Back To CourseBasics of Astronomy
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I use a calendar every day to plan and record important events. And while I used to do this on a paper calendar when I was a kid, now I've got a virtual app for that.
But it doesn't matter whether you use one on your phone or you still circle stuff on one hanging on the wall, a calendar is, was, and always will be the same thing at its core level. A calendar is a way of organizing units of time in order to take account of time over a long period.
There are many different calendars that exist today or have existed in the past. We'll take a look at one of each, the Julian and Gregorian calendars.
I came. I saw. I conquered. Julius Caesar, the man who conquered France and helped establish the Roman Empire, was more than a brilliant general and a cunning politician.
He also introduced what's known as the Julian calendar, a civic calendar instituted by Julius Caesar that took effect in 45 BC. It's something he pondered over as he stayed in ancient Egypt and studied the Egyptian solar calendar that would form the basis for his own calendar. Like his impressive military tactics and political success, his calendar was no less revolutionary because it ignored the moon.
The earliest Roman calendars, the calendars of the greatest republic and empire at the time, were based on the lunar phase cycle. This was a problem because calendars based on this go out of sync with the actual seasonal year quite a bit. And so, two important elements in our modern calendars actually go back to Caesar's brilliant ideas as a result.
The first concept is the division of a year into twelve months, with each month having a set number of days, which when added together equal the number of whole days in a seasonal year. That number of days is 365.
The second important innovation is the idea of a leap year. This is when an additional day is added once every four years. Thus, the average length of a Julian calendar year became 365.25 days, which is basically the same length as the actual year. This calendar became the most important calendar everywhere from the Middle East to England after it was established.
Even in the Christian dominated world thereafter, this calendar, developed by a pagan, was so good that it survived for over 1,500 years. However, slowly all over the world, the Gregorian calendar began to replace the Julian calendar, with the last countries finally adopting the former in the 20th century.
The Gregorian calendar is a calendar introduced by Pope Gregory XIII in 1582 that was gradually adopted by all countries around the world as the international calendar of choice. This change was made because even though the Julian calendar was way better than calendars that preceded it, it still very gradually fell out of step with the seasonal year.
Such a problem would not have occurred if the average length of the year was precisely 365.25 days, or 365 days and 6 hours. But the true mean length of the year is a bit shorter. It's 365 days, 5 hours, 48 minutes, and 46 seconds. That means that after 1,500 years, the Julian calendar was 10 days behind the actual solar year.
So, Pope Gregory came to the rescue! He decided to solve this problem with two approaches that were introduced to him by others.
First, he made an official decree that the day after October 4, 1582, would be October 15. You see how powerful he was. He could literally change the numbers on the calendar and everyone would listen! If I did that, no one would even notice.
Such a brilliant move put the calendar back in line with the seasonal year. And in order to prevent Julian calendar-like errors from accumulating later on, a second solution was introduced. This solution involved removing three leap days every 400 years.
Every century year that is not divisible by 400 is no longer a leap year as a result. Meaning the years 1700, 1800, 1900, 2100, and so on, did not and will no longer have a leap day. However, because the year 2000 is divisible by 400, it was a leap year.
Such a reform to the calendar tells us that the mean length of the calendar year is now 365 days, 5 hours, 49 minutes, and 20 seconds. That's only 34 seconds longer than the actual (that is to say tropical) year. So much so, that it will take 2,500 years for the Gregorian calendar to fall out of step by one day.
A calendar is a way of organizing units of time in order to take account of time over a long period. We went over two very important calendars in this lesson.
The Julian calendar was a civic calendar instituted by Julius Caesar that took effect in 45 BC. It fixed many problems associated with ancient calendars by adopting the notion of a leap year and dividing the year into twelve months, each of which had a set number of days. And although it was a very good calendar, it did gradually run behind the actual solar year over the coming centuries.
Thus, the world adopted the Gregorian calendar, which is a calendar introduced by Pope Gregory XIII in 1582 that was gradually adopted by all countries around the world as the international calendar of choice. The Gregorian calendar is one where three leap years are removed every 400 years in order to keep the calendar much more in line with the true length of the year.
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Back To CourseBasics of Astronomy
28 chapters | 325 lessons