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.