The Lunar Calendar & Metonic Cycle

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  • 0:02 The Lunar Phase Cycle
  • 0:37 The Lunar Calendar
  • 2:23 The Metonic Cycle
  • 3:24 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov

Artem has a doctor of veterinary medicine degree.

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. 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.

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