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What equinoxes and solstices are
Four days a year mark the turning points of the seasons, and they are caused by Earth’s tilt, not by its distance from the Sun. Earth leans about 23.4 degrees on its axis, and as it orbits, that lean points sometimes toward the Sun and sometimes away. At the June solstice the northern hemisphere leans most toward the Sun, giving the longest day of the year in the north and the shortest in the south. At the December solstice the lean reverses. At the two equinoxes, in March and September, the lean points neither toward nor away, the Sun stands over the equator, and day and night are nearly equal everywhere.
This calculator gives the dates of all four events for any year from 1900 to 2100. Enter the year, and the tool returns the March equinox, June solstice, September equinox, and December solstice in UTC, computed from the Sun’s declination: the equinoxes are the days it crosses zero, and the solstices are the days it reaches its extremes. The dates are approximate to about a day, because the exact moment depends on your time zone. An event at 1 am UTC on 21 June is still 20 June in the Americas, so always check the moment, not just the date, if the exact day matters to you.
Why the dates drift
Equinoxes and solstices do not fall on the same date every year, because a year is not a whole number of days. Earth takes about 365.2422 days to orbit the Sun, so the calendar, with its 365 day years and leap days, slowly slips against the seasons and is then corrected. That is why the March equinox usually falls on 20 March but sometimes on 19 or 21 March, and why the June solstice wanders between 20 and 22 June.
The seasons themselves are unequal in length, which surprises many people. From the March equinox to the June solstice is about 92 days, but from the September equinox to the December solstice is only about 89 days. The reason is Earth’s elliptical orbit: Earth moves faster near the Sun in early January and slower in early July, so northern summer lasts longer than northern winter. Southern seasons mirror this, with a longer summer in December to February terms reversed. If you ever wondered why summer feels longer than winter in the north, the orbit, not your imagination, is the answer.
A worked example
Take 2026. The March equinox falls around 20 March, the June solstice around 21 June, the September equinox around 22 September, and the December solstice around 21 December, all in UTC. In the northern hemisphere those dates mark the starts of spring, summer, autumn, and winter. In the southern hemisphere the same dates mark autumn, winter, spring, and summer, because the seasons are reversed.
Now try a distant year, such as 2050. The pattern looks familiar, but individual dates may sit a day earlier or later than in 2026, because the leap-year cycle has shifted the calendar against the orbit. That drift is small, a day or two, but it matters for anyone planning an event tied to the exact solstice, like a sunrise alignment at a stone circle, where being a day out means missing the alignment.
What equinox really means
Equinox comes from Latin words meaning equal night, but equal day and night do not actually happen on the equinox. The Sun is a disc, not a point, and the atmosphere bends its light, so the Sun appears to rise a little early and set a little late. The true equal day, called the equilux, falls a few days before the March equinox and a few days after the September one, depending on latitude. The equinox itself is defined by geometry, the Sun crossing the celestial equator, not by the length of daylight you measure with a clock. It is a neat example of a popular fact that is almost, but not quite, true.
Frequently asked questions
Is the June solstice the hottest day? No. The longest day brings the most sunlight, but land and sea take weeks to warm up, so the hottest days usually fall in July and August in the north. The solstice is a sunlight extreme, not a temperature extreme.
Why is Earth closest to the Sun in January? Because the orbit is elliptical and its closest point, perihelion, happens to fall in early January. Distance has a small effect on seasons compared with tilt, which is why January is winter in the north despite Earth being nearest the Sun.
Do other planets have equinoxes? Yes, any tilted planet has them. Mars, with a tilt similar to Earth’s, has equinoxes and solstices too, and its seasons last much longer because its year is 687 Earth days.
Use the calculator above to find the season changes for any year you care about: enter the year, read the four UTC dates, convert the one that matters to your time zone, and remember that the southern hemisphere reads the same table in reverse.
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