The fall equinox marks the beginning of autumn in the Northern Hemisphere each September. Here are some answers to the most common questions about this.

When is it?

In 2026, the autumnal equinox happens at 8:05 p.m. EDT Tuesday, Sept. 22.

What happens at that exact time?

The sun's most direct rays will shine on the equator on Tuesday. That only happens one other time of the year, during the spring equinox in late March.

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So, if you happen to be along the equator, the sun will be directly overhead in the middle of the day Tuesday.

That means you'll see virtually no shadow cast on objects along the equator.

If you care for a more geeky definition and love geometry, NASA is happy to oblige:

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What does this moment mean?

It's the official start of astronomical fall in the Northern Hemisphere.

However, south of the equator, it’s their start of spring and it’s called their spring, or vernal equinox.

The beginning and end of the satellite loop below shows what the line marking the transition from day to night looks like over one year's time. That line known as the "terminator" is straight north-south on both the fall and spring equinoxes.

But didn't fall start already?

To meteorologists, fall began when September did.

Meteorologists group the seasons slightly differently than astronomers.

That's because average temperatures tend to fall in more neatly-packed three-month buckets, as meteorologist Rob Shackelford explained in a previous piece.

In many Northern Hemisphere locations, June through August are the warmest months, while December through February are the coldest. The three-month transition period in between those is known as "meteorological fall."

Are days and nights each 12 hours long on the equinox?

While the word equinox comes from the Latin for "equal night," that's not quite what happens on that day.

That's because the time of the equinox is calculated based on the center of the sun.

Daylight is the time between when the sun's top edge first noses above the horizon (sunrise) and when the last edge of the sun disappears below the horizon (sunset).

The atmosphere also refracts, or bends sunlight, which makes the sun appear higher in the sky than its actual position.

This means the day with exactly 12 hours of daylight, known as "equilux," happens a few days after the fall equinox in the Northern Hemisphere.