Start moments of the 24 solar terms, 1900 to 2100

Conventions taken

MEASURED ERROR: 3.2 MINUTES EARLY ON AVERAGE, 14 MINUTES AT MOST relative to astronomical values. Checked on 27 September 2026 over all 4,824 marks from 1900 to 2100 against the Sun's apparent longitude from the INPOP ephemeris of IMCCE (Paris Observatory), and over 804 equinox and solstice marks against USNO: the marks here are at most 14 minutes early and 9 minutes late, and close to four in five are early. Version 1.0.0 was 8.4 minutes early on average and 20 at most (measured over 148 marks from 2024 to 2030); version 1.1.0 adds aberration, nutation and ΔT; version 1.1.1 only changes the reference source, the data is unchanged; version 1.2.0 adds four per-mark error columns, described at the end of this note. Start dates match for 4,814 of the 4,824 marks; the 10 that differ by a day are all less than 15 minutes from midnight. The remaining error is because the engine's solar position model uses a truncated series, good enough to fix the DAY of a term but not enough to call this minute-accurate. The sat_nua_dem column flags moments within thirty minutes of midnight: on exactly those rows the error above can shift the starting DAY, so do not settle anything on them without checking an astronomical source. Times are Vietnam time, UTC+7. The mark is the instant solar longitude reaches an exact multiple of 15 degrees, found by bisection down to under one minute. Each row carries its own error: lech_so_voi_imcce_phut is this dataset's time minus the IMCCE time (negative means early), so subtracting it gives the astronomical time to about a minute; phut_toi_nua_dem is the signed distance to the nearest midnight; sai_so_uoc_phut is the 15-minute error bound the cross-check enforces on every mark; rui_ro_bien is cao when the mark is less than 15 minutes from midnight (the error can change the day), trung-binh for the rest of the sat_nua_dem band (up to 30 minutes), and thap for every other mark.