Questions and bridges

What Is an Equinox? A Short Answer

An equinox is the moment the Sun crosses the celestial equator, the projection of Earth’s equator onto the sky. It happens twice a year, around March 20 and September 22–23, when day and night are nearly equal. In the astrological tradition, the March equinox marks the start of Aries.

The short answer

An equinox is the instant the Sun’s center passes through the celestial equator. At that moment Earth’s axis is tilted neither toward nor away from the Sun. It occurs twice a year, in March and September, and the exact date and time shift slightly from year to year.

Why “equal” is only approximate

The word comes from Latin for “equal night”. In practice, daylight lasts a few minutes longer than night, because the Sun is a disc rather than a point and the atmosphere bends its light, lifting its image above the horizon.

Link to the seasons

Seasons come from the tilt of Earth’s axis, not from our distance from the Sun. The March equinox starts astronomical spring in the Northern Hemisphere and autumn in the Southern; the September equinox does the reverse. See the seasons and the solstice for more.

In the astrological tradition

The tropical zodiac begins at the March equinox point, which tradition calls 0° Aries. The September equinox corresponds to the start of Libra. This is a symbolic framework offered for reflection and inspiration; it does not claim that celestial bodies cause events. To explore the geometry behind it, read about the ecliptic and precession.

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