astronomy Astronomy () is a natural science that studies astronomical object, celestial objects and phenomena. It uses mathematics, physics, and chemistry in order to explain their origin and chronology of the Universe, evolution. Objects of interest ...

, an octaeteris (, plural: octaeterides) is the period of eight

solar year A tropical year or solar year (or tropical period) is the time that the Sun takes to return to the same position in the sky of a celestial body of the Solar System such as the Earth, completing a full cycle of seasons; for example, the time fro ...

s after which the

moon phase Concerning the lunar month of ~29.53 days as viewed from Earth, the lunar phase or Moon phase is the shape of the Moon's directly sunlit portion, which can be expressed quantitatively using areas or angles, or described qualitatively using the t ...

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 Venus is the second planet from the Sun. It is sometimes called Earth's "sister" or "twin" planet as it is almost as large and has a similar composition. As an interior planet to Earth, Venus (like Mercury) appears in Earth's sky never fa ...

ian visibility cycles (the Venusian

synodic period The orbital period (also revolution period) is the amount of time a given astronomical object takes to complete one orbit around another object. In astronomy, it usually applies to planets or asteroids orbiting the Sun, moons orbiting planets, ...

) and thirteen Venusian revolutions around the sun (Venusian

sidereal period The orbital period (also revolution period) is the amount of time a given astronomical object takes to complete one orbit around another object. In astronomy, it usually applies to planets or asteroids orbiting the Sun, moons orbiting planets, ...

). This means, that if

is visible beside the

moon The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...

, after eight years the two will be again close together near the same date of the

calendar A calendar is a system of organizing days. This is done by giving names to periods of time, typically days, weeks, months and years. A date is the designation of a single and specific day within such a system. A calendar is also a physi ...

. : The octaeteris, also known as ''oktaeteris'', was noted by

Cleostratus Cleostratus ( el, Κλεόστρατος; b. c. 520 BC; d. possibly 432 BC) was an astronomer of ancient Greece. He was a native of Tenedos. He is believed by ancient historians to have introduced the zodiac (beginning with Aries and Sagittarius ...

ancient Greece Ancient Greece ( el, Ἑλλάς, Hellás) was a northeastern Mediterranean civilization, existing from the Greek Dark Ages of the 12th–9th centuries BC to the end of classical antiquity ( AD 600), that comprised a loose collection of cult ...

as a day cycle. The octaeteris is the calendar used for the

Olympic games The modern Olympic Games or Olympics (french: link=no, Jeux olympiques) are the leading international sporting events featuring summer and winter sports competitions in which thousands of athletes from around the world participate in a var ...

; if one

Olympiad An olympiad ( el, Ὀλυμπιάς, ''Olympiás'') is a period of four years, particularly those associated with the ancient and modern Olympic Games. Although the ancient Olympics were established during Greece's Archaic Era, it was not until ...

was 50 months long, the next would be 49 lunar months long. This octaeteris calendar is used for the Olympic dial of the

Antikythera mechanism The Antikythera mechanism ( ) is an Ancient Greece, Ancient Greek hand-powered orrery, described as the oldest example of an analogue computer used to predict astronomy, astronomical positions and eclipses decades in advance. It could also be ...

, to determine the time of the Olympic games and other Greek festivities. The 8 year short lunisolar cycle was probably known to many ancient cultures. The mathematical proportions of the octaeteris cycles were The Three Kings panel also contains more accurate ratios, ratios related to other planets, and apparent astronomical symbolism.

References

* {{Greek astronomy Ancient Greek astronomy Calendars Time in astronomy

See also

References