This year, 2020, is a leap year, meaning that rather than the usual 365 days of the year, we actually have 366. Leap years occur every four years, and this extra vital day of the year ensures that our calendars stay in working order.
Forcing our human calendars to stay in sync with nature’s cycles has required some awkward and confusing science. In this article, we’ll take a look at this and history behind leap years and why they are necessary.
Why do we need leap years?
A full solar year is 365.2422 days, so at the 365-day mark, the planet hasn’t quite reached its original starting point. This extra 0.2422 of a day might seem tiny, but it all adds up. If we kept every year fixed at 365 days, the months and seasons would shift out of place.
In around 700 years time, June in the northern hemisphere would fall in the middle of winter. All future planning would become very complicated and religious holidays such as Easter and Christmas would be out of sync with their respective seasons.
Thus, in the modern calendar, we add the extra 0.2422 accumulated over 4 years into 1 extra day that becomes February the 29th. However, if a year is divisible by 100, there’s no extra day. But if that year is divisible by 400, there is an extra day after all.
It’s all a bit of a complicated mess, but necessary to ensure our calendar keeps in sync with the solar orbit.
The history of the leap year
The history of the leap year and our modern calendar can be traced by to Ancient Rome and Julius Caesar. In the first century BCE, the Roman calendar had slipped out of alignment with the seasons by two months. In 46BC, Caesar declared that the current year would last 445 days to bring the calendar back into alignment, which initially caused all sorts of confusion among Roman citizens.
To avoid any further confusion, Caesar created the Julian Calendar that added a leap year every three years. Caesar’s heir, Emperor Augustus adjusted this by making a leap year every four years during his reign. But this calendar still wasn’t correct, since the Roman year was shorter than 365.25 days.
Finally, Pope Gregory XIII replaced the Julian calendar with the much more refined, Gregorian one in the 16th century with the modern schedule of leap days. He also restored the seasons and religious holidays to their original place by forcing a leap in time. October 4th in 1582 was followed by October 15th rather than the 5th.
Since this was a decision made by the Catholic church, Protestant and Orthodox countries initially resisted the change. Greece was the last country to do so, which didn’t make the reform until 1923. In other cultures, the measuring of years has differed. The Hindu, Hebrew and Chinese ones have holidays and traditional calendars centred around a lunar pattern, which is why they move around relative to the Gregorian one.
500 years before Pope Gregory changed the calendar, the Persian astronomer, Omar Khayyam, measured the length of a year as 365.24219858156 and created his own leap year system around this figure. This has since been deemed far more accurate than the current Gregorian calendar!
Will the calendar change again?
According to Russian math historians, Adolph Yushkevich and Boris Rosenfeld, it might need to! The current system is accurate to one day in 3,333 years, so by the year 5,000, we’ll have to decide whether to add more leap years or to change the whole calendar system again.
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