What Is an Equinox? The Two Days the Sun Crosses the Equator

An equinox is a moment, not a day: the instant the sun’s center crosses Earth’s celestial equator, moving north in March and south in September. Around 20 March and 22–23 September each year, Earth’s tilted axis points neither toward nor away from the sun, so the terminator — the line dividing day from night — runs straight through both poles, and every latitude on the planet receives close to twelve hours of daylight.

The name means “equal night,” but the equality is slightly oversold. Sunrise is timed from the sun’s upper edge, not its center, and Earth’s atmosphere bends sunlight around the horizon, letting us see the sun before it geometrically rises. Both effects add daylight, so the true 12-hour split — the “equilux” — actually falls a few days before the March equinox and after the September one, with the exact date depending on latitude.

The equinoxes anchor the seasons and the calendar. The March equinox defines the start of astronomical spring in the north (autumn in the south), and it’s the hinge for major cultural dates: Nowruz, the Persian New Year, begins at the exact equinox moment; Easter is set as the first Sunday after the first full moon on or after it; Japan observes both equinoxes as public holidays; and monuments from Chichén Itzá to Angkor Wat were built to stage precise equinox light shows.

Two elegant facts hold true only near these dates. First, the sun rises almost exactly due east and sets due west for every observer on Earth — the year’s best free compass calibration. Second, at the equator the sun passes through the very zenith at noon, casting no shadow from a vertical pole.

The equinoxes also drift over millennia: Earth’s axis wobbles like a slowing top over a 26,000-year cycle called precession, sliding the equinox points slowly through the zodiac — the reason astrological signs no longer match the constellations behind the sun.

Both upcoming equinoxes appear with exact dates in the holidays and astronomy list at itsdately.com, alongside the solstices that complete the year’s four turning points.

The dates ripple through cultures in quieter ways too. Japan pauses twice a year for Higan, a week of grave-visiting centered on each equinox, when tradition holds the boundary between worlds is thinnest. Space engineers mark “eclipse seasons”: near the equinoxes, geostationary satellites pass daily through Earth’s shadow and must run on batteries, while satellite TV viewers experience brief “sun outage” interruptions as the sun lines up directly behind broadcast satellites. Even auroras cooperate — geomagnetic activity statistically peaks near the equinoxes, making them prime season for northern lights.

The word itself has traveled far beyond astronomy: languages worldwide borrowed the Latin, and the dates anchor civic life in unexpected places — the equinoxes mark the official start of spring and autumn on most weather services’ “astronomical seasons,” even as meteorologists prefer tidy whole months. The gap between the two definitions produces a perennial newsroom argument every March and September about when the season “really” begins; both camps are right, measuring different things — the sky’s geometry versus the year’s temperature rhythm.

Related on itsdately: Daylight Hours by Latitude: Why Summer Days Are Longer the Farther North You Go · How Long Is a Day on Other Planets? · Why Are There 60 Seconds in a Minute? Blame Babylon

Further reading: U.S. Naval Observatory Astronomical Applications For readers asking “What Is an Equinox? The Two Days the Sun Crosses the Equator”, the answer comes down to the specific factors covered above.