Why do we have a leap year? We have a leap year because the calendar year doesn’t exactly match the solar year.
Earth orbits the sun at an average distance of 149,600,000 km. We travel around the sun in a clockwise direction and we travel about 940 million km with every orbit. One orbit of the sun is called a sidereal year and it takes 365.256 days. And this is the reason why we have a leap year. 0.256 days is 5 hours 48 minutes and 56 seconds. That means, every year, it takes us almost 6 hours longer to orbit the sun than our calendars say. Over 4 years, that almost 6 hours has become almost 24 hours, which is pretty much one full day. If there were no leap years, the solar year and the calendar year would go out of sync by 1 full day every four years. That doesn’t sound that bad, but in 400 years, the solar year would be 100 days out of sync with our calendar and February would be roughly where December is now. All of the seasons would no longer match the calendar year. Luckily, it was an easy thing to fix.
In 45 BC, Julius Caesar reformed the calendar. The Roman calendar was a mix of a lunar calendar and a solar calendar, with a load of extra months thrown in. It only had ten months, which is why December has the Latin for 10 (decem) in it. To try and make the calendar fit the solar year, officials would add days to any month to make it longer. However, they would usually add days to a month that favored them, such as election months, or the last month of their rule. Caesar wanted to do away with this and have a standard calendar, so he brought an Alexandrian astronomer who had experience of the calendar system used in Egypt to make a solar calendar for Rome. The one that the astronomer came up with is the Julian calendar. They added two months to make it twelve and a leap day every fourth year to make the calendar year fit the solar year. To make them fit, Caesar added 67 days to 45 BC, bringing it in line and, coincidentally, increasing his rule as consul.
There is a small problem with the leap year, though. It doesn’t completely fix the problem. One sidereal year takes 365.242190 days. Adding a 24 hour day every four years actually makes the calendar longer because 0.242190 times four is 0.96876, which is not exactly a day. Every time we add a day, we are making the calendar 0..03124 days longer, which is about 44 minutes. Again, that doesn’t sound like a lot, but over hundreds or thousands of years, those 44 minutes start to add up. In 1,000 years, our calendar would be 183 hours ahead of the solar calendar. However, this also is easily fixed. To compensate, there isn’t a leap year every four years. If a year is divisible by 100, but not by 400, the leap year will be skipped. So, 1100, 1300, 1400, 1500, 1700, 1800, 1900 didn’t have leap years while 1200, 1600, and 2000 did.
We call the extra day a leap year, but it has another name. It is called a bissextile. Today, the leap day is on February 29th, but it used to be on February 24th. This was the sixth day before the first day of March on the Roman calendar. The sixth day before March was called “ante diem sextum Kalendas Martias”. In a leap year, rather than adding a second day as we do now, they doubled this sixth day before March. It became known as “ante diem bis sextum Kalenda Martias”. “Bis” means twice and “sextum” means sixth. Over time, bis sextum became bissextile.
The phrase leap year was first used in the late 14th century. It probably came from northern Europe, either German or Dutch, where the day was called “leap forward” because it caused all of the festival days to leap forward. A year is 52 weeks and 1 day long, so without a leap year, every year the day will advance by one. If Christmas is on a Saturday in one year, it will be on a Sunday the next. Except, when there is a leap year, it will jump another day forwards. And this is what I learned today.
https://www.timeanddate.com/calendar/monthly.html?year=2024&month=2&country=26
Sources
https://www.etymonline.com/word/leap%20year
https://en.wikipedia.org/wiki/Roman_calendar
https://airandspace.si.edu/stories/editorial/science-leap-year
https://edition.cnn.com/interactive/2024/02/world/leap-year-meaning-explained-dg-scn/
https://www.dictionary.com/e/word-of-the-day/bissextile-2024-02-29/
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