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The September Equinox

There are two equinoxes every year: one in September and one in March. In September, the Sun crosses the equator from north to south.

illustration of Earth's position in relation to the Sun's rays at the September equinox
illustration of Earth's position in relation to the Sun's rays at the September equinox

At the equinox, Earth’s axis is perpendicular to the Sun’s rays as the Sun is directly above the equator.

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Sun Crosses Celestial Equator

The September equinox is the moment the Sun crosses the celestial equator—an imaginary line in the sky above Earth’s equator—from north to south. This happens on September 22, 23, or 24 in most years.

Equinox Local Time & Date

In Washington DC, District of Columbia, USA: Wednesday, September 22, 2021 at 3:21 pm EDT (Change location)
This corresponds to Wednesday, September 22, 2021 at 7:21 pm UTC.

Why Does the Sun Move from North to South?

During the course of a year, the subsolar point—the spot on the Earth's surface directly beneath the Sun—slowly moves along a north-south axis. Having reached its northernmost point at the June solstice, it starts moving southward until it crosses the equator on the day of the September equinox. The December solstice marks the southernmost point of its journey.

So, why does the subsolar point move north and south? This happens because the Earth’s axis is tilted at an angle of about 23.4° in relation to the ecliptic, an imaginary plane created by Earth’s path around the Sun. In June, the Northern Hemisphere is tilted toward the Sun, and the subsolar point is north of the equator. As the Earth travels toward the opposite side of its orbit, which it reaches in December, the Southern Hemisphere gradually receives more sunlight, and the subsolar point travels south.

Equinox and solstice illustration.
Equinox and solstice illustration.

Our Earth is tilted as it orbits the Sun, which is why equinoxes and solstices happen.

Why Is It Called “Equinox?”

On the days of the equinoxes, the Earth’s axis is perpendicular to the Sun’s rays, meaning that all regions on Earth receive about the same number of hours of sunlight. In other words, night and day are, in principle, the same length all over the world. This is the reason it’s called an “equinox,” derived from Latin, meaning “equal night.”

However, this isn’t entirely true. In reality, equinox days don’t have exactly 12 hours of daylight.

The Equinoxes and the Seasons

The March and September equinoxes mark the beginning of the spring and autumn seasons on Earth, according to one definition. The equinox in September is the start of fall in the Northern Hemisphere and the beginning of spring south of the equator.

Topics: Astronomy, Sun, Seasons, September, Equinox, Earth