This called intercalation to rectify any differences.

This lesson will discuss the lunar calendar, luni-solar calendar, the Metonic cycle, and how they relate to keeping time in the ancient world and even some calendars today.

The Lunar Phase Cycle

As another lesson points out, the lunar cycle was an ancient way of keeping track of time. A lunation, also known as one lunar phase cycle, a synodic month, is completed approximately every 29.53 days.

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It is believed that the lunar phase cycle may have been observed and kept track of as far back as 30,000 years ago! Even today, the lunar phase cycle is still used by the traditional Jewish and Islamic calendars. This lunar calendar, as well as something known as the Metonic cycle, will be the subject of our lesson.

The Lunar Calendar

In most lunar calendars, the new month starts after the new moon, with the thin crescent’s first appearance after sunset. This event was likely chosen for the start of a new month because it was easily observable and recognizable.

But such a definable event is obviously not without its problems, such as cloudy skies on and around such a day.The lunar calendar is also not without its issues as a whole, independent of any clouds, storms, and the thunderous wrath of Zeus. If an attempt is made to match the lunar calendar with the seasonal year, then it can drift out of step with the seasons. But some lunar calendars, like the Islamic calendar, go on about their cycle independently of the seasonal year.But other lunar calendars do take into account the seasonal (or solar) year. Such calendars are known as luni-solar calendars. Since I just said that the lunar calendar and seasonal year don’t match up very well, the luni-solar calendars use a process called intercalation to rectify any differences.

Intercalation is the periodic subtraction or addition of a month, and such a month is called an intercalary month.But, how do you determine when to add or subtract an intercalary month? Many civilizations had their own answers based on astronomical observations that are linked to the seasonal year. As an example, ancient Greeks linked the appearance or disappearance of certain stars with seasonal developments, like stormy seas, at least as far back as the 8th century B.

C. They even made star calendars, called parapegmata, which were used as reference points in order to keep the lunar calendar aligned with, and adjusted to, the seasonal year.

The Metonic Cycle

In the 5th century BC, another method of intercalation came to the forefront in ancient Greece.

An ancient Greek mathematician and astronomer, called Meton, propagated this method, one that came to be known as the Metonic cycle. It actually had its roots in a very old Babylonian tradition that not only used precise astronomical observations but also some intricate arithmetic.Regardless, what Meton observed was that if you periodically added seven intercalary months every 19 years, you’d keep the seasonal year pretty much aligned with the lunar calendar. It wasn’t perfect, but it was remarkable enough, such that the error between the two amounted to just over a day in 100 years! The significance of the Metonic cycle went beyond what I just said; it also paved the way for future generations to use mechanical, mathematical, and logical rules to understand nature and time.

Lesson Summary

A lunation, also known as one lunar phase cycle, a synodic month, is completed approximately every 29.

53 days. Lunar calendars were some of the earliest ways by which to measure time in the ancient world. But if lunar calendars were attempted to be matched with the seasonal year, they would drift out of step.Lunar calendars that take into account the seasonal (or solar) year are known as luni-solar calendars.

To keep the two aligned as best as possible, a process called intercalation was used to rectify any differences. Intercalation is the periodic subtraction or addition of a month, and such a month is called an intercalary month. One such method that used intercalary months, called the Metonic cycle, was propagated by an ancient Greek mathematician and astronomer, called Meton.

Learning Outcomes

Now that you are finished with this lesson, you can achieve these goals:

  • Describe lunation and the drawbacks of the lunar calendar
  • Discuss luni-solar calendars and discuss understand how they could be modified to follow the seasons
  • Recall the significance of the metonic cycle
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