The calendar is born

When we want to check the day of the week, or find out what date of month it will be on a particular day next week, we look at the calendar. Today, we have calendar on our watches, on our computers so, it is very simple. But it wasn't that simple thousands of years ago. Our earliest ancestors kept track of time and the passing of days by watching the sun, the moon and the stars. They saw the sunrise and knew that another day had begun. By watching the change in the length of shadows they could tell whether it was early or late in the day. The phases of the moon: new, quarter, half or full, give them some idea of how many days had passed.

Gradually, man came to realize it took the moon a little more than 29 days to complete a full cycle-from new moon to new moon. From this discovery came the idea of the lunar month based on the cycle of the moon. Man also learned that it took the Earth about 365 days to make a full circle around the sun. And so the idea of the solar(sun) year was born. But a true calendar was still a long way off. Dividing time according to the movements of the moon and the sun created a problem. A year based on 12 lunar months of 29 1/2 days came to 354 days which was 11 days short of a solar year of 365 days.

THE ROMAN CALENDAR

The Romans used a calendar of 12 lunar months. But they quickly discovered that this made the year 11 days short. By adding an extra month every few years they hoped to make up the difference. However, when Julius Caesar became ruler of Rome in the century B. C., he found that the calendar was off by 3 months. Caesar consulted a leading astronomer called Sosigenes and learned that a year had 365 1/4 days. A new calendar was created. Instead of the lunar months of 29 1/2 days , the new months, except for February, had either 30 or 31 days. A year would thus have 365 days. To make up the extra quarter day, one day would be added to February every fourth(leap) year.

THE CALENDAR IS CHANGED

But even the Roman calendar, also known as the Julian calendar, was not accurate. Sosigenes' estimate of 365 1/4 days to a year was off by 11 minutes and a few seconds. This was not a great deal of time but over a period of hundreds of years it added up to a number of days. In later centuries, people found that the vernal equinox, the first day of spring, when day and night are equal, kept coming earlier and earlier. In 1582, it came 10 days ahead of time, on March 11 instead of March 21.

Pope Gregory XIII called upon the leading astronomers and mathematicians to figure out a way of correcting the error in the Roman calendar. An astronomer named Christopher Clavius suggested dropping 10 days from the year and Pope Gregory agreed. The day after October 4, 1582, was declared to be October 15 and not October 5. But that didn't solve the problem for the future. So the Pope decreed that from then on century years(the first year of a century) would have no extra day in February during leap years. The exceptions would be those century that could be divided by 400, such as the year 2000.

So the problem was solved-well, almost. The Gregorian calendar, which is used in most countries of the world gains 25 seconds a year, or one day in every 3 300 years. But I don't think we have to worry much about that. :D