If you Google "first day of spring", it will tell you March 20. But don't be fooled! For many people in the US, the first day of spring will actually take place on March 19. As it turns out, this year marks the earliest spring we've seen since 1896.

That's thanks to the fact that 2000 was a leap year, which was unusual since most century years, like 1700, 1800, and 1900, were not leap years. Don't worry, there are no unnatural forces at work. It's simply a consequence of our imperfect Gregorian calendar system that can't quite account for Earth's annual revolution around the sun.

Long ago, our ancient human ancestors decided that spring officially began when the sun shone directly on Earth's equator (illustrated below). As a result, the first day of spring is sometimes referred to as the spring, or vernal, equinox.

However, there is one problem with establishing seasons based on Earth's movement through space, which is that the time it takes Earth to complete one revolution around the Sun is not exactly 365 days. In fact, it actually takes 365 days, 5 hours, 48 minutes, and 45 seconds.


The Gregorian calendar tells us, however, that one year is equal to 365 days exactly - no more and no less. That is, unless we have a leap year, in which case it becomes 366 days.

Leap years are a great way to account for this discrepancy and keep our calendar, as well as our seasons, in check. Without leap years, we'd quickly fall out of step. Within 100 years our calendars would be 24 days off schedule.

Special exceptions

Hosting a leap year every four years, however, doesn't quite cut it. That's why there are two special exceptions:

  1. There's no leap year at the turn of the century. The years 1700, 1800, and 1900 were not leap years, even though they fell within the regular 4-year cycle.
  2. Unless the turn of the century is divisible by 400. This is why the year 2000 was a leap year.

"Our calendar system is set up in a 400 year cycle so that it makes up for all the little fractions that are left over since Earth doesn't spin in 365 and a quarter days," astronomer Bob Berman - who runs Overlook Observatory in New York and owns a giant book containing tens of thousands of past and future equinox dates - told Business Insider.

Berman is also a regular guest on presentations hosted by Slooh, which is a collection of observatories around the world that stream live cosmic events online. Slooh hosted a special presentation on Sat. March 19 at 5pm ET where Berman and other experts discussed the spring equinox.

"What this tends to do," Berman continued, "is make leap days reset themselves so that the equinoxes and solstices all happen [on schedule]."

The big difference about the 21st century was that the year 2000 was a leap year. While this didn't affect our daily lives, it did make one difference: "Instead of everything being reset so that all the dates of the equinoxes and solstices get knocked back to their usual dates … that did not happen," said Berman.

The result is that the first day of spring will be moving to earlier times throughout this century. This year, the spring equinox officially starts at 4:30 UT March 20, which means it will take place at 12:30am March 20 ET and 9:30pm March 19 PT.

Before 2100 rolls around, it won't just be the US - everyone across the globe will experience spring equinoxes on March 19.

By the time we reach the 2300s, however, we'll be back on track and most spring equinoxes will fall on March 21st, again, according to Berman. "This is all part of the plan to keep the dates from getting too far out of whack," Berman said. "And because of this, it really keeps everything accurate to about one day in 4,000 years. It's very impressive."

This article was originally published by Business Insider.

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