Equinox and solstice dates

Ask for a year and you get the four instants to the minute, computed rather than looked up — along with the length of each season, the cross-quarter days, and the dates the Earth is nearest and farthest from the Sun. Name a city as well and the same year comes back on its clock, with its longest and shortest days. Free, no account, nothing stored.

What it does, and why it is here

An equinox is not a day. It is the instant the Sun crosses the celestial equator — a moment, with an hour and a minute, the same one for the whole planet. A solstice is the instant it reaches its extreme. Which calendar day that lands on depends on where you are: an equinox at 03:15 UTC is still the previous evening in Chicago, which is why published dates disagree by one.

And the instants move. The year is 365.2422 days, so each event arrives about six hours later than the last, then jumps back a day when February takes a twenty-ninth. Over a century the pattern drifts far enough that no printed table stays right — the March equinox has been on the 19th, the 20th and the 21st within living memory.

So the tool solves for the instant rather than reading it from a list, and gives it in UTC, which is the only form that is unambiguous. The optional city then translates the whole year onto a local clock, and adds the things a place has that a planet does not: your longest day, your earliest sunset, your latest sunrise.

What you fill in

• Year — any calendar year the calculation covers, 1550 to 2650.
• From a place, optional — a city. Pick one and the instants come on its clock, with its longest and shortest days, its earliest sunset and latest sunrise, and the length of the day across the whole year.

What you get back

• The four instants, to the minute, in UTC — March equinox, June solstice, September equinox, December solstice.
• Length of the seasons — and they are not equal. The Earth moves faster near the Sun, so the northern winter is several days shorter than the northern summer.
• Perihelion and aphelion — the dates the Earth is nearest and farthest from the Sun, with the distance. Early January and early July, which is the direct refutation of the idea that the seasons are made by distance: they are made by the tilt.
• Cross-quarter days — the Sun halfway between an equinox and a solstice, at longitudes 315°, 45°, 135° and 225°. These are the midpoints the old calendars kept as Imbolc, Beltane, Lughnasadh and Samhain, and they are computed here rather than approximated to the first of the month.
• The year from here, when you named a city — the four instants on the local clock, the daylight on each of those four days, and the year’s longest and shortest days, earliest sunset and latest sunrise. Those last two are not the solstice, and finding out why is most of the reason people come looking.
• The length of the day through the year, drawn — a point every five days, twelve hours marked, the equinoxes and solstices as verticals, and the longest and shortest days as dots.

What it does not do

It is not a seasons calendar in the meteorological sense. Meteorologists start their seasons on the first of March, June, September and December, for whole-month statistics; astronomical seasons start at these instants. Neither is wrong — they answer different questions.

And it does not do the whole sky: this is the Sun’s year. Eclipses, moon phases and planetary events are separate calculations.

Why the dates move, and why the extremes do not line up

Questions

Is it free? Do I need an account?

Free, no account, no limit.

Which years can I ask for?

This year and next are served without an account; the calculation itself covers 1550 to 2650.

Why are the times given in UTC?

Because the instant is one moment for the entire planet, and UTC is the only way to state it without ambiguity. Name a city and you get it on that city’s clock as well.

Are these the first days of the seasons?

Astronomically, yes. Meteorologically, no — that convention starts the seasons on the first of March, June, September and December.

What are the cross-quarter days?

The midpoints between an equinox and a solstice, computed from the Sun’s longitude. The old European calendars kept them as Imbolc, Beltane, Lughnasadh and Samhain, and their modern descendants sit near them.

Why is the earliest sunset not on the shortest day?

Because solar noon drifts across the year by up to a quarter of an hour — the equation of time. Sunset reaches its earliest a week or two before the December solstice, and sunrise its latest a week or two after.

Is the September equinox always on the 22nd or 23rd?

Usually, and not always. Ask for the year rather than assuming; that is what the tool is for.

Does it work in the southern hemisphere?

The instants are the same everywhere — only the names of the seasons swap. Name a southern city and the local view flips accordingly.

Sources and further reading

• Equinox and solstice — the definitions the instants are solved for.
• Equation of time — why the earliest sunset misses the solstice.
• Perihelion and aphelion — the distance that does not make the seasons.