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What Do You Know About Our Calendar?

Calendars are important for things like turning homework in on time, but how was our calendar made?

So many things that we do depend on the calendar. It tells us what day of the week it is, if we have to go to school or work, if it’s a holiday, when farmers should plant crops, when we earn our allowance or paycheck, and when it’s our birthday. Calendars are important, and it took a lot of history, math, and astronomy to create the calendar we use now.kid holding umbrella

The Gregorian calendar, the one we use today, was introduced in 1582—over 400 years ago. Before this, countries in Europe followed the Julian calendar, named after the Roman emperor Julius Caesar. Like the Gregorian calendar, the Julian calendar tried to divide the solar year, the time it takes the Earth to make one rotation around the sun with, into months, weeks, and days. One rotation around the sun takes exactly 365 days, 5 hours, 48 minutes, and 45.51 seconds, but the Romans didn’t have the scientific tools to measure some of those minutes and seconds, so their calendar was short by 11 minutes.

Eleven minutes might not seem like a big deal, but after hundreds of years, they really add up.

Another problem with the Julian calendar was that it got leap years wrong. The Romans knew the math of their calendar was a little off, and so they added an extra day in February every four years to try to fix the problem—except they still weren’t quite right, and so by the 1500s, the calendar was ten days behind the solar year!  

When scientists realized these mistakes, they knew they needed a new calendar that fixed these problems. With new tools and knowledge—like big telescopes and more complicated math—the Italian scientist Aloysius Lilius and the German mathematician Christopher Clavius figured out how to create a more accurate calendar.

Their solution was to only include the extra day in February in years divisible by four, unless the year can also be divided by 100, then the extra day isn’t added. If the year can be divided by 400, the leap day is added even though it can also be divided by 100. Whew! That’s some fancy math.

Even though the new calendar was created in 1582, it wasn’t accepted by most people across Europe until 200 years later! In order for everyone to begin using the new calendar, overnight the date switched from September 2 to September 14, 1752. Understandably, some people were mad about losing 11 days of their year, but Benjamin Franklin thought the idea was wonderful!

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