Organizing Time - Calendar Systems

A historical timeline

The big units: days, weeks, months

The structures that sort out the passage of time

Whether it starts on the first of January, mid-February or in another month, the year is organized into months, weeks and days.

This system is known as a calendar. The method of ordering the years is also part of that system. How did it all start?

Celestial cycles inspired the creators of the first calendars. All early systems, except the ancient Egyptian one, were lunar. Ancient Egypt used a solar calendar.

The word calendar comes from Latin and the name given in ancient Rome to the first day of the month, calendae, when the appearance of the New Moon was solemnly announced.

The Horae - daylight hour goddesses

In classical antiquity, each hour was ruled by one of twelve goddesses and their tasks. Each goddess had a specific role for the duration of a certain daylight hour.

These deities were called, as a group, the Horai (Ὡραι) in Greek or Horae in Latin. Their task was to assist the Sun (Helios) in his daily passage from sunrise to sunset.

Rather than today's fixed lengths, the hours of the classical world varied in length: shorter in winter to reflect the shorter periods of daylight. Check our special timepiece for the current Hora in several cities.

Did you know?

Different city-states in ancient Greece were observing different calendars. The start of the year would be, for most of them, between autumn and winter. Athens was the exception.
The Athenian or Attic calendar is said to have used the first sighting of a New Moon after the Summer Solstice, to mark the beginning of a new year.
The first month of the year had a name associated in modern vocabulary with great loss, but in ancient Greece it meant "offering of a hundred oxen". Month number one was called Hekatombaion.

From Julian to Gregorian calendar

Calendars have continually been altered and corrected and sometimes completely redefined.

The Western or Gregorian calendar was preceded by the Julian calendar, instituted by Julius Caesar in 46 BC.

The Julian system was adjusted by Pope Gregory XIII in 1582, who eliminated the accumulated error caused by a faulty calculation of the length of a year. Its recurrence was avoided by restricting century leap years to those divisible by 400.

The Gregorian calendar was adopted in Britain and its colonies 170 years later, in 1752. Three September days were skipped: 3 September became 14 September, and at the same time the beginning of the year was moved from 25 March to 1 January.

Russia did not adopt it until the October Revolution of 1917, so the event itself (24-25 Oct in the Julian calendar at the time) is celebrated on 7-8 November.

Jewish calendar

The Jewish calendar is a complex combination of lunar and solar cycles, varied by considerations of religious observance. A year may have 12 or 13 months, each of which normally alternates between 29 and 30 days. The New Year (Rosh Hashanah) falls between 5 Sept and 5 Oct. Read a fuller explanation of the Hebrew calendar system, with today's date shown in Hebrew, transliteration, and English.

Chinese calendar

The Chinese calendar is lunar, with a cycle of 60 years. Both the traditional one and, from 1911, the Western calendar are in use in China. Read a fuller explanation of the Chinese calendar system, with today's date shown in Chinese, pinyin, and English.

Muslim calendar

The Muslim calendar, also lunar, has 12 months of alternately 30 and 29 days, and a year of 354 days. This results in the calendar rotating around the seasons in a 30-year cycle.