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Leap Year: Why You Can Thank Julius Caesar

Today is Feb. 29 — the bissextle or leap day, an artifact that dates back to the year 46 B.C.

Today is Feb. 29 - the bissextle or "leap day," an artifact that dates back to the year 46 B.C.

Back then, Julius Caesar took the advice of the learned astronomer Sosigenes of Alexandria, who knew from Egyptian experience that the tropical year (also known as the solar year) was about 365.25 days in length. So to account for that residual quarter of a day, an extra day - a leap day - was added to the calendar every four years.

This new "Julian" calendar was used throughout the Roman Empire and by various Christian churches. At that time, February was the last month of the year. [Earth Quiz: Do You Really Know Your Planet?]

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Initially, in order to make a proper transition from the Roman calendar (which had 355 days and which was basically a lunar calendar) to the Julian calendar, and to get the months and various feast days and holidays back into their normal seasons, 90 extra days were inserted into the year 46 B.C. Caesar divided these 90 extra days into three temporary months.

One month was added between February and March. Two other months (Intercalaris Prior and Intercalaris Posterior) were added after November. The end result was a year that was 15 months and 445 days long, and was nicknamed Annus Confusionus - the Year of Confusion.

Then, to honor his contribution to timekeeping, Julius Caesar later renamed the fifth month (formerly known as Quintilis) after himself (July). See what sweeping changes you can make when you're an emperor?

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Flawed timetable

The Julian calendar worked so well at first that many countries adopted it. Unfortunately, it was flawed, being 0.0078 of a day (about 11 minutes and 14 seconds) longer than the tropical year.

So, the Julian calendar introduced an error of one day every 128 years, which means that, every 128 years, the tropical year shifts one day backward with respect to the calendar. This made the method for calculating the dates for Easter inaccurate.

As a result, by the year 1582 - thanks to the overcompensation of observing too many leap years - the calendar had fallen out of step with the solar year by a total of 10 days. It was then that Pope Gregory XIII stepped in and, with the advice of a German Jesuit mathematician and astronomer named Christopher Clavius, produced our current Gregorian calendar.

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First, to catch things up, 10 days were omitted after Thursday, Oct. 4, 1582, making the next day Friday, Oct. 15. This edict was most unpopular; many people felt that 10 days had been taken from their lives. There were riots in the streets throughout Europe, and workers demanded their 10 days' pay - forgetting, conveniently, that they hadn't worked those 10 days! Thankfully, the hubbub eventually died down.

Next, to more closely match the length of the tropical year, "century years" were declared not to be leap years (though they had been leap years in the old Julian calendar). The exceptions were those century years divisible by 400.

And that's why the year 2000 was a leap year, but 1700, 1800 and 1900 were not.

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The Gregorian calendar, however, was not adopted by the American Colonies until 1752. That's why George Washington was not born on Washington's Birthday.

In our time, we celebrate Washington's Birthday on Feb. 22. But the United States' first president was born in 1732 - and by that time, the error in the Julian calendar had increased to 11 days. So a calendar hanging on the wall where Washington was born would have read Feb. 11, 1732.

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And if you think the 20 years that it took the American Colonies to finally ratify the Gregorian calendar was a long time, that was nothing compared to Russia, which finally accepted calendar reformation in 1918.

And Greece held out even longer - all the way to 1923!

The Gregorian calendar has proven to be far superior to the Julian calendar. Over a span of one year, it runs 26 seconds too fast, but that's an error so slight that it will not be necessary to eliminate a day from the calendar until around the year 5300.

Gaius Julius Caesar, Art History Museum, Vienna, Austria. | Andrew Bossi/Wikimedia Commons

Suggested ‘improvements'

Still, some people would like to see our calendar changed yet again. One of the more popular proposals is the World Calendar created by Elisabeth Achelis of The World Calendar Association in 1930.

The World Calendar consists of 364 days. The year would be divided into four quarters, with each quarter consisting of three months. The first month of each new quarter (January, April, July and October) would have 31 days and would always begin on a Sunday. All the remaining months would have just 30 days.

In such a setup, each date would fall on the same day of the week every year. So if you were born on a Tuesday, your birthday would always fall on a Tuesday. Independence Day would always fall on a Wednesday; Christmas Day would be a Monday; and Thanksgiving would finally have a fixed date: Nov. 23, since the fourth Thursday in November on the World Calendar would always be on that date.

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Triskaidekaphobes likely would not like this new setup; it would mean four Friday the 13ths every year. (Currently, the maximum number for any given year is three.)

But wait! This is a 364-day calendar. What happens to day 365? And what about leap years?

