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Autumnal Equinox – Fall Equinox

Equinoxes are opposite on either side of the equator, so the autumnal (fall) equinox in the Northern Hemisphere is the spring (vernal) equinox in the Southern Hemisphere and vice versa.

Equinox and solstice illustration.

Equinoxes and solstices happen twice a year.

Equinoxes and solstices mark the start of astronomical seasons. The equinoxes start spring and fall, while solstices start astronomical summer and winter.

Not Entirely Equal Day & Night

On the two equinoxes every year the Sun shines directly on the Equator and the length of day and night is nearly equal – but not exactly.

The September equinox marks the moment the Sun crosses the celestial equator – the imaginary line in the sky above the Earth’s equator – from north to south and vice versa in March.

Autumnal Equinox – Northern Hemisphere

(USA, Central America, Canada, Europe, Asia, northern Africa)

First Day of Fall?

In the Northern Hemisphere, the fall equinox marks the first day of fall (autumn) in what we call astronomical seasons. There's also another, more common definition of when the seasons start, namely meteorological definitions, which are based on average temperatures rather that astronomical events.

Autumnal Equinox – Southern Hemisphere

(Australia, New Zealand, South America, southern Africa)

Topics: Astronomy, Seasons, September, March, Equinox

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