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.