The spring equinox has arrived.

The spring equinox falls on Thursday, marking the beginning of spring for the Northern Hemisphere and fall in the Southern Hemisphere. At the equator, the Sun will be directly overhead at noon. The equinox is the only time when both the North and South Poles are illuminated by sunlight at the same time.

The occasion has been celebrated and celebrated across the world for centuries. For example, Nowruz, the Iranian New Year, is based on the spring equinox. And at the Mayan site Chichen Itza in Mexico, people gather during the equinox to watch the sun create a shadow pattern that resembles a snake descending from a building called El Castillo.

But what is happening in the sky? Here's how we divide the year using Earth's orbit.