Spring equinox: Why Spring 2020 is the earliest in over 100 years

The moment winter becomes spring is upon us. Sometimes also known as the "vernal equinox," the day marks the day when day and night are almost equal everywhere around the world.

But this year, spring is early. For a series of complicated reasons, this year's equinox is on March 19, not March 20 or 21 like usual.

The vernal equinox this year takes place on Thursday, March 19, is the earliest in 124 years.

For the northern hemisphere it marks the start of longer days, and the opposite for the southern hemisphere. And in 2020, it is at least a day earlier than usual.

So why is this equinox so early? Although warmer global temperatures may make it feel like spring has come early, this astronomical phenomenon has nothing to do with climate change. Instead, it is partly to do with how leap years have altered the calendar over the years.

It's easier to understand this if you don't think about the equinox as a day, but as a point in time.

The Earth tilts on its axis as it orbits the Sun, so you're tilted away from the Sun in winter, and towards the Sun in summer. The March equinox is the point where it crosses over, meaning the Earth is almost upright toward the Sun in that moment.

Considering the equinox as a moment helps explain why day and night might not be exactly equal, and why it's come a bit early this year.

TimeAndDate lists all the March equinoxes of recent years, showing how it's moved over time.

• In 2015 it was at 18:45 Eastern time on March 20.
• In 2016 it was at 00:30 Eastern time on March 20.
• In 2017 it was at 06:28 Eastern time on March 20.
• In 2018 it was at 12:15 Eastern time on March 20.
• In 2019 it was at 17:58 Eastern time on March 20.
• In 2020 it's at 23:49 Eastern time on March 19.

Why does the spring equinox change?

It doesn't actually take 365 days exactly to move around the Sun. Rather, it is more like 365.24 days. That means the moment when the Sun is facing the Earth dead-on changes in terms of the Earth's rotation on its axis, even though it doesn't change in terms of the Earth's orbit around the Sun.

Our calendar is designed to account for this discrepancy, but it does mean the time moves around a bit more. Every four years we add an extra day to the calendar, the February 29 leap year. But to account for that 0.24 difference rather than 0.25, we only add a leap year day to years at the end of a century that divides evenly by 400.

That means 1900 is not a leap year, even though four years prior, 1896, was a leap year, because it doesn't divide evenly by 400. By contrast, the year 2000 was a leap year because, even though it's at the end of a century, it divides evenly by 400.

What does this have to do with the start of spring? Because that extra leap year in 2000, designed to keep the calendar in sync with the Sun, also kicked the equinox back a few. That means we can expect a lot more March 19 equinoxes in the coming years.

There's another reason why the equinox is earlier this year.

Daylight savings time came in the United States before the equinox. Prior to 2007, the Pacific time zone states, as well as Alaska and Hawaii, were the only ones to change their clocks at the start of March, but the other states changed away from the early April clock change after that year.

Thanks to a combination of 16th-century and 21st-century decision-making, and a little astronomical science, spring has arrived a day early.