Solstice and Equinox Dates Explained: Why They Move

Ask most people when the longest day of the year is and they'll say 21 June. Ask about the shortest and they'll say 21 December. They're often right. But not always, and the reasons it shifts are more interesting than you'd think.

For a start, a solstice isn't really a day at all. It's a moment.

What a solstice and an equinox actually are

The Earth is tilted on its axis by about 23.4 degrees, and it keeps that tilt pointing the same way as it travels around the sun. For half the year the northern hemisphere leans towards the sun and gets longer days. For the other half it leans away.

The solstices are the two moments when that lean is at its most extreme. In June, the North Pole is tipped as far towards the sun as it gets, which gives the northern hemisphere its longest day. In December it's tipped furthest away, giving the shortest.

The equinoxes are the two moments in between, in March and September, when the sun sits directly above the equator and neither hemisphere is leaning towards it.

Because these are points in the Earth's orbit, each one happens at a specific instant, worked out to the minute. The winter solstice in 2026 is at 14:50 UTC on 21 December. It's that precise moment our Winter Solstice 2026 countdown runs to.

Why the date moves

There are two reasons the solstice can land on the 20th, 21st, 22nd or even the 23rd.

The first is the length of the year. The Earth takes about 365 days and 6 hours to go round the seasons, but the calendar year is 365 days. So each year the solstice happens around six hours later on the clock than the year before. After three years it has crept almost a day later. Then a leap year adds 29 February and pulls it back again.

That's why solstices and equinoxes wobble around in a four-year pattern. Over the long run, the 100-year leap year exception nudges things too, which is why the dates drift slightly earlier across a century.

The second reason is time zones. Because the solstice is a single moment, the date it falls on depends on where you are. Take the 2027 winter solstice. It happens at 02:42 UTC on 22 December. In London that's 2:42am on the 22nd. In New York it's 9:42pm on the 21st. Same moment, different date.

The usual date ranges

Over the current century, give or take the odd edge case, you can expect:

March equinox: 19, 20 or 21 March, most often the 20th.June solstice: 20, 21 or 22 June, most often the 21st.September equinox: 21 to 24 September, most often the 22nd or 23rd.December solstice: 20 to 23 December, most often the 21st or 22nd.

In 2027, measured in UTC, the March equinox is on 20 March at 20:25, the June solstice is on 21 June at 14:11, the September equinox is on 23 September at 06:02, and the December solstice is on 22 December at 02:42. You'll find live countdowns to each one in our Seasons section.

When does spring actually start?

This is where people tend to argue, because there are two answers and both are official in their own way.

Astronomical seasons use the equinoxes and solstices. Spring starts at the March equinox, summer at the June solstice, autumn at the September equinox and winter at the December solstice. The dates move a little from year to year.

Meteorological seasons use whole calendar months. Spring is March, April and May. Summer is June, July and August, and so on. Weather services like the Met Office use these because they're tidy. Each season is the same length every year, which makes it much easier to compare one year's weather with another.

So if someone tells you it's the first day of spring on 1 March, and someone else says it's 20 March, they're both right. They're just using different rulers.

Is day and night really equal at the equinox?

Not quite, which surprises a lot of people. "Equinox" means "equal night", but on the day itself most places get slightly more than 12 hours of daylight.

There are two reasons. Sunrise and sunset are measured from when the top edge of the sun appears and disappears, not its centre, which adds a little at each end. And the atmosphere bends sunlight, so we can see the sun for a few minutes when it's technically just below the horizon. The day when daylight and darkness are actually closest to equal comes a few days before the spring equinox and a few days after the autumn one.

The shortest day isn't the darkest evening

Here's another oddity. If you live in the UK, you might notice that the evenings start getting lighter before Christmas, even though the shortest day hasn't arrived yet or has only just passed.

That's because the solstice is about the total amount of daylight, not when the sun sets. Thanks to the shape of the Earth's orbit and its tilt, the earliest sunsets come in mid-December, before the solstice, while the latest sunrises come around the turn of the year. The day as a whole is shortest on the solstice, but the darkest evenings and darkest mornings fall either side of it.

Why people still mark them

Long before anyone could calculate the moment to the minute, people tracked these turning points carefully. Stonehenge lines up with the midsummer sunrise and the midwinter sunset, and thousands of people still gather there for both. Midsummer is a major celebration in Sweden and Finland, and many winter festivals around the world fall close to the December solstice, when the days finally start to lengthen.

It's a nice thing to know when you're in the depths of December. However dark it gets, once that moment passes, every day is a little longer than the one before.