The first day of fall has arrived. Here's how the equinox marks the changing of seasons.

ReutersA view of Earth taken by NASA’s Earth Polychromatic Imaging Camera.
  • This year’s fall or autumnal equinox happens Monday, September 23.
  • Earth’s rotation does not cause the fall equinox. Rather,equinoxes occur because the planet has a tilted axis.
  • Fall comes when the sun’s warming rays line up perpendicular to Earth’s axial tilt. During an equinox at Earth‘s equator, the sun appears almost directly overhead.
  • Visit Business Insider’s homepage for more stories.

Monday is this year’s fall equinox.

Also called the September or autumnal equinox, this astronomical event signals the arrival of fall, summer’s end, and a trend toward increasingly cold and dark days as winter comes.

At least, that’s the case for people who live in Earth’s northern hemisphere, which roughly 90% of all human beings call home. (Blame Earth’s shifting land masses for that fun fact.)

For those in the southern hemisphere, the milestone marks the official beginning of spring. The days down under are growing longer, the weather is warming, and sunlight is growing brighter as winter approaches.

Two factors drive these all-important seasonal shifts: Earth’s tilted axis and the planet’s orbit around the sun.

How the fall equinox works

The Earth orbits the sun once every 365 days and 6 hours. Our planet also rotates once per day around a tilted axis.

That tilt is about 23.5 degrees (for now), which means different parts of the world get bathed with various intensities of light over the course of a year. Meanwhile, the planet’s rotation keeps the heating even – it’s sort of like a 7,917-mile-wide rotisserie chicken made of rock and a little water.

The fall equinox occurs when the sun’s warming rays line up perpendicular to Earth’s axial tilt:

Earth during equinoxShayanne Gal/Business Insider

If you were to stand directly on the equator at the moment the equinox peaks – which came at precisely 3:50 a.m. ET Monday morning, according to the National Weather Service – the sun would appear more or less directly overhead. Your shadow would also be at its absolute minimum. The sun sets and rises roughly 12 hours apart on this day, too.

But this moment doesn’t last, since the Earth makes its way around the sun at a speed of roughly 66,600 mph.

Uneven seasons

Our planet’s orbit is elliptical and its centre of gravity slightly offset from the sun, so the time it takes to cycle through the seasons isn’t perfectly divvied up.

Read more: The speed of light is torturously slow, and these 3 simple animations by a scientist at NASA prove it

About 89 days and 19 hours after the fall equinox, the Earth will reach its winter solstice – when the most direct sunlight strikes the Southern Tropic (or Tropic of Capricorn). Another 89 days later, the spring or vernal equinox will occur.

Then it’s another 93 days and 18 hours to the summer solstice – when the most direct rays of the sun reach their northernmost latitude, called the Northern Tropic (or Tropic of Cancer) – and another 92 days and 16 hours to get back to the fall equinox.

Earth equinoxes and solstices graphicShayanne Gal/Business InsiderHow Earth, its axial tilt, and the sun work to create solstices, equinoxes, and seasons.

The animation below, from NASA’s Goddard Space Flight Centre, shows this seasonal progression.

It was created using geosynchronous satellite images taken over Africa; such satellites fly around Earth in a geosynchronous orbit, which means they move fast enough to hover above one spot on the planet. This creates a great opportunity to photograph Earth over the course of the year and see how the the angle of sun changes.

Take a look:

via Gfycat

The truth about the egg-balancing trick

While we’re talking about equinoxes and solstices – that whole business of only being able to balance an egg on-end during a solstice is a myth. You can balance an egg any time you please, thanks to very small pores in its shell.

Those pores create nearly invisible dimples in the shell upon which a (very, very) patient person can stand up the egg.

Don’t look for any gravitational interplay between Earth and the sun to help you out either; that’s far too weak to make a noticeable difference.

This is an updated version of a story that was originally published on March 20, 2019.

Business Insider Emails & Alerts

Site highlights each day to your inbox.

Follow Business Insider Australia on Facebook, Twitter, LinkedIn, and Instagram.