Editing Octaeteris

{{Short description|Astronomical cycle of eight solar years}}
In [[astronomy]], an '''octaeteris''' ({{Langx|el|ὀκταετηρίς}}, plural: '''octaeterides''') is the period of eight [[solar year]]s 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 synchronicity with five [[Venus]]ian 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]].

:{| class="wikitable"
|+ Comparison of differing parts of the octaeteris
! Astronomical period
! Number in an<br/>octaeteris
! Overall duration<br/>([[Earth]] days)
|-
| [[Tropical year]] 
|align=right|  8 &emsp; &emsp; 
| &nbsp; 2 921.93754
|-
| [[Synodic month|Synodic lunar month]] 
|align=right|  99 &emsp; &emsp; 
| &nbsp; 2 923.528230 
|-
| [[Sidereal month|Sidereal lunar month]] 
|align=right|  107 &emsp; &emsp; 
| &nbsp; 2 923.417787
|-
| [[Venus]]ian [[synodic period]] 
|align=right|  5 &emsp; &emsp; 
| &nbsp; 2 919.6
|-
| Venusian [[sidereal period]]
|align=right|  13 &emsp; &emsp; 
| &nbsp; 2 921.07595
|}

The octaeteris, also known as ''oktaeteris'', was noted by [[Cleostratus]] in [[ancient Greece]] as a {{sfrac|2 923| 1 |2}} day cycle. The octaeteris is the calendar used for the [[Ancient Olympic Games|Olympic games]]; if one [[Olympiad]] was 50&nbsp;months long, the next would be 49&nbsp;lunar months long. This octaeteris calendar is used for the Olympic dial of the [[Antikythera mechanism]], to determine the time of the Olympic games and other Greek festivities.

The 8&nbsp;year short lunisolar cycle was probably known to many ancient cultures. The mathematical proportions of the octaeteris cycles were {{Citation needed span| noted in Classic Vernal rock art in northeastern [[Utah]] by J.Q. Jacobs in 1990.|date=January 2020}} The Three Kings panel also contains more accurate ratios, ratios related to other planets, and apparent astronomical symbolism{{clarify|date=January 2020}}.

== See also ==
*[[Metonic cycle]]
* [[Eclipse cycle]]

== References ==
* {{cite book |title=Mathematical Astronomy Morsels |first=Jean |last=Meeus |author-link=Jean Meeus |publisher=Willmann-Bell |year=1997 |at=Chapter&nbsp;9, p.&nbsp;51, Table&nbsp;9.A "Some eclipse Periodicities" }}

{{Greek astronomy}}

[[Category:Ancient Greek astronomy]]
[[Category:Calendars]]
[[Category:Time in astronomy]]