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2020, like many election years, is a leap year. What does this mean, exactly? Well, it means that February, the shortest month of the year, is a little longer than usual this year. Instead of ending on the 28th, like it normally does, every fourth year February gets a 29th day. But why is this the case?
Well, as it turns out, without Leap Day, the calendar would get far out of whack. In fact, without Leap Day, the seasons would gradually slip back in the year, and months would be all but meaningless for tracking seasons. Here’s the science.
Timekeeping on Earth is based on the movement of our Solar System. We measure a day by the length of time it takes for our planet to complete one rotation on its axis. This creates the illusion of the sun “rising” and “setting,” as the Earth itself blocks the light from the sun on one half of the planet.
Years, of course, are measured by the length of time it takes for the Earth to complete a rotation around our sun, Sol. The calendar you’re likely most familiar with is the Gregorian calendar, which divides the year into twelve months and each month into roughly four weeks. Importantly, each year has exactly 365 days, except for each year divisible by four, in centuries that are divisible by 400.
The Gregorian calendar, introduced by Pope Gregory XIII in 1582, addressed a slight problem with the earlier Julian calendar.
A year isn’t exactly 365 days: it’s 365 and one quarter days, roughly. That “roughly” was a bit of problem 1,500 years after Julius Caesar introduced his calendar. Caesar’s calendar assumed that a year was exactly 365 and a quarter days long, even though it’s slightly less than a quarter of a day.
By 1583, this slight discrepancy had added up to a huge 10 day difference in the calendar and the actual day. So, Gregory fixed it with the unusual “divisibility” solution, which accounted for the “roughly” one quarter day difference.
Leap Day, which is on February 29 once every four years, is traditionally known as a day for women to propose to men. This bucks the traditions of men being the ones to propose the wedding. Another, lesser-known tradition involves green emerald diamond engagement rings, as their unusual color denotes the unusual nature of the “extra” day and the “reversed” proposal.