Dec. 31 would be recognized as "Worldsday" (a world holiday). It would come between Saturday, Dec. 30 and Sunday, Jan. 1. As for leap years, the extra day would be inserted not at the end of February as it is now, but at the end of June. June 31 would thus become a second World Holiday; like the Olympics, it would be celebrated every four years.

In the Jan. 17, 2016, issue of Parade magazine, Marilyn vos Savant answered a question from a reader who wanted to know if there were a "less clunky" alternative to our present calendar.

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Vos Savant mentioned the Symmetry454 calendar, a perennial solar calendar that conserves the traditional seven-day week, has symmetrical, equal quarters, and starts every month on Monday. All holidays, birthdays, anniversaries and the like are permanently fixed. All ordinal day and week numbers within the year are also permanently fixed; Friday the 13th never occurs under this calendar.

"But there's a teensy drawback," vos Savant wrote. "Every five or six years, you would have to add a week at the end of December!"

So it seems that, like it or not, we are stuck - at least for now - with our current calendar.

Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for Natural History magazine, the Farmer's Almanac and other publications, and he is also an on-camera meteorologist for News 12 Westchester, N.Y. Originally published on Space.com.

Copyright 2016 SPACE.com, a Purch company. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

March 30, 2012

--A Bronze Age calendar could be hidden in this 3,400-year-old bronze statue, according to a new study into the artifact's finely carved decorations. Known as the Trundholm sun chariot, the sculpture shows a disk mounted on four wheels and pulled by a horse. Also known as Solvognen, the sun chariot is now in the collection of the National Museum of Denmark in Copenhagen.

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The sculpture was discovered in 1902 in a bog at Trundholm on the Danish island of Seeland. Experts believe that it was buried there as a ritual offering around 1400 BC.

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Approximately 10.23 inches across, the sun disk is made of two bronze plates decorated with concentric circles and spirals. The plates are soldered together using an outer ring: one side is gold plated, representing the sun during the day. "The decorations in both sides of the disk include circular grooves, spirals and circles ordered in various rings. They could simply be some beautiful ornamentation. But they could well contain a mathematical puzzle," Amelia Carolina Sparavigna, assistant professor at the department of physics of Turin's Polytechnic University, told Discovery News. She detailed her research in a paper published on the arXiv physics website.

The two sides of the sun disk have been interpreted as a depiction of the sun as it is drawn across the heavens from East to West during the day, presenting its bright side to the Earth and then returns from West to East during the night, when the dark side faces Earth. Representing the daily journey of the sun and the progression of the year and the recurrence of the seasons, the bronze sculpture might contain a 360-day calendar.

The theory that the disk might have a "calendar knowledge" was first proposed by Klavs Randsborg, professor of archaeology at the University of Copenhagen. He suggested that decorations in certain Danish Bronze Age artifacts were based on the knowledge of years, months and days.

For example, for the night side of the disk, Randsborg pointed out that the sum of spirals in each circle of the disk, multiplied by the number of the circles in which they are found, counted from the middle (1x1 + 2x8 + 3x20 + 4x25), results in a total of 177. That figure comes very close to the number of days in six synodic months. The synodic cycle is the time that elapses before a specific object will reappear at the same point in the sky when observed from the Earth, so it is the apparent orbital period observed from Earth. The math formula could then represent a Moon calendar (and a Sun-calendar for the diurnal side of the disk).

Sparavigna proposed a different, possibly easier, interpretation for the night disk. In her theory, the eight concentric circles of the first ring would represent the days of a week. Containing a total of 45 circles, 20 in one ring and 25 in the other, the outer two rings would represent the weeks of the year. "If we consider the winter solstice as the beginning of the year, the two groups could represent‭ ‬two seasons: one a 'young sun' made of 20 weeks, followed by the season of a 'mature and then old sun' made of 25 weeks," Sparavigna said. By multiplying the eight days of the week by 45 weeks, Sparavigna obtained a 360-day calendar.

The Trundholm disk would then become a nundinal calendar. On the left: the calendar shows a red marker for a day of the week (the first) and a blue marker for a week (the second in the "first season"). The calendar would thus read: the first day of the second week of the first season. On the right: the calendar shows a red marker for a day of the week (the third) and a blue marker for a week (the fifth in the "second season"). The calendar would thus read: the third day of the fifth week of the second season.

As for the gilded day-side of the disk, Sparavigna believes that the total number of spirals, central included, (52) may hint at another mathematical puzzle, possibly resulting in the Trundholm disk being a two-cycle calendar. "If we assume that each spiral is representing a week having seven days, we can obtain a 364-day calendar. The larger central "week" could contain one or two extra days, depending on years," she said.

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