Equinox dates — spring & autumn

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Related astronomy & calendar pages

FAQ

Frequently asked questions

What is an equinox?

An equinox is the moment when the Sun crosses Earth's celestial equator — when its apparent geocentric longitude is exactly 0° (March equinox) or 180° (September equinox). On these two days, every point on Earth's surface receives roughly equal hours of daylight and darkness, and the terminator (the line between day and night) runs from pole to pole.

When is the next equinox?

The next equinox is shown in the countdown card at the top of this page. The March equinox falls between March 19 and March 21; the September equinox falls between September 21 and September 24. The exact UTC instant shifts about 5 hours 49 minutes later each common year and resets on leap years.

Are day and night really equal at the equinox?

Almost, but not exactly. The word "equinox" comes from Latin *aequus* (equal) + *nox* (night), and the moment of equinox does see the geometric center of the Sun above the horizon for 12 hours everywhere. But two effects extend daylight slightly: the Sun is a disc (not a point), so we see it before its center rises and after its center sets, and atmospheric refraction lifts the apparent position of the Sun by about 0.5° at the horizon. The actual day of "equilux" — equal sunrise-to-sunset length — falls a few days before the March equinox and a few days after the September equinox at most latitudes.

What's the difference between vernal and autumnal equinox?

The naming is hemisphere-dependent. In the Northern Hemisphere the March equinox is called the *vernal* (spring) equinox and the September equinox is the *autumnal* equinox. In the Southern Hemisphere the labels are reversed: March is autumnal, September is vernal. To avoid confusion, astronomers and almanacs use the neutral names "March equinox" and "September equinox."

Why does Easter depend on the equinox?

By the rule established at the First Council of Nicaea in 325 CE, Easter Sunday is the first Sunday after the first full moon on or after the March equinox. The Catholic and Protestant churches use a fixed ecclesiastical equinox (always March 21) and a tabular full moon, so their Easter is purely arithmetical, not astronomical. The Eastern Orthodox churches use the Julian calendar for the rule, which is why Orthodox Easter is usually one to five weeks later than Western Easter.

What is the precession of the equinoxes?

Earth's rotation axis wobbles slowly like a spinning top, completing one full circle every roughly 25,772 years. Because the equinoxes are defined by the intersection of the equator and the ecliptic, they slide westward along the zodiac at about 50 arcseconds per year — a phenomenon Hipparchus discovered around 130 BCE. This is why the astrological "signs" no longer line up with the constellations: the system was anchored 2,000 years ago.

How accurate are the equinox times on this page?

The values are computed using Meeus's algorithm (Astronomical Algorithms, 2nd ed., Chapter 27) and are accurate to about 30 seconds for years 1000 through 3000. They match published USNO and IMCCE almanacs to within a minute, and are given in UTC. Convert to your local timezone with our world clock or time-zone converter.