Why the Solar Terms Shift Every Year — and the Setsubun That Moved

Many people in Japan memorize Setsubun as February 3. Yet in 2021 and 2025 it fell on February 2. Setsubun — and the solar term Risshun that it precedes — moves because the 24 solar terms are fixed by the position of the Sun alone, independent of the calendar date.

The 24 solar terms are points every 15° of solar longitude

The 24 solar terms (nijūshi sekki) divide the Sun's yearly path along the ecliptic into 15° steps — 24 exact instants. Spring equinox = 0°, summer solstice = 90°, autumn equinox = 180°, winter solstice = 270°. Risshun, the traditional start of spring, sits at 315°; the day containing that instant is Risshun. Setsubun is defined as the day before it, so when Risshun moves, Setsubun moves with it. In Japan these points still anchor seasonal greetings and events like the Setsubun bean-throwing.

Why the dates aren't fixed

A tropical year is not exactly 365 days but about 365.2422 days — roughly 5 hours 49 minutes longer than the 365-day calendar year. So the moment the Sun reaches 315° slips that much later each year, until a leap day pushes it back about a full day. That sawtooth wobble is the main motion. But the fraction that piles up over four years (about 23 h 16 m) does not exactly cancel the leap day's 24 hours, so the terms creep about 45 minutes earlier every four years. That small residual accumulates, so across decades Risshun has crept from Feb 5 → Feb 4 → Feb 3 (the leap-year rule for century years nudges it back over the longer term).

Distribution of Risshun dates (1970–2027)

Counting the last 58 years of Risshun with our own calendar engine:

Feb 5 vanished after 1984 (it does not return before 2099 in our calculation), while Feb 3 debuted in 2021 — a clear drift from "the 5th still appeared" in the past, to "centered on the 4th" now, to "the 3rd growing common" ahead.

2021 and 2025 had a "February 2 Setsubun"

When Risshun lands on Feb 3, the Setsubun before it moves up to Feb 2. 2021 was the first year since 1900 with Risshun on Feb 3 (our calculation), so a Feb 2 Setsubun was a first for most people alive. Japanese media widely reported that "the 2021 Feb 3 Risshun was the first in 124 years, since 1897" — that year is outside our calculation range, so we pass it along only as a reported figure, not as our own result.

February 3 will only grow more common

In our calculation, Risshun falls on Feb 3 in a run of years spaced four apart, growing more frequent toward the end of the century:

• 2021 → 2025 → 2029 → 2033 → 2037 … (every four years for now)
• By late in the 21st century, Feb 3 appears almost every other year

So "Setsubun is February 3" will slowly stop being reliable. Check any year's exact dates with the 24 Solar Terms calendar.

The lunar calendar has a related problem

Because the old Japanese lunisolar calendar assigns its month names using these same solar terms, it can reach a case where a month name isn't uniquely defined — the kyureki 2033 problem. We cover the mechanism in the kyureki 2033 article.

FAQ

The Risshun and Setsubun dates here come from our own calendar calculation (from the instant the Sun reaches solar longitude 315°), as of 2026-07-16. The "first in 124 years, since 1897" figure is reported by others, not computed here.