The calendar

The day | Week | Month | Year | Other calendars | Calendar reform

All human societies have developed ways to determine the length of the year. These systems – calendars – also dictated when the year should begin, and how to divide the year into manageable units of time. A calendar is a system of reckoning the time over extended intervals by combining days into longer groupings which are linked to the way in which we live. The groupings often have religious significance and some of the groups are linked to astronomical periods.

For our ancestors, estimating the passage of time was essential to everyday life. It was important to be able to predict sunset in order to get to safety – out of reach of enemies and predators who could approach unseen in the dark. Understanding the passing of the seasons was vital for farmers – planting seeds at the wrong time of year or missing a harvest would mean starvation.

It is difficult for many of us to imagine how much more important the cycles of the days, months and seasons must have been for people in the past than today. Most of us never experience the true darkness of night, notice the phases of the Moon or feel the full impact of the seasons.

Our calendar is made up of days, weeks, months and years. The day is the rotational period of the Earth; the week is a purely artificial period linked to the Biblical story of creation; the month is linked to the period of the Moon and the year is linked to the Earth's period of revolution around the Sun.

The day

The definition of the day seems very simple. It should be the time taken for one passage of the Sun across the meridian to its next crossing. Unfortunately the Earth's movement around the Sun is not circular and this causes the time indicated by a sundial to be different from that measured by a clock. (The difference is called the equation of time.)

Nor is the Earth's rotation period constant, although the variations are very small. So we use a length for the day which represents the Earth's average rotation period. Occasionally we have to insert 'leap-seconds' to correct from this exact measure to the slightly irregular rotation of the Earth.

The week

Mosaic law forbidding work on every seventh day established a seven-day period as a measure of Jewish time. This passed over into the Christian church and gradually became established in the Roman calendar. The astrological practice of naming the days using the names of the Sun, Moon and the five known planets also yielded a seven day period.

The names that we give to the days of the week are still based on this nomenclature. In English Sunday and Monday are named after the Sun and Moon while the other days of the week are named after the teutonic versions of the gods that correspond to each planet (with Saturday keeping its Latin connection with Saturn):

  • Tiu - Mars
  • Woden - Mercury
  • Thor - Jupiter
  • Freya - Venus

The month

Many ancient calendars were based on the lunar cycle. The lunar month is 29.530589 days and the number of lunations in a year is 12.368267. The most common scheme was to have 12 months of 30 days with either arbitrary or calculated additional days or months to bring the system into accord with the solar year.

Originally the Roman calendar had 10 months; Martius (named after Mars and indicating the time for resumption of war), Aprilis (after the word for `to open', hence Spring), Maius (after Maia, the goddess of growth), Junius (after juvenis, meaning youth), Quintilius, Sextilis, Septembris, Octobris, Novembris and Decembris (meaning the fifth to tenth months). The Romans had a dislike of even numbers as these symbolized death and so the months had either 29 or 31 days. King Numa increased the number of months to 12 by introducing Januarius (after the god Janus, the protector of doorways and hence the opening of the year) and Februarius (after Februalia, the time for sacrifices at the end of the year). The order of the months was later changed.

In order to reconcile the solar and lunar years, at the recommendation of Sosigenes Julius Caesar altered the lengths of the months and the change to our present format was completed by Caesar Augustus. They both claimed the names of one of the months and this gave rise to Julius and Augustus which replaced Quintilius and Sextilis.

The year

The number of days in one revolution of the Earth around the Sun is 365.2422 days. Unfortunately, this is not a whole number and so if we wish to keep the calendar in track with the seasons we must adopt some form of variable year length.

The first major attempt to accomplish this was by Julius Caesar. Besides pinching a day from the last month of the year (then February) to make `his' month, July, have 31 days he introduced the Julian calendar. February was further despoiled by Augustus Caesar who also purloined a day so as to make `his' month, August, also have 31 days. As some sort of compensation the leap-day was given to February.

The Julian calendar introduced a year of length 365 days with a leap-year every 4 years. It also moved the start of the year from March 25 to January 1. The adopted length of the year, 365.25 days, is only slightly different from the actual length, 365.2422 days, but over the centuries the difference mounts up and by the 16th century had become noticeable. The effect is to move Spring and the date of Easter, which is related to the vernal equinox, closer to the date of Christmas.

The Gregorian reform to the calendar altered the rule for determining if a year should be a leap-year by stating that centenary years should only be leap-years if they were divisible by 400. It also dropped several days from the calendar so that the vernal equinox was brought back to March 21. The mean length of the calendar year is now 365.2425 days and the error compared with the true value amounts to only 3 days in 10,000 years.

Other calendars

The Jewish calendar and the Muslim calendar are intimately connected with the Moon. The Jewish calendar is now a fixed calendar with rather complex rules for its construction. The length of the Jewish year may be 353, 354 or 355 days or 383, 384 or 385 days. Each month has 29 or 30 days.

The Muslim calendar is also a fixed calendar, but the religious festivals depend on visual sightings of the New Moon. The length of the Muslim year is 12 months of alternate lengths 30 and 29 days, except for the 12th month which can have either 29 or 30 days. The calendar is kept in adjustment with the Moon using a cycle of years of different length.

Calendars from around the world

Download Calendars from around the world (PDF, 389KB)

You can click on the chapters in the contents page to jump to a particular section and use the hyperlinks within the e-book.

Find out about the different types of calendars, the different calendars used throughout history and those still in use today. And discover more about the astronomical cycles all calendars are based upon.

Calendar reform

Many attempts have been made to reform the calendar using different ways of dividing up the year so as to give a perpetual calendar. Each suggestion has its advantages but in general these have not been seen to outweigh the disadvantages caused by the changes they would involve.

For anyone with a liking for historic associations the present calendar, for all its apparent complexities is a remarkably good approximation to the non-simple relationship between the periods of the Earth's rotation, its period about the Sun and, to a lesser extent, the period of the Moon.