The 2018 spring equinox has arrived — here's what it is and how it works

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The vernal or spring equinox of 2018 happens Tuesday, March 20.
Earth's rotation does not cause equinoxes.
Equinoxes occur when Earth's tilted axis is perpendicular to the sun's rays.
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 Tuesday at precisely 12:25 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, it's now the fall: The days are growing shorter, the weather is cooling off, and sunlight is growing 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 6 hours. Our planet also rotates once per day around a tilted axis.

That tilt is about 23.5 degrees today 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, 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 graphics

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, or 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 graphics

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: