The name calendar arose in those days, when Roman priests expected the appearance of the new Moon in the sky. Then they could announce the beginning of the new calendar month. The first day in the month was then called calendar and later this term started to be used as the appellation of the whole year’s partition.

Day and year are the natural elements of the calendar. A day is determined by the speed of Earth’s axial rotation and a year by the duration of Earth’s circulation around the Sun. Week and month were devised by people. Weeks approximately correspond with the periods between the Moon’s phases. Approximately means, that the lunar year (which is determined by the Moon) differs with the solar year (which is based only on the movement of the Sun). The difference is about 11 days…

The importance of a calendar for practical life is great. Therefore, we cannot wonder, that the oldest civilisations created their own calendars.

The overflow of Nile in the ancient Egypt happened at the same time when the shining star Sirius appeared. But the east of Sirius was also announcing the arrival of the summer solstice. Thanks to this coincidence was the length of the Egyptian calendar the same as the length of the solar year. The Egyptians divided the year into 12 months, each month had 30 days and the remaining days were feast days. But the solar year is nearly a quarter of a day longer. Because the Egyptians did not use the leap year, as we do it today, their calendar was every four year delayed for one day. And so the New Year Day was wandering through the whole year and in 1460 (but actually in 1461) year it returned to its original place. The saddest thing is, that the Egyptians did know about it, but they kept this state because of religious reasons. A Pharaoh Ptolemaist III. tried to enforce the addition of one extra day every four years – but unsuccessfully.

But the Maya’s calendar is marked by an incredible exactness. It is very complicated, but after proper studying we find out, that this calendar is logic and harmonic. The Mayas knew the length of a solar year very exactly: 365,2430 days. Only 2 ten thousandths less then today recognized length of a year.

Let’s blow over other calendars: e.g. those in India (here they had had more than 30 of them until the year 1953!), or Muslim calendar (every 33-year-old Muslim woman would be only 32 years old according to our calendar).

Only Roman calendar is important for us, because our present-day calendar was developed from it. The Roman calendar had only 304 days and 10 months (the names of these “numbered” months were preserved in the names of our last four months). Later the Roman king Numa Pompilius added two other months and prolonged the year up to 355 days. Then, because of the temporal compensation, he used to add the 13^th month, very often according to someone’s wishes. This disorder was eliminated by Julius Caesar in 46 BC with the help of a famous Greek astronomer Sósigen. So a new calendar came into being. It was later called Julian – in honour of Julius Caesar. This calendar took account of a year, which was 365 and one quarter of a day long and it had 12 months and one leap year, just like today (It is interesting, that the Greeks had known the length of a year more exactly at that time, but Sósigen didn’t use it for unknown reason.). The original names of months were preserved, only February and January were added and so the beginning of a year was moved on too. Later the names of the seventh and eighth month were changed in honour of Julius Caesar and his successor Augustus (these names were kept in many foreign languages up to now).

At the beginning of the Roman history all important events were dated according to the consuls, who were in office at that time. And nobody minded, that the consuls didn’t start their office on 1st January but in June and so there were two consuls every year. Under the emperor Augustus dating has began since the rise of Rome. After a longer period, the opinion about rising the town in 753 BC, was accepted.

After the fall of Roman empire, most of the territory was mastered by Christianity. Therefore the era couldn’t remind of the rise of the heathen and sinful Rome. For the Christian calendar only one event came in question as the beginning of new era: The birth of Jesus Christ. And so around 487 a monk called Dionysus Exiguus (as you can see – heathen names weren’t prejudicial) was entrusted with finding out the year of Christ’s birth. Sometimes around 525 the new year is being used. The alleged year of Christ’s birth was indicated as 1 A.D. From today’s point of view it doesn’t seem to us, that the monk was so learned because he forgot the year 0 (it might be the heritage of the Roman way of counting) and secondly Christ was born a few year earlier.

The Julian old style calendar was a masterpiece at that time. It had taken 700 year until someone found out, that the calendar is retarding. And not before another 700 years (1414), at the council in Constance, cardinal Pierre d´Ailly asks for the reform of calendar. At that time the calendar was 10 days delayed. But the council in Constance found it more important to burn Master Jan Hus. And so the reform had to wait until 1582.

Gregorius XIII.
(picture from web page http://www.greenheart.com/billh/gregory.html)