Dates and Days

Equinoxes and solstices

When the seasons actually turn, to the minute

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The four turning points: times in UT, with each season's length
EventDateTime (UT)Day of yearSeason length
How this was counted
March equinox— June solstice— September equinox— December solstice— Harvest moon—

Checked on every build against 32 equinox and solstice instants from JPL Horizons. How it is checked

Enter a year from 1800 to 2150 for the exact instant of both equinoxes, both solstices and the harvest moon, in UT, with the length of each season. An equinox is a single moment (the March one is the instant the Sun’s apparent longitude crosses zero), so its clock time differs in every time zone.

Why the first day of spring moves

The Earth takes about 365.2422 days to go round the Sun and the calendar gives it 365, so the equinox drifts about six hours later each year and jumps back a day at each leap year. Add a time zone and the same instant lands on different dates: an equinox at 00:30 UT is the 20th in London and still the 19th in New York.

The seasons are unequal in length too, as the table shows.

Why published equinox times disagree, measured

Compare a few sites and the same equinox appears a minute or two apart. Part is rounding. For any year far from the present, the larger part is ΔT, the gap between the smooth time astronomers compute in and the time the Earth keeps, because its rotation is slightly irregular and slowly slowing. ΔT is measured for the past and predicted for the future, and reasonable sources predict it differently.

Checked against JPL Horizons at one-minute resolution, this page agrees to within 53 seconds anywhere from 1800 to 2050, then parts by about two minutes at 2100 and four at 2150. That later gap comes entirely from the ΔT prediction, and nobody knows how fast the Earth will be turning in 2150. Times are given in UT.

The solstice is also a calendar anchor

In the Chinese calendar the December solstice fixes the numbering of every month in the year: the month containing it is month 11, and Chinese New Year follows two or three new moons later. There the solstice has to be computed to the minute, because it must land on the right side of a midnight in China.

How the harvest moon is chosen

Usually one full moon is plainly nearer the September equinox, because full moons are 29.5 days apart and the equinox sits well inside one gap. In 2025 it was close: one candidate was only about half a day nearer than the other, and the answer depended on knowing the equinox to better than a few hours. The table reports that margin, so you can see when the choice was close.

Questions

Why is the first day of spring sometimes the 19th and sometimes the 21st?
Because the calendar year and the Earth's orbit do not match, and because of time zones. The equinox comes about six hours later each year until a leap day pulls it back, and one instant can be the 20th in London and still the 19th in New York.
Why do different websites give slightly different equinox times?
Partly rounding, but mostly delta-T, the gap between uniform astronomical time and the time kept by the Earth's slightly irregular rotation. Past values are measured, future values are forecasts, and forecasts differ from source to source. This page follows the Espenak and Meeus long-term parabola, while JPL's Horizons service holds delta-T near its present value of about 69 seconds, so the two part by about two minutes at 2100 and four at 2150.
How accurate are these times?
Under a minute for any year from 1800 to 2050. The instants were compared against JPL Horizons' own apparent solar longitude, stepped at one-minute resolution, and the worst of 24 checked instants was 53 seconds out. Beyond 2050 the delta-T forecast dominates and the uncertainty is a few minutes.
Why are the four seasons different lengths?
The Earth's orbit is an ellipse, and a planet moves fastest when it is closest to the Sun. Perihelion falls in early January, so the Earth races through the northern winter and dawdles through the northern summer. Northern winter is the shortest season, nearly five days shorter than summer.
What is the harvest moon?
The full moon nearest the September equinox, before or after it. When the two candidates nearly tie, the choice depends on the exact equinox instant, and the table shows how many days nearer the chosen full moon was.