Solar equinox

A solar equinox is the moment in the year when the Sun appears directly above Earth’s equator, rather than to its north or south. On these dates, the Sun appears to rise exactly in the east and set exactly in the west, making the equinoxes important astronomical markers. They occur twice annually, around March and September, and correspond to the times when Earth’s rotation axis is neither tilted toward nor away from the Sun. In modern astronomy, equinoxes are defined by the Sun’s geocentric longitude reaching 0° or 180° on the ecliptic.
During an equinox, day and night are approximately equal in length across the planet, though not precisely equal due to factors such as atmospheric refraction, the Sun’s apparent diameter and the rapid seasonal change in daylight length. Historically, many cultures identified the equinox by observing the day when sunrise and sunset occur due east and due west.
In the Northern Hemisphere, the March event is commonly known as the spring or vernal equinox, and the September event as the autumnal equinox. In the Southern Hemisphere, the terminology is reversed. Modern hemisphere-neutral terms include northward equinox for March and southward equinox for September. These identify the direction in which the Sun appears to cross the celestial equator.

General Characteristics of Equinoxes on Earth

Centuries of observation revealed that the Sun’s rising point on the horizon shifts between extreme northern and southern positions and that the midpoint between these extremes occurs when day and night are nearly equal. The term equinox comes from the Latin aequus (equal) and nox (night). For many cultures, particularly in the northern hemisphere, the vernal equinox marked the beginning of spring and the start of the new year, as seen in the Assyrian, Hindu and Persian calendars. Ancient Greek calendars also anchored their new year either on an equinox or a solstice. The predictive capabilities of ancient devices such as the Antikythera mechanism included the ability to forecast equinoxes and solstices.
The equinoxes are the only times when the terminator—the boundary dividing day and night—is perpendicular to the equator, causing both hemispheres to be illuminated equally. This is also when the Sun rises at one pole and sets at the other, marking the transition between polar night and polar day. For a short interval of several days, both poles experience continuous daylight. The subsolar point—where the Sun is at zenith—lies directly on the equator and crosses it moving northward in March and southward in September.

Historical Adjustments to the Equinox Date

In 45 BC, Julius Caesar established 25 March as the date of the spring equinox in the Julian calendar. However, because the Julian year exceeded the tropical year by around 11 minutes, the equinox gradually drifted earlier over the centuries. By the First Council of Nicaea in AD 325, it fell around 21 March, and by the late sixteenth century it had shifted back to 11 March. To correct this drift, Pope Gregory XIII introduced the Gregorian calendar in 1582, reducing the number of leap years and restoring the equinox to a date close to 21 March. Minor variations still occur due to the structure of the leap-year cycle.

Modern Dates and Long-Term Patterns

Because the Gregorian calendar does not perfectly match Earth’s orbital period, the dates of the equinoxes shift slightly from year to year, repeating in a 400-year cycle. In the twenty-first century, the March equinox typically occurs between 19 and 21 March, and the September equinox between 21 and 23 September. These variations arise from leap years and subtle gravitational influences on Earth’s orbit.

Naming Conventions

Traditional names such as vernal equinox and autumnal equinox are widely used but can be misleading in the Southern Hemisphere, where the seasons are reversed. Terms like spring equinox and fall equinox are also ambiguous. Hence, month-specific names—March equinox and September equinox—have become common because they avoid hemispheric confusion. Hemisphere-neutral alternatives, northward and southward equinox, are sometimes used in astronomy and planetary science, describing the direction of the Sun’s apparent motion across the equator.
Astrological references such as “first point of Aries” and “first point of Libra” originate from historical constellations, though precession has shifted these points into Pisces and Virgo.

Length of Day and Night at the Equinox

On the equinox, the centre of the Sun spends nearly equal time above and below the horizon, resulting in almost equal day and night. However, day is typically slightly longer than night because:

• The Sun is a visible disc rather than a point.
• Atmospheric refraction lifts the Sun’s image above the horizon before its centre rises.
• The rate of change in daylight length is steep near the equinox.