The Earliest Spring of Our Lives: Leap Year Shifts the Equinox

Spring 2016 arrives exceptionally early—March 19 or 20, depending on your time zone—thanks to leap-year quirks. Notice the equinox used to fall on the 21st? Let's unpack why.

February 29 does more than create rare birthdays. Our calendar is nearly perfect: atomic clocks divide seconds into 9+ billion parts, synced globally via phone, web, GPS. But the year 2000 introduced a twist. For the first time in four centuries, a century year divisible by 100 did not skip its leap day. There was a February 29, 2000.

Gregorian calendar rules: years divisible by 4 are leap years. But if divisible by 100, they skip—so 1700, 1800, 1900 had no Feb 29. However, if divisible by 400, they are leap years. Thus 2000 kept its leap day—the first century leap year since Galileo's time. That decision prevented the usual one-day "jump back" of equinoxes/solstices, letting them creep earlier.

Equinoxes/solstices generally land on the 21st (fall equinox on Sept 23). But they've been shifting earlier. Spring began on March 21 in our childhoods, then the 20th for recent years. In 2016, it hits March 19 for most US/Canadian zones—the earliest since 1896!

Why Spring Begins Early This Year

A year isn't an even 365.25 days—it's 365 days, 5 hours, 48 minutes, 46 seconds (365.2422 days). If it were exactly 365.25, adding a day every four years would work forever. But Earth spins slightly slower, so we omit three leap days every 400 years (skipping century years not divisible by 400). This keeps us accurate to one day in ~3,300 years (we'll even skip Feb 29 in year 4000).

The old Julian calendar (all century years = leap years) accumulated an 11-minute annual error, pushing equinoxes to March 11—Easter Bunny in snow! The Gregorian reform fixed this. This leap year gives us the earliest seasons of our lives (so far).