Insight & Analysis

Where do leap years come from?

Published: Mar 2020

Last Saturday 29th February, many parts of the world saw a leap day. Leap years aren’t exclusive to a particular sect and are known by all who follow the Gregorian calendar. We thought we’d share some facts about the quadrennial day.

Wooden block calendar displaying 29 February

Julius Caesar was the first to establish leap years, along with the Greek astronomer Sosigenes of Alexandria. The traditional Roman calendar had fallen out of sync with the seasons, so Caesar asked Sosigenes to reform it. The Julian calendar included a leap year every four years – with no exceptions. This has since been reformed to reflect the fact that a full solar year is actually just under 365.25 days, so having leap years every four years without exception eventually caused it to get out of sync anyway.

It was in 1577, when the Julian calendar had fallen ten days out of alignment, that Pope Gregory XIII took action and the Gregorian calendar was established – which we still use today. Leap years still occur every four years, but in the Gregorian calendar there are exceptions. “Century years”, like 1700, 1800, and 1900, did not have leap days, but 2000 did. This is because 2000 is divisible by 400.

Other calendars use different methods to keep in sync with the seasons, such as adding an entire leap month in the Chinese calendar. Approximately every three years, a leap month is added when there are 13 new moons from the start of the 11th month in the first year to the start of the 11th month in the next year.

There are also leap seconds, which either add or subtract seconds to the time – although all leap seconds so far have been positive and it’s unlikely there will ever be a need for a subtraction. Coordinated Universal Time (UTC) and International Atomic Time (TAI) don’t quite match up. TAI deviates by only one second in up to 100 million years, whereas the slowing of the Earth’s rotation causes a discrepancy between times of around one second every year and a half. Leap seconds are added to ensure this discrepancy does not become too large and the time we use is synchronised as much as possible with the Earth’s rotation.

But of course, the most well-known tradition of a leap year is that it’s the day that women can propose marriage to men – instead of the other way around. Now, in the face of feminism and equality, it’s an outdated tradition, but it supposedly dates back to the initial implementation of the Gregorian calendar, where a British play joked that the idea of adding February 29th every four years was so ridiculous that it should also be a day when women trade their dresses for ‘breeches’ and act like men. By the 1700s, women were using leap days to propose to the men in their lives.

Outdated traditions aside, leap years are a necessity for the measurement of time to function effectively and for dates to stay on track, and who doesn’t love a free day?

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