Equinoxes & Solstices: What Actually Happens When the Seasons Turn

Every world calendar carries four astronomical dates a year, and nearly everything casually said about them is slightly wrong. The solstice isn't the day the Sun "stands still" for a day; the equinox doesn't split day and night exactly in half; and none of it has anything to do with how close we are to the Sun. What's really happening is cleaner, stranger and easy to see once you watch the geometry instead of the calendar.

One instant, whole planet

Earth orbits the Sun with its axis tilted 23.4° and pointing — over a human lifetime — at the same patch of sky. That means that as we travel around our orbit, the Sun appears to slide along a great circle through the constellations, the ecliptic, completing the lap in one year. The four seasonal markers are nothing more than the Sun's crossing of four evenly spaced points on that circle: longitude 0° and 180° (the equinoxes) and 90° and 270° (the solstices).

A crossing is an instant. It does not happen "on the 21st" any more than midnight happens all evening. When the Sun's apparent longitude passes 90°, that is the June solstice — one moment, shared by the entire planet, that each time zone merely stamps with a different local label. That's why the seasons pages on this site lead with a single UTC minute and then translate it city by city: the translation is the only part that varies.

What the instant looks like on the ground

At the equinoxes the Sun stands directly over the equator, and the day/night line — the terminator — momentarily runs pole to pole. Every latitude is sliced into a day and a night of nearly equal length; at the poles themselves, the Sun skims the horizon in the half-year handover between polar day and polar night.

At the solstices the Sun stands over one of the tropics — 23.4°N in June, 23.4°S in December, the tilt angle made visible on the map. Those two latitude lines are defined by this: the tropics are simply as far as the overhead Sun ever gets. North of the equator, the June instant falls inside the longest day of the year and the December instant inside the shortest; south of it, precisely the reverse. One instant, two opposite seasons — which is why these pages say "June solstice," never "summer solstice," in their URLs. Summer is a hemisphere's opinion.

The equinox's little lie

"Equal night" — that's what aequinoctium means — is an approximation the atmosphere refuses to honour. Air bends light, so when you watch a sunrise the Sun is geometrically still below the horizon, lifted into view by refraction; and we time sunrise and sunset from the Sun's upper edge, not its centre. Both effects gift extra minutes to the day. On the equinox itself, day beats night by roughly 6–10 minutes at mid-latitudes and more near the poles. The genuinely equal date — the equilux — arrives a few days before the March equinox in the northern hemisphere and a few days after it in the southern (and swaps for September).

The solstices carry a stranger small print: the year's earliest sunset does not fall on the shortest day. Clock time and Sun time drift apart through the year — the equation of time — and near the December solstice that drift pushes solar noon later by about half a minute a day. Sunset therefore bottoms out around two weeks before the solstice and sunrise peaks around two weeks after it, while the day in between is shortest on the instant itself. If you've ever felt the evenings brightening before Christmas while the mornings kept getting darker, you weren't imagining it.

The distance myth, retired by the calendar itself

Here is the cleanest evidence that distance doesn't drive the seasons: Earth is closest to the Sun in early January and farthest in early July — perihelion in northern midwinter, aphelion in northern midsummer, the exact opposite of what the myth needs. The orbit's ellipse only swings our distance by about 3%. Tilt, by contrast, changes both how high the Sun climbs and how many hours it spends above your horizon; at 50°N the daily solar energy arriving in June is several times December's. Distance whispers; tilt decides. (The apsides do leave one fingerprint: southern seasons are slightly more intense and slightly shorter, because Earth moves fastest near January's perihelion.)

Why the minute is knowable — and where its edge is

The instants on the seasons pages are the published minutes of the U.S. Naval Observatory's Astronomical Applications Department, and this site independently recomputes each one as a cross-check (they agree within about a minute and a half across 104 events, and the 2026 set was further verified against JPL's DE441 ephemeris to half a minute). One honest caveat travels with any table like this: a decade or more out, converting the physics to a clock reading depends on predictions of Earth's slightly irregular rotation, so the minute for, say, 2039 can still legitimately shift a little as the years arrive. The geometry is exact; our planet's timekeeping is the wobbly part.

The next turning of the seasons is always counting down on the equinoxes & solstices page — the exact instant, your city's clock, and how long that day will be where you stand.