In astronomy, an octaeteris 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 year) and thirteen Venusian revolutions around the sun (Venusian sidereal year). 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	2921.9376
Synodic lunar month	99	2923.5282
Sidereal lunar month	107	2923.24
Venusian synodic year	5	2919.6
Venusian sidereal year	13	2921.1

Octaeteris, also known as Oktaeteris, was first published by the ancient Greeks. They denoted it as 2923.5 days at best, which is 1.5 days more than 8 Julian calendar years.

See also

• Metonic cycle