The calendar is a necessary tool to organize time in relation to agricultural activities, to religious obligations and to social and civil life.

Our calendar currently is twelve months and 365 days a year. Any careful reader, however, may have wondered why the "twelfth" month is called "December," word rather means "tenth." The questions multiply if we observe that the eleventh is called "November",  related word  to "novem = nine ', the tenth is called" October, related word to " octo” = eight', the ninth is called "September "obviously related word to " septem = seven '.

There is therefore a mismatch between the name of the months and their order on the calendar. This confusion increases if we know that the month of August was called at the time "Sextilis = the sixth"  and June was called "Quintilis = fifth".

The explanation is that on the early history of Rome the year began in March and the year had  ten months. From this premise, the names of the Roman months corresponded with their order.

But the story of the calendar is really complicated and interesting. We will outline some data. When organizing the cycle of time man has in mind the number of days it takes the earth to circle the sun. They are more than 365 days; 365.25 approximately. But it also takes into account the phases of the moon, that we see full of light or on increasing or decreasing phases.

Rome had throughout its history two lunar calendars l and one solar. So somehow coexist lunar calendar and solar. As well the cycles are not measured with pinpoint accuracy, are accumulated errors and diversions on  the annual cycle; the errors become huge. This forced repeatedly to reform the calendar to upgrade to astronomical reality.

According to the mythical history was Romulus, the founder of Rome in 753 BC, who made  also the calendar, lunar  at that time. The year began in March, the month in which also began farming tasks and military campaigns. The year was 304 days in ten months.

It was necessary to reform, that the mythic tradition attributed to King Numa, who ruled from 715 to 673 BC, but probably corresponds to the middle of V century BC. At this time, are added two more months, while previously it was not relevant to form the calendar. The year was 355 days in 12 lunar months. The Pontifex Maximus added  the intercalary month every two years; its duration was fixed at 20 days, to catch up with the so-called "tropical year" or sidereal year, the time it takes the Earth to revolve around the Sun

However, for various reasons there was a important lag. In time of Julius Caesar (100 BC-44 BC) confusion between certain celebrations and the time according to the current schedule was huge.
Caesar was in Egypt with the famous Queen Cleopatra; from her  probably had his son Caesarion. There he met the solar calendar and understood the utility that its use could have. According to Suetonius, advised Sosigenes, philosopher and astronomer of Alexandria, who had set the solar year of 365 days and six hours, that is, with a very small error of less than one second per day. It is this accuracy that still amazes us considering scientific instruments which then possessed.

Caesar made the necessary adjustments and imposed accordingly solar calendar of 365 days a year, inserting an extra day every four years. From the magnitude of reform gives us an idea that in the year 44 BC were inserted  90 days and this year was 455 days.

This calendar, quite logically called "Julian" is basically still used today and has been imposed in most of the world.

Pope Gregory XIII reformed again the calendar by liturgical reasons in 1582, so that Easter would be celebrated on the Sunday following the full moon after the spring equinox, as was ruled by the Council of Nicaea in AD 325. The gap with the tropical year had accumulated approximately 10 days then. That reform is called "Gregorian".

But the mismatch continues to exist because the tropical year actually lasts 365.242189, it is, 365 days, 5 hours, 48 minutes and 45.16 seconds; that is equivalent to about 1 day every 128 years . So continuing need for new reforms.

Perhaps this article certainly seems complex. Not so in reality compared to the many details and clarifications to which the interested reader can get to delve into these issues.

We continue to be governed by the calendar of Julius Caesar

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