Measuring Time by Days and Months

Moon Cycles, Solar Years and Seasonal Rhythms

There are many lunar calendars in operation. For a selection of Lunar New Years click here.

We do not know how the concept of measuring time was first grasped, but we can see that early calendars are based on natural cycles. These could be:

  • daily sunrise
  • seasons
  • human lifespans
  • animal or plant behaviour
  • the changing path of the sun and the stars
  • the phases of the moon.

Three cycles which were constant across the inhabited globe were:

  • the daily sunrise/sunset
  • the phases of the moon
  • the female menstrual cycle

The moon’s cycle (lunation) at about 29 days was short enough to record accurately. Many early societies adopted this as a unit of time, known now as a synodic month.

The synodic month enabled societies to plan and reflect more effectively.

Full moon photograph telephoto
"Blood Moon" September 2015
© Dan Cotruta

Measuring Time by Solar Years

Early people saw that the sun’s path changes but it seems to return to its starting point every 365 days. This long cycle became the solar year.

Sky watchers observed that the sun’s path crossed the equinox twice in each cycle.  At the times when the sun rose and set on the equator there was equal day and night for the whole planet (equinox). These days of equinox became the fixing points for calendar makers.

The sun’s cycles were clearly connected to changing seasons whether from dry to wet in the tropics or warm to cold in higher latitudes.  A solar calendar was therefore very useful to farming societies to plan and record annual events. And of course it was useful for tax collectors too!

It is important for amateur historians to note that the official start of the solar year changed through time and counting from January is a relatively recent development.

From Months to Lunar Years

People came up with three main types of lunar calendar:

The lunisidereal calendar tied the lunar year to the position of stars. This system used by astronomers in ancient India underlies the Hindu lunar calendars. Modern Hindu lunar calendars tend to use the lunisolar system.

The pure lunar calendar has twelve synodic months. The average length of a pure lunar year is 354 days – eleven days shorter than the solar year. Thus lunar years change faster than solar years so there are more lunar than solar years within a span of time. An example is the Islamic Calendar which reached 1437 in the same time as 1394 solar calendar years.

The lunisolar calendar reconciles the lunar year with the solar year by inserting a thirteenth month every few years. The Chinese Lunar Calendar is lunisolar as are the Jewish Calendars. These calendars are mainly kept now to mark religious years and traditional festivals. There are several different starting points for Lunar New Years as you can see from our Moon Time.

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