Dates and Days

Equinoxes and solstices

When the seasons actually turn, to the minute

An equinox is an instant, not a day: the moment the Sun’s apparent longitude crosses zero. It lands at a different clock time in every time zone, which is why the first day of spring is sometimes the 19th and sometimes the 21st. Both equinoxes, both solstices and the harvest moon, for any year from 1800 to 2150.

March equinox— June solstice— September equinox— December solstice— Harvest moon—
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The four turning points — all times UT, and the length of each season
EventDateTime (UT)Day of yearSeason length

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 for different people: an equinox at 00:30 UT is the 20th in London and still the 19th in New York. Nothing is wrong in either case — they are the same moment, read on two clocks.

The seasons are not equal in length either, which the table above shows. The Earth’s orbit is an ellipse and it moves fastest when closest to the Sun, in early January, so the northern winter is the shortest season by nearly five days.

Why published equinox times disagree, measured

Compare a few sites and the same equinox appears a minute or two apart. Part is rounding; the larger part, for any year far from the present, 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 — entirely from that prediction, not from any disagreement about where the Sun is. Neither is wrong: nobody knows how fast the Earth will be turning in 2150. Times are given in UT rather than a local zone, which would imply precision the number does not have.

The harvest moon is a definition, not a name

The harvest moon is the full moon nearest the September equinox — whichever side of it falls closer. Usually that is obvious, because full moons are 29.5 days apart and the equinox sits well inside one gap. Occasionally it is close: in 2025 the two candidates were separated by only about half a day of difference, and the answer genuinely 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 and when it was not.

Questions

Why is the first day of spring sometimes the 19th and sometimes the 21st?
Because an equinox is an instant, not a day, and the calendar does not divide evenly into the year. The Earth takes about 365.2422 days to orbit the Sun while the calendar gives it 365, so the instant drifts roughly six hours later each year and jumps back a day at each leap year. Time zones do the rest: an equinox at 00:30 UT falls on the 20th in London and is still the 19th in New York. Both are correct; it is one moment read on two clocks.
Why do different websites give slightly different equinox times?
Partly rounding, but mostly delta-T: the gap between the uniform time astronomers compute in and the time the Earth actually keeps, because its rotation is slightly irregular and slowly slowing. Delta-T is measured for the past and predicted for the future, and reasonable sources predict it differently. This page follows the Espenak and Meeus long-term parabola; JPL's Horizons service holds delta-T near its present value of about 69 seconds. Checked against Horizons at one-minute resolution the two agree within 53 seconds from 1800 to 2050, then part by about two minutes at 2100 and four at 2150 — purely from that assumption, not from any disagreement about where the Sun is.
How accurate are these times?
Under a minute for any year from 1800 to 2050, measured rather than claimed: the instants were compared against JPL Horizons' own apparent solar longitude, stepped at one-minute resolution, and the worst disagreement across 24 checked instants was 53 seconds. Beyond 2050 the delta-T prediction dominates and the honest figure is a few minutes. Times are given in UT rather than a local zone, because converting would imply a precision the underlying number does not have.
Why are the four seasons different lengths?
Because the Earth's orbit is an ellipse, not a circle, 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. The difference is not small: winter in the northern hemisphere is nearly five days shorter than summer, and the table above gives the exact lengths for whichever year you ask about.
What exactly is the harvest moon?
The full moon nearest the September equinox, on whichever side of it falls closer. Usually the choice is obvious, since full moons are 29.5 days apart and the equinox sits well inside one gap. Occasionally the two candidates nearly tie, and then the answer depends on knowing the equinox instant properly rather than approximately. The table reports how much nearer the winning full moon was, so a close call is visible instead of hidden.