We all grew up being taught that the seasons changed on the 21st, so did it strike anyone else a little odd that this spring began this year on March 19?
Sure enough, I checked and the spring equinox at 10:14 PDT on March 19 was the earliest spring equinox since 1896!
But get this -- every leap year, the spring will get roughly 30-45 minutes earlier until it peaks at 2096 when spring will arrive on March 19 -- at 7:02 a.m.! (PDT)
Why? It's due to the same reasons we have leap years -- the exact length of a year isn't a nice round number. According to the internet (because, who remembers this stuff off-hand?) the length of a year is 365.2422 days -- or about 11 minutes and 14 seconds short of 365 days and exactly 6 hours.
By adjusting for leap year every four years, it averages out to 365.25 days. The earliest season start times occur on leap year (because what is March 19 this year would be March 20 in non-leap years). Each year after a leap year, the start time of the season gets roughly 6 hours later until the next leap year pulls it back 24 hours.
But since .25 isn't the same as .2422, the season start times drift just a little earlier each year due to this minor error.
Remember back in 2000 when there was talk about how it was a rare leap year because usually we don't do a leap year in years divisible by 100, unless it's divisible by 400? For example, we don't do leap years in 1700, 1800 or 1900, but we did in 2000 (divisible by 400!) but we won't in 2100? That correction gets us closer to 365.2422
So in 1896, we had a spring start time of 6:23pm on March 19 (Pacific Standard Time, because they hadn't thought of PDT yet). Spring came about 6 hours later each year but in 1900 -- when there was no leap year, because 1900 divided by 400 requires decimal points, the spring start time just kept getting 6 hours later until 1904 when it finally got pushed back 24 hours due to leap year.
But in 2000, we didn't skip the leap year reset, so the spring start time will keep getting earlier each leap year until 2096 because in 2100, we won't do a leap year, in essence allowing the spring start time to keep pushing back another full day on our calendar.
It's not until we complete the entire 400 year cycle that the equinox start times truly reset and the drift starts over.
But in the meantime, enjoy this early start to spring! Just don't pay any attention to the fact that this drift also means autumn begins a bit earlier as well.
P.S. Check out this NASA video that shows how the Earth's tilt affects the sunlight on the planet through the seasons. Images acquired December 21, 2010 - September 20, 2011.