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In astronomy, an octaeteris (plural: octaeterides) is the period of eight solar years after which the moon phase occurs on the same day of the year plus one or two days.

This period is also in a very good sychronicity with five Venusian visibility cycles (the Venusian synodic period) and thirteen Venusian revolutions around the sun (Venusian sidereal period). This means, that if Venus is visible beside the moon, after eight years the two will be again close together near the same date of the calendar.

Comparison of differing parts of the Octaeteris
Astonomical period Number in an Octaeteris Overall duration (Earth days)
Tropical year 8 2921.93754
Synodic lunar month 99 2923.528230
Sidereal lunar month 107 2923.417787
Venusian synodic period 5 2919.6
Venusian sidereal period 13 2921.07595

The Octaeteris, also known as Oktaeteris, was noted by Cleostratus in ancient Greece as a 2923.5 day cycle. The 8 year short lunisolar cycle was probably known to many ancient cultures. The mathematical proportions of the Octaeteris cycles were noted in Classic Vernal rock art in northeastern Utah by J. Q. Jacobs in 1990. The Three Kings panel also contains more accurate ratios, ratios related to other planets, and apparent astronomic symbolism.

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