Tuesday, March 19 at 11:06 p.m. EDT marked the vernal equinox for the Northern Hemisphere, when the sun was directly over the equator and its energy was in balance between the Northern and Southern Hemispheres, according to the National Weather Service.

Most years, the season typically changes on March 20th or 21st. So, why did spring start a few hours earlier in 2024?

First day of spring 2024

The reason the first day of spring was March 19 was because 2024 is a leap year. Leap years are caused by Earth’s rotation. A year is 365 days, but technically it takes the Earth slightly longer to orbit around the sun.

The Earth takes 365 days, 5 hours, 48 minutes and 46 seconds — or 365.2422 days — to fully orbit the sun, according to NASA. Those extra hours are eliminated from the calendar most years. But every four years, an extra day is added to February so the calendar and seasons don’t get out of sync. If this didn’t happen, the extra hours would add up over time and seasons would start to change.

Those leap years cause the first day of spring to happen earlier than normal.

In 2020, another leap year, the first day of spring was also on March 19, with the vernal equinox occurring at 11:50 p.m. At the time, it was the earliest first day of spring since 1896.

But the vernal equinox of 2024 had it beat. Because spring began even earlier, at 11:06 p.m. ET, all the time zones in the continental U.S. experienced the first day of spring on the 19th — at 10:06 p.m. in the Central Time Zone, 9:06 p.m. Mountain Time and 8:06 p.m. Pacific Time.

During the next leap year, 2028, spring will again start on March 19. And spring will continue to start at earlier and earlier times on March 19 every leap year until 2103.

In 2025, which isn’t a leap year, the spring equinox will occur on March 20 at 5:01 a.m. EDT, and in 2026 it will occur March 20 at 10:46 a.m. EDT, according to National Weather Service.

What is the spring equinox?

The seasons are marked by either an equinox or a solstice and occur because the Earth rotates on an axis, so different parts of the planet get more or less exposure to the sun as it orbits the star throughout the year.

Spring and fall are marked by an equinox, which means “equal night” in Latin. The sun passes directly above the equator on the equinox and there are about an equal 12 hours of daylight and 12 hours of night, NASA explains.

During the vernal equinox that marks spring in the Northern Hemisphere, the Southern Hemisphere is experiencing its autumnal equinox, which ushers in fall for that part of the world.

The autumnal equinox for the Northern Hemisphere usually happens on Sept. 22 or 23.

During the solstices that mark summer and winter, the Earth is reaching the greatest angles of its axis. Typically on June 20 or 21, the summer solstice occurs in the Northern Hemisphere because this area of Earth is tilted toward the sun. The opposite happens on Dec. 21 or 22 with the winter solstice.

Meteorological spring

To make matters more confusing, meteorologists follow a different system for the seasons. Spring for weather forecasters starts on March 1, because that’s typically when the climate begins to become more spring-like in most areas. Meteorological summer starts June 1, meteorological fall begins Sept. 1 and meteorological winter begins Dec. 1.

With this method, the length of the seasons is more even. During non-leap years, they’re all 90 to 92 days, the NWS explains.

But the astronomical seasons that follow the equinoxes and solstices are not as even. Spring has 92.771 days, summer has 93.641 days, fall has 89.834 days and winter has 88.994 days, according to the Old Farmers’ Almanac.