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.