The March equinox is the astronomical start of spring that occurs at 11:33 a.m. on Sunday, March 20, and marks the beginning of warmer, longer days in the Northern Hemisphere, and shorter, colder days in the Southern half.

These celestial events, equinoxes and solstices, are caused by the Earth’s 23.5-degree tilt on its axis and its constant motion. It only happens twice a year when the sun crosses the equator. During the spring, or vernal equinox, the Northern Hemisphere goes from tilting away from the sun to bending toward it. That moment is the zero point of the sidereal time system, which astronomers use to locate heavenly objects. It’s based on how fast our planet is rotating.

“We're in our transition moment,” said Jackie Faherty, an astrophysicist at the American Museum of Natural History. “We reached the Equinox, it's kind of like an equal moment for both hemispheres where you have equal day, equal night. So we swapped with the Southern hemisphere ⁠— when it's our spring Equinox, it's their fall Equinox.”

Which became the perfect basis for ancient calendars and ritual celebrations. The Persian year begins on the March equinox. The first day of the Babylonian calendar was on the first new moon following the spring equinox.

‘Equinox is a wonderful thing,” said Bart Fried, executive vice president of the Amateur Astronomers Association of New York. “Farmers look at that to tell them to get ready to start planting.”

In Western Christianity, the vernal equinox is used to calculate Easter, which is the first Sunday following the first full moon on or after the March equinox, April 18.

But there are observable signs of the equinox. Sunsets and sunrises occur faster on those days because the ascension happens due east, and the setting due west – when the sun is at its steepest angle in reference to the horizon. And no matter the earthly coordinates, the sun is directly overhead at noon, as it would be at the equator.

For Faherty, she plans to celebrate all day – wake up early to watch the sun rise, and watch the sun in the sky all day until it disappears beneath the horizon line.

“You’re kind of marking the day,” Faherty said. “Mark where the sun is, and then watch it change position. I’m truly marking the day in its length and watching the sun’s position.”

This year, before sunrise on the March equinox, three planets will be visible in the sky, due east. Venus, the brightest and Earth’s nearest neighbor, will be at its greatest elongation ⁠— its farthest from the sun. Close by, Mars will appear as a small red dot, and Saturn as a golden spot.

The next equinox, Autumn, will occur at 9:03 p.m. on September 22, when the days will get shorter and colder.