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Off the Beaten Track

##### Day By Day

An Aztec calendar stone

Ancient Egyptian calendar in the Karnak Temple

The stone obelisk had been in its place for longer than anyone could remember. It was about as tall as three men and it occupied a central position in front of the temple.
The crowd stared intently at the shadow cast by the obelisk, waiting. When the Sun reached its zenith, a sigh of disappointment ran through the onlookers as it was realized that, once again, the shadow was not where it was supposed to be. Something was wrong.

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Calendars have existed since the dawn of civilization. Ever since humans first felt the need to keep track of their activities and to plan key events in their lives, they adopted natural units of time that were directly observable and tied to the movements of the Earth, Sun and Moon. At the beginning, these phenomena seemed to be sufficiently precise and served their purpose well. Night followed day, the full moon appeared about once every 30 days and the Sun passed through its zenith once every twelve lunar cycles. However, there were two main problems. Firstly, these three time units are not exactly divisible by each other, i.e. the ratios among them do not give whole numbers. Secondly, the duration of each movement is not constant and fluctuates for various reasons.
There was also another problem tied to the method of measurement. To start with, the length of a day can be measured in two different ways. We have a "sidereal day", which is the time taken by the Earth to rotate on its axis relative to the stars. It lasts exactly 23 hours, 56 minutes and 4.0905 seconds. But we also have a "solar day", which is the time between two consecutive passages of the Sun over a fixed point on the Earth. A solar day is almost four minutes longer than a sidereal day. By convention, nowadays the length of the solar day is considered to be exactly 24 hours.
The length of a month can also be measured in two different ways. A "lunation", or "lunar month", is the time between two consecutive new moons. It lasts 29 days, 12 hours, 44 minutes and 2.8 seconds. This is the most ancient method of measuring the duration of a month. The other method is based on the "sidereal month", which is the time it takes the Moon to orbit once around the Earth with respect to the stars. It lasts 27 days, 7 hours, 43 minutes and 11.5 seconds.
The duration of a year can also be measured with either the solar method or with the sidereal method. A solar year lasts 365 days, 5 hours, 48 minutes and 47 seconds. It's based on the observation of the Sun's position in the sky at noon over the various seasons. A sidereal year lasts 365 days, 6 hours, 9 minutes and 9 seconds. It's easier to measure because it's easier to observe fixed stars in the night sky and for this reason some calendars start the year on the first day that a particular star becomes visible.
The only unit of time that is not related to celestial phenomena is the week. Some calendars divide the month into four quarters, thus producing weeks of seven days, whereas others have ten-day weeks. Some calendars don't have weeks at all!
Over the years, calendars used in various parts of the world showed greater and greater discrepancies between the dates on the calendars and the observed seasons and position of the Sun. Equinoxes, which occur twice a year and mark the moment in which day and night are of equal length, were not falling on the dates they were expected to fall. This was causing problems with the organization of work and agriculture and with the correct observation of religious rituals. Corrective measures typically involved adding or subtracting days or even months to bring calendars back in line with the seasons.
In ancient Rome, the year started on the day of the spring equinox and had months of either 29 or 31 days; however, an extra month was added every two years. Some centuries later, Julius Caesar made a sweeping reform and had to extend the year 46 B.C. to 445 days by imperial decree.
The calendar that is now most widely used in the western world is called the Gregorian calendar. It made some important corrections to Julius Caesar's calendar, which had drifted behind the solar calendar. In order to bring the new calendar in line with the seasons, the day after Thursday, October 4th 1582 was Friday, October 15th.

By Giovanni Gregorat, Contributing Editor MFN

Author: Giovanni Gregorat