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The Gregorian Calendar

We now know that the solar cycle is 365 days, 5 hours, 48 Minutes and 45.51 seconds. The period is known as the astronomical year, or tropical year, and t is measured from one spring equinox to the next. The fact that the astronomical year is not a precise whole number of days has created interesting problems with calendars throughout the ages.

Our prehistoric ancestors had a very different concept of time than we have today. They would have recognised night and day, of course, and they used a lunar calendar. The interval of time between two new moons, around 29.5 days, would have been their equivalent of one month. A year was probably a more difficult concept. In temperate zones where there are seasons, they probably noted when the new leaves sprouted in the spring or when the leaves fell from the trees in autumn. These indications were important for agriculture and religious festivals.

The lunar calendar caused problems because it was out of phase with the solar year. The 12 lunar months of 29.5 days totalled 354 days. This number is 11 days less than the solar year, so it means that each calendar year is starting earlier than the previous one. It took 32 years to get back in line with the solar year! The Sumerians, who lived in Mesopotamia around 3000 BC, left written evidence of using a lunar calendar that worked on 30 days cycles. Even the Sumerians calendar was 5 days short each year.


Moon phase calendar

This is called a Moon phase calendar © NASA


By about 1750 BC the Babylonians had created a solar calendar. This calendar had 12 months, one of 29 days followed by one of 30 days. This left them 11¼ days short in the solar year, so they added one month seven times every 19 years! It was the Babylonians who introduced the week of seven days. It is thought that the reason seven was the number of days used to make one week is because of the seven planets known at that time.

Around 1300 BC the Egyptians began to use a solar calendar that started when the Dog Star (Sirius) rose in the East just before the sun. This gave them a solar year of 365 days. They divided this into 12 months of 30 days and then just tagged on 5 days at the end! As we now know, the solar cycle is precisely 5 hours, 48 minutes and 45.51 seconds more than the Egyptians had counted on. Their calendar also slipped backwards, although not so dramatically; it would take 1460 years for the Dog Star to be at the same starting point on the eastern horizon!

By 49 BC the Roman calendar was four months behind the solar cycle. The Emperor, Julius Caesar, employed the astronomer Sosigenes to solve the problem. He calculated the solar year to be 365 days and 6 days. To compensate for these extra days he introduced a 365¼ day cycle. This is how the concept of the "leap year" started. Every four years, in February, an extra day was added. To bring this new Julian Calendar into operation the Romans had to jump forward 90 days overnight! The new calendar still had an 11½ minute backwards slip every year. This is not very much over a short period of time but it starts to have effect after a few hundred years. For example, after 200 years there is a 38.3 hour difference with the solar cycle.

By the 1500's this difference had started to be a real problem. In 1581 Pope Gregory XIII turned to the Jesuit priest, Father Christopher Clavius to put things right. Clavius calculated that the calendar and the solar cycle are 3 days apart every 400 years. He made an adjustment to the Julian calendar by making the century years (those years ending in 00, such as 2000) only leap years if they could be divided by 400. So the year 2000 was a leap year but the years 1800 and 1900 were not!

All of Catholic Europe followed the Papal Bull of 1582 that ordered the new calendar to be adopted. However, most of the Protestant nations ignored the Gregorian calendar and continued with the Julian calendar. By the 1700's the population of the UK was living 11 days earlier than most of the continent. The British Calendar Act of 1751 adopted the Gregorian calendar for the following year. On the night of September 2nd 1752 the British public went to sleep and woke up on September 14th!

It is worth noting that the Gregorian calendar is still 26 seconds short of the solar cycle and that means it is one day behind the solar cycle every 3323 years. In the last century it was decided to make an adjustment to compensate for this. Century years that can be divided by 4000 will not be leap years. What this means is that the year 4000 AD will not be a leap year, but I would not worry abut that too much if I were you.

It is interesting that the history of the calendar tells us about the origins of the names we use for the days of the weeks and the months of the year. In Babylonian times each day was named after one of these planets. Even today we still use these ancient names, even though their origins are not always obvious. Sunday for the sun, Monday for the moon, Tuesday for Mars, Wednesday for Mercury, Thursday for Jupiter, Friday for Venus and Saturday for Saturn. The months of the year are mainly from Roman times; January after the Roman god Janus, March after the Roman god Mars, June after the god Juno, July after Julius Caesar, August after the Emperor Augustus Caesar. February comes from the Latin word febrou that means to purify by sacrifice, September from septem meaning seven (since it was the seventh month in the old Roman year that started in March), October from octo meaning eighth, November from novem meaning ninth and December from decem meaning tenth.

In the UK there is a rhyme that children use to remember the number of days in the months of the year:

Thirty days hath September,
April, June and November:
All the rest have thirty one
Except for February alone
Which has twenty-eight days clear,
But twenty-nine each Leap Year.


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