok.

The table is checked against published dates before it ships, and the same anchors are asserted by the test suite.

Related

The lunar calendar converter turns any date into its 農曆 equivalent and shows the term it sits in. The zodiac calculator uses 立春 for the astrological boundary, which is the one that disagrees with Lunar New Year.