When is equinox? Fall officially begins in the Northern Hemisphere tonight, Tuesday, Sept. 22, with the arrival of the September equinox.

Although meteorologists consider fall to begin on Sept. 1, astronomical autumn starts at the equinox. This year, it occurs at 8:05 p.m. EDT on Tuesday, Sept. 22, according to Timeanddate.com , when the sun crosses the celestial equator, and Earth’s axis is tilted side-on to the sun.

At this specific moment, neither of Earth’s hemispheres is tilted toward or away from the sun, meaning both receive almost equal amounts of sunlight.

This celestial geometry marks the change of seasons — from summer to fall in the Northern Hemisphere and from winter to spring in the Southern Hemisphere. The word “equinox” comes from Latin words meaning “equal night” — though day and night are not exactly 12 hours long on the equinox.

What Time Is The Fall Equinox?

The equinox is not an all-day event. It occurs at one precise moment worldwide, so the local time depends on where you are.

In North America, the September equinox occurs on Tuesday, Sept. 22 at:

  • Eastern Time: 8:05 p.m. EDT
  • Central Time: 7:05 p.m. CDT
  • Mountain Time: 6:05 p.m. MDT
  • Pacific Time: 5:05 p.m. PDT
  • Alaska Time: 4:05 p.m. AKDT
  • Hawaii: 2:05 p.m. HST

What Causes The Fall Equinox?

The equinoxes and solstices are consequences of Earth’s axis being tilted by about 23.5 degrees relative to its orbit around the sun. As Earth travels around the sun, that tilt causes the amount and intensity of sunlight reaching each hemisphere to change, creating the seasons.

After tonight’s equinox, the Northern Hemisphere will continue tilting progressively away from the sun, bringing shorter days and longer nights until the December solstice — the longest night of the year north of the equator.

There’s no sudden change in the sky at the instant of the equinox, but there is an easy way to observe its effects.

The key is to watch where the sun rises and sets. Around the equinox, the sun rises almost exactly due east and sets almost exactly due west. That’s because the sun is crossing the celestial equator — an imaginary projection of Earth’s equator into the sky.

For skywatchers in the Northern Hemisphere, the September equinox also signals something else extremely valuable — the nights will now be longer than the days, with increasingly more time for stargazing as fall progresses toward winter.

Wishing you clear skies and wide eyes.