It all happens because the number of days in a year isn’t even. A year lasts 365 days, 5 hours, 48 minutes, and 46 seconds. Call it 365.2422 days. If only the year were 11 minutes longer, or 365.25000 days, we could simply add one day every fourth year and take care of the fraction forever.
But because Earth spins a hair less than 365 ¼ times per year, we must sometimes omit that extra once‑every‑four‑year day, and that’s what creates all this fussing. Skip three leap years every four centuries and you’re accurate to one day in about 3300 years. (We even deal with THAT little glitch by skipping February 29 in the year 4000.)
A calendar that doesn’t accurately divide days into the year starts going weirdly out of sync. Seasons start happening at odd times. In the previous Julian calendar (where all century years were leap years) the annual 11‑minute error accumulated to where equinoxes were happening around March 11. The Easter Bunny was hopping around in the snow!
The present calendar takes care of everything. This leap year will make 2016 have the earliest seasons of our lives (thus far!).
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