The year's spring equinox, also called the March or vernal equinox, falls on Monday March 20, at 5:24 p.m. EDT.
To people who live in Earth's northern hemisphere, this astronomical event signals the arrival of spring, winter's end, and the increasingly warm and brighter days that come with the pending arrival of summer.
For those in the southern hemisphere, though, fall will arrive: The days grow shorter, the weather cools, and the sunlight grows dimmer as winter approaches.
What drives this all-important seasonal clock?
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 six hours. Our planet also rotates once per day around a tilted axis.
That tilt is about 23.5 degrees (for now) and bathes different parts of the world with various intensities of light over the course of a year. Meanwhile, the planet's rotation keeps the heating even, like a 7,917-mile-wide rotisserie chicken made of rock and a little water.
Earth's rotation does not cause equinoxes. The spring equinox occurs when the sun's warming rays line up perpendicular to Earth's axial tilt:
If you stand directly on the equator at noon in the Eastern Time time zone at noon, the sun will appear more or less directly overhead. Your shadow will also be at its absolute minimum.
The sun also sets and rises roughly 12 hours apart during the equinox.
But this moment won't last as the Earth makes its way around the sun at a speed of roughly 66,600 mph.
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.
About 92 days and 19 hours after the spring equinox, the Earth will reach its summer solstice. Another 93 days and six hours later, the fall or autumnal equinox will occur.
Some 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 the planet over the course of the year and see how the the angle of sun changes.
NASA's Goddard Space Flight Center created the animation below using geosynchronous satellite images taken over Africa, and it clearly shows the seasonal progression:
Equinox is the first day of spring, but not for meteorologists
March 20 is officially the first day of spring, but meteorologists take a very different view of our seasons. For them, spring started on March 1.
That's because the calendar seasons don't line up with the astronomical seasons. The coldest three months of the year, which start in December, usually end in late February in the Northern hemisphere. so it makes sense that meteorological spring should start at the beginning of March.
Springing forward may mess with your head
Though most of us will be looking forward to the warming weather, spring is also associated with the dreaded daylight-savings time, which happened on March 12 in the US.
Research has previously shown that heart attacks, strokes, and fatal car crashes all spike around the start of the time shift.
It may come down to our sleep rhythms, our body is shocked by even an hour's difference in its sleep schedule, and that may make us more prone to deadly mistakes, Insider previously reported.
A recent YouGov survey found 62% of Americans would rather drop daylight saving time.
What about the egg-balancing trick?
You may hear people saying that it's possible to balance an egg on its end on equinox because of the way it is positioned in relation to the sun.
Spoiler alert: it is a myth.
You can balance an egg any old time you want, 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.
And don't look for any gravitational interplay between Earth and the sun to help you out; it's far too weak to make a noticeable difference.
This story was originally published on 6:19 p.m. EDT on March 19, 2018. Dave Mosher wrote a previous version of this article.
