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The September Equinox

There are 2 equinoxes every year – in September and March – when the sun shines directly on the equator and the length of day and night is nearly equal.

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

Earth's position at the September equinox.

Illustration of Earth's position in relation to the Sun's rays at the September equinox (Illustration is not to scale.)

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Opposite Sides & Seasons

Seasons are opposite on either side of the Equator, so the equinox in September is also known as the autumnal (fall) equinox in the Northern Hemisphere, and is considered the first day of fall.

In the Southern Hemisphere, it is known as the vernal (spring) equinox, and marks the first day of spring.

Equinox Local Time & Date

September Equinox in Washington DC, District of Columbia, USA is on
Friday, September 22, 2017 at 4:02 pm EDT (Change city)

September Equinox in Universal Coordinated Time is on
Friday, September 22, 2017 at 20:02 UTC

Sun Crosses Celestial Equator

The September equinox occurs the moment the Sun crosses the celestial equator – the imaginary line in the sky above Earth’s Equator – from north to south. This happens either on September 22, 23, or 24 every year.

10 Facts About the First Day of Fall

Earth's Axial Tilt

Illustration showing Earth's position in relation to the Sun at the equinoxes and solstices.
Earth orbits the Sun at a slant.
Earth orbits the Sun at a slant, which is why equinoxes and solstices happen.
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Earth's axis is tilted at an angle of about 23.4° in relation to the ecliptic, the imaginary plane created by Earth's path around the Sun.

On any other day of the year, either the southern hemisphere or the Northern Hemisphere tilts a little towards the Sun. But on the 2 equinoxes, the tilt of Earth's axis is perpendicular to the Sun's rays, like the illustrations show.

Why "Equinox"?

On the equinox, night and day are nearly exactly the same length – 12 hours – all over the world. This is the reason it's called an "equinox", derived from Latin, meaning "equal night". However, even if this is widely accepted, it isn't entirely true. In reality

equinoxes don't have exactly 12 hours of daylight

Traditions and Folklore

In the Northern Hemisphere, the September equinox marks the start of fall (autumn). Many cultures and religions celebrate

holidays and festivals around the September equinox.

The Snake of Sunlight

Illustration image
The snake of sunlight at Chichen Itza, Mexico.
The snake of sunlight on the stairs of the main pyramid at Chichen Itza, Mexico.
©iStockphoto.com/CostinT

A famous ancient equinox celebration was the Mayan sacrificial ritual by the main pyramid at Chichen Itza, Mexico.

The pyramid, known as El Castillo, has 4 staircases running from the top to the bottom of the pyramid's faces, notorious for the bloody human sacrifices that used to take place here. The staircases are built at a carefully calculated angle which makes it look like an enormous snake of sunlight slithers down the stairs on the day of the equinox.

Topics: Astronomy, Sun, Seasons, September, Equinox

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Vernal & Autumnal Equinox

  1. Vernal (Spring) Equinox
  2. Autumnal (Fall) Equinox
  3. March Equinox
  4. 10 Facts: March Equinox
  5. March Equinox Celebrations
  6. September Equinox
  7. 10 Facts: September Equinox
  8. September Equinox Celebrations
  9. Nearly Equal Night & Day

Equinox & Solstice Worldwide

Sunrise & Sunset Times

The Science of Seasons

  1. What Causes Seasons?
  2. Earth's Axis Is Tilted
  3. Meteorological vs. Astronomical Seasons
  4. What Is a Solar Analemma?

Look Up Seasons

Winter & Summer Solstices

  1. What is the June Solstice?
  2. Facts about June Solstice
  3. What is the December Solstice?
  4. Facts about December Solstice
  5. When is the Summer Solstice?
  6. When is the Winter Solstice?
  7. June Solstice Celebrations
  8. December Solstice Celebrations

Equinox & Solstice Worldwide

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