An equinox is the moment when the sun crosses the celestial equator — the imaginary line directly above Earth's equator. At this moment, the sun's rays strike the equator perpendicularly, and every location on Earth receives approximately the same amount of daylight. There are two equinoxes each year:
| Equinox | Northern Hemisphere | Southern Hemisphere | Approximate date |
|---|---|---|---|
| March equinox | Spring (vernal) equinox | Autumn (autumnal) equinox | March 19–21 |
| September equinox | Autumn (autumnal) equinox | Spring (vernal) equinox | September 22–23 |
As with the solstices, the exact date shifts slightly from year to year due to the difference between the calendar year (365 days) and the solar year (365.25 days).
The name equinox comes from Latin: aequus (equal) + nox (night). But in practice, day and night are not exactly equal at the equinox — daylight is actually a few minutes longer. There are two reasons:
First, atmospheric refraction: Earth's atmosphere bends sunlight over the horizon, making the sun visible for a few minutes before it geometrically rises and after it geometrically sets. This adds roughly 5–7 minutes of extra daylight per day.
Second, the definition of sunrise and sunset: sunrise is defined as the moment the top edge of the sun first appears, and sunset as the moment the top edge disappears. Since the sun has a measurable diameter (about 0.5°), this adds a further 1–2 minutes at each end. The combination means that at the equinox, most locations experience about 12 hours and 6–8 minutes of daylight rather than exactly 12 hours.
The date when day length is exactly 12 hours — the true equilux — is typically 3–4 days before the spring equinox and 3–4 days after the autumn equinox.
The equinox is the time of year when the length of the day is changing most rapidly. At 51°N (London), day length changes by about 3–4 minutes per day around the equinoxes — the fastest rate of the year. By contrast, around the solstices, the change slows almost to zero: right at midsummer and midwinter, the day length barely changes for several weeks.
This accelerating rate of change near the equinox is why spring and autumn can feel so dynamic. The evenings grow noticeably longer from one week to the next — and in autumn, the light disappears with surprising speed.
Astronomically, the equinoxes mark the boundaries of spring and autumn. But meteorologically — based on temperature patterns — the seasons are usually defined differently. Meteorological spring in the Northern Hemisphere begins March 1 (regardless of the equinox), and autumn begins September 1. The two systems are about three weeks out of phase.
Many ancient cultures built monuments aligned with the equinox sunrise. Angkor Wat in Cambodia, for example, aligns precisely with the spring equinox sunrise. Stonehenge in England, famous for its summer solstice alignment, also has equinox-related features.
The equinox is a global event — but what it signals differs by hemisphere. For the Northern Hemisphere, the March equinox means spring: days are getting longer, temperatures will rise, and the longest day lies ahead. For the Southern Hemisphere, the same date marks the arrival of autumn: days are shortening, and the shortest day lies ahead.
Near the equator, the equinox has almost no practical significance for day length, since daylight is close to 12 hours year-round regardless.
Use the tool on the homepage to see exactly how long the day is at your location today — and how it will change day by day from here.
A few days after the equinox. Because the atmosphere bends light, and because sunrise is measured from the moment the upper edge of the sun's disc appears, the day on the equinox itself still runs slightly longer than the night — about 12 hours and 9 minutes in London. The date when day and night really are equal is called the equilux, and in the UK it falls around 18 March and 26 September. The further north you go, the wider the gap between equinox and equilux.
Around the equinoxes, day length shifts faster than at any other point in the year. London gains about 4 minutes of daylight per day around the spring equinox, Edinburgh nearly 5, and Oslo more than 5.5. That is why March and September feel like the months when the light genuinely moves, while June and December can pass without a noticeable difference from one week to the next.
The reason is geometric. At the equinox the Sun crosses the equator at the steepest angle it reaches all year, so its path across the sky shifts most from day to day. At the solstices that motion reverses and the change falls close to zero.
The equinoxes get less attention than the solstices, which is arguably the wrong way round. A solstice marks an extreme, and extremes are where change is slowest — a fortnight either side of midsummer can pass with a few minutes of difference in total. The equinoxes sit at the steepest part of the annual curve, where the day is gaining or losing three to four minutes every twenty-four hours at British and Danish latitudes.
That is around half an hour a week. It is the fastest daylight ever changes, and it is fast enough to notice from one weekend to the next — which is why March feels transformative and late June does not, despite June having far more light in absolute terms. The whole shape is visible on the year chart.
The equinox is defined astronomically: the instant the sun's centre crosses the celestial equator. On that date the geometric day and night are equal everywhere. But as the sunrise and sunset times on this page show, the actual observed day comes out longer than twelve hours — by around nine minutes at the latitude of Britain and Denmark, and more further north.
The date on which observed daylight really does equal twelve hours has its own name, the equilux, and it falls a few days before the March equinox and a few days after the September one. How far off it lands depends on latitude, but not in the direction most people expect. The excess at the equinox itself does grow northward — 12 h 06 min at 10°N, 12 h 08 min in Copenhagen, 12 h 12 min in Tromsø — yet the gap between the two dates shrinks: about twelve days at 10°N, four at 30°N, three at 40°N and two from 50°N upwards. Further north the day is also changing faster, and the two effects very nearly cancel. Very close to the equator it does not occur at all, because the day never drops to twelve hours.
Both causes are covered above and both are about the definition of sunrise rather than about the astronomy: we measure from the sun's upper edge rather than its centre, and the atmosphere refracts the light so the sun is visible while still geometrically below the horizon.
The equinoxes are the only dates on which every place on Earth gets roughly the same amount of daylight. On every other date, latitude divides us: the northern and southern hemispheres get opposite deals, and the poles get extremes that nowhere else experiences.
At the equinox the day–night boundary passes exactly through both poles, so every line of latitude is cut in half. Even at the North Pole the sun is on the horizon — rising there in March for a six-month day, and setting in September for a six-month night. There is nowhere on the planet at that moment where the day is dramatically longer or shorter than anywhere else, which does not happen on any other date of the year. The 3D globe shows this clearly if you set the date to late March and spin it from pole to pole.
The spring equinox falls on 20 or 21 March and the autumn equinox on 22 or 23 September. The dates shift slightly from year to year because the year is not exactly 365 days long.
No. On the equinox itself the day is slightly longer than the night — about 12 hours and 9 minutes in London. This is because the atmosphere bends light, and because sunrise is measured from the moment the upper edge of the sun's disc appears.
The equilux is the day when day and night really are equal in length. In the UK it falls around 18 March and 26 September, a few days offset from the equinox itself.