So why do we even have a leap year? It might surprise you to learn that the answer hides in one simple fact about our planet: the time Earth takes to spin around the Sun, what we call following the solar year.
Wait, doesn't Earth complete its revolution in exactly three hundred and sixty-five days? If so, how does that explain an extra day?
The not-quite-365 days
Here's the catch. Earth doesn't take exactly three hundred and sixty-five days to come full circle around the Sun. It takes roughly three hundred and sixty-five point two four days. To make up for that little extra time, we add an extra day every four years.
The rule for spotting a leap year is fairly simple. Any number divisible by four counts as a leap year, with one exception: century years. A century year only counts if it's divisible by four hundred. That's why nineteen hundred was not a leap year, but two thousand was. And this system, the Gregorian calendar, carries a history around two thousand years long.
How we got here
To understand how it all came together, we need to travel back a couple of millennia. Before the Gregorian calendar, which we use now, there was the Julian calendar, and it served as the base for what followed. The Julian calendar used a three hundred and sixty-five day year, but the advisors to the Roman emperor Julius Caesar noticed that the years weren't aligning with the seasons. Around forty-five BC, it's said, the emperor decreed that, from then on, there would be an extra day every four years.
That might sound like the whole secret to leap years. But there's more to the story.
The small but stubborn error
The trouble was that the actual difference wasn't exactly zero point two five days each year; it was closer to about zero point two four. That difference might seem tiny, but it added up over time. And it eventually caught up with everyone when, in fifteen eighty-two, Pope Gregory XIII was informed that the alignment of the seasons had drifted by ten days. That's what a small difference between zero point two four and zero point two five can do over the centuries.
The Pope's scientific advisors worked out the rule of treating century years as leap years only when divisible by four hundred, and normal years as leap years when divisible by four. But what to do about those ten lost days? Simply delete them. That's what happened: the Pope chalked off the days between fourth October fifteen eighty-two and fifteenth October fifteen eighty-two.
Not the only way to keep time
The Gregorian calendar follows the solar year, but other cultures do things differently. Some follow a lunar year, as with the Chinese lunar calendars, while others follow a mix of both solar and lunar reckoning.
Here's a fun thought: it's said that the probability of someone being born on a leap day is roughly zero point zero zero zero six seven. And for another curious mind, there's this: Earth's orbit around the Sun is said to be getting slightly longer over the years. By a small difference, mind you, but it means there may come a time when we have to add a day or two to our calendar year.
Speaking of dinosaurs, since Earth's orbit around the Sun may have been shorter back then, did their days run to less than twenty-four hours? Now there's a question to let your curiosity wander over until the next deadline rolls around.