This Week’s Equal Night, Equal Light: What Is Equinox, How Does It Work And Why Does It Matter To You?

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At exactly 13:31 p.m. UTC on Tuesday, September 22, 2020 the Sun will cross the celestial equator.

So what?

The “southward equinox”—known in the northern hemisphere as the fall or autumnal equinox—is an important “quarter day” in the Earth’s journey around the Sun.

You can’t specifically see equinox occur and yet it has a powerful and profound effect on our lives.

Here’s everything you need to know about Tuesday’s equinox:

What is September’s equinox?

An equinox marks the day when the midday Sun crosses the equator. They occur twice per year, in late March and lat September. It’s a global event occurring simultaneously for everyone on the planet.

Why is September’s equinox important?

It heralds the beginning of astronomical fall or autumn in the northern hemisphere and astronomical spring in the southern hemisphere.

What is a solstice?

This is all a story about Earth’s axis, which is titled by 23.5º, and to appreciate it fully you need to know about solstices as well as equinoxes. A solstice occurs when Earth’s axis is titled such that one of the planet’s hemispheres receives the maximum sunlight and the other its minimum. At June’s solstice the midday Sun appears to be at its northerly point, directly overhead the Tropic of Cancer, so days are long in the northern hemisphere, and short in the southern hemisphere. At December’s solstice, the Sun is at its most southerly point south of the equator above the Tropic of Capricorn, so the opposite occurs.

Why do we have equinoxes and solstices?

Earth takes 365 days to orbit the Sun. The tilt means the proportion of sunlight that falls on any part of the planet is constantly changing. That means we have way markers—seasons. It’s about light and heat, who gets it, and when. Though the actual difference in temperature is down to many other factors (there’s a lot more ocean in the southern hemisphere, for example), the critical factor is the height of the Sun in the sky. It’s that which determines its intensity and the length of the day.

So the season is divided into two solstices and two equinoxes, with cross-quarter days in between them. This is what’s coming up:

  • September 22, 2020: September equinox (southward)
  • October 31, 2020: Halloween and All Souls’ Day.
  • December 21, 2020: December solstice
  • February 2, 2021: Groundhog Day and Candlemas, a holy day in the Christian calendar.
  • March 20, 2021: March equinox (northward)
  • May 1, 2021: May Day, a traditional spring holiday in the northern hemisphere.
  • June 20, 2021: June solstice
  • August 1, 2021: Lammas, a traditional pagan celebration of the first harvest of the season.

Wishing you clear skies and wide eyes.

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