The first day of spring is finally here — for about 90% of us. Here's why equinoxes mark the changing of seasons.

NASA/FlickrA view of Earth taken by Apollo 11 astronauts on July 20, 1969.
  • This year’s spring or vernal equinox will happen on Wednesday, March 20.
  • Earth’s rotation does not cause the spring equinox. Rather,equinoxes occur because the planet has a tilted axis.
  • Spring 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.

The year’s spring equinox, also called the March or vernal equinox, falls on Wednesday at precisely 5:58 p.m. ET, according to the National Weather Service.

This astronomical event signals the arrival of spring, winter’s end, and the trend toward increasingly warm and bright days that come with the pending arrival of summer.

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 fall. The days down under are growing shorter, the weather is cooling off, and sunlight is growing dimmer as winter approaches.

What drives these all-important seasonal shifts? Technically, two things: Earth’s tilted axis and the planet’s orbit around the sun.

How the spring 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, sort of like a 7,917-mile-wide rotisserie chicken made of rock and a little water.

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

Spring equiniox sunlight earth axis tilt bi graphicsShayanne Gal/Business InsiderAn illustration of the spring equinox.

If you stand directly on the equator as the equinox peaks, the sun will appear more or less directly overhead. Your shadow will also be at its absolute minimum. The sun sets and rises roughly 12 hours apart, too.

But this moment won’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 center 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 92 days and 19 hours after the spring equinox, the Earth will reach its summer solstice – when the most direct rays of the sun reach their northernmost latitude, called the Northern Tropic (or Tropic of Cancer). Another 93 days and six hours later, the fall or autumnal equinox will occur.

Then it’s another 89 days and 19 hours to the winter solstice – when the most direct sunlight strikes the Southern Tropic (or Tropic of Capricorn) – and another 89 days to get back to the spring equinox.

Earth equiniox solstice seasons spring summer fall winter sun bi graphicsShayanne Gal/Business InsiderAn illustration of the spring and fall equinox and the summer and winter solstice.

The animation below, from NASA’s Goddard Space Flight Center, 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:

The truth about the egg-balancing trick

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 19, 2018.

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