The calendar: a necessary tool for men

For all civilized men, or on the path of civilization, it is important to find one's bearings in time. These  temporal landmarks allow different peoples to set certain common events  and plan agriculture while being in line with astronomy.

Over  time people have adapted calendars to their uses, customs and needs, 1  for example, all countries do not celebrate the New Year at the same  time.

There are 3 types of calendar2:

• the lunar calendar: it follows the phases of the moon, 354 days distributed in 12 months (Muslim calendar);
•  the solar calendar: based on the seasons, 365 days spread over 12 months (Julian and Gregorian calendar);
•  the luni-solar calendar: mixture of 2, so 365 days, but the months coincide with the lunation (Chinese calendar).

History and changes in the calendar

At first, people had to divide time into days, months and years. For this they have based themselves on several astronomical phenomena3:

1- the solar day that separates the 2 sunrises or 2 sunsets (about 24 hours);

2- the lunation which is the interval of time separating 2 new moons.

The  first calendar was developed in the 3rd millennium BC by the cities of  Babylon4, although a calendar dated to the 5th millennium BC was  discovered in southern Egypt, in Nabta Playa. This brand new calendar was based on moon movements and consisted of 12 months of 29 or 30 days. It was possible to add days, or even additional months to stay in line with the different seasons of the year.

Then,  the Egyptians decided to replace this lunar calendar with a solar  calendar composed of 12 months and which counted 365 days. For the solar calendar, the designers have based on 4 points of reference6:

• equinox of spring: duration of the day and night identical, about 12 hours;
•  equinox of autumn: duration of the day and the same night, about 12 hours;
•  winter solstice: the night is the longest of the year and the shortest day;
•  summer solstice: longer day and shorter night.
  

The first Roman calendar was born in the 7th century BC. This lunar calendar counted 304 days and began in March. Days were then added to equal the solar year. This added period then becomes the months of January and February7.

In 46 BC J.  - C., to return in adequacy with the seasons, Julius César decided to  adapt the Egyptian calendar to the agricultural calendar to create the  Julian calendar. With the latter the year had 365 days and a leap year every 4 years3.

Several  centuries later, astronomers realized that Caesar had made some  mistakes and that the calendar was out of step with the cycle of the  stars. In 1582 Pope Gregory XIII decided to reform the calendar again and created the Gregorian calendar. Since that time, the beginning of the year has been set for January 1st7.

In  1785, in France, the poet François Fabre d'Eglantine composed the  republican calendar, also called revolutionary calendar or calendar of  the French. This  new calendar associates each day with a product of the soil and no  longer with a saint, the weeks become decades and the months have names  related to the seasons.

With this new calendar, François Fabre d'Eglantine shows his will to weaken Christian rites. In 1806 Napoleon 1st put in place the Gregorian calendar. Today,  the Gregorian calendar is used in most countries, but the lunar  calendar is still used in the Muslim religion for example.