Up: Index

The Metonic Cycle and the Saros

Particle Physics and Astronomy Research Council

Royal Greenwich Observatory

Information Leaflet No. 5: 'The Metonic Cycle and the Saros'.
_________________________________________________________

The Metonic Cycle:

The Greek astronomer Meton, in the fifth century BC, discovered that the dates of the phases of the Moon repeated exactly after a period of 19 years.
Mathematically, it uses the fact that 19 tropical years contain 6,939.60 days while 235 synodic months contain 6,939.69 days.

Since it is almost equal to 20 eclipse years, 6,932.4 days, it is possible for a series of four or five eclipses to occur on the same dates 19 years apart.
The metonic cycle was used to determine how intercalary months could be inserted into a lunar calendar so that the calendar year and the tropical (seasonal) year were kept in step.

The Saros Cycle:

Edmund Halley, whose name is associated by most people with the comet carrying his name, was interested in classical writings, especially those concerning astronomy. He mistakenly connected the naming of a cycle of 223 synodic months by the tenth century Greek lexicographer Suidas with the eclipse cycle of the same period. The name given to the cycle by Suidas was the Saros.

This cycle was almost certainly known to the ancient Babylonians and was possibly used by Thales around 585 BC.

Eclipses of the Sun and Moon can only occur at New or Full Moon respectively and these have to occur close to the nodes of the Moon's orbit.
The nodes are the places in the orbit where the plane of the Moon's orbit and the ecliptic cross.
The time between successive passages by the Moon through one of its nodes is called the Draconic month and equals 27.212220 days.
The time between successive New or Full Moons is called the Synodic month and equals 29.530589 days.

If we take 223 synodic months (6,585.321 days) and compare them with 242 draconic months (6,585.357 days) we can see that they are almost the same. This period is the Saros and it amounts to 18 years, 10 and a third days.

This means that eclipses can be expected in families whose members are separated by the length of the Saros. Thus knowing the date of one eclipse allows the prediction of others.

It also happens that the Saros is also nearly equal to 239 anomalistic months (the time between successive closest approaches of the Moon to the Earth) and so the length of the eclipses in each cycle will be approximately the same.

Glossary:

Synodic month: The interval between two successive New Moons.

Draconic month: The interval between two successive passages of the Moon through the same node of its orbit.

Anomalistic month: The time between successive perigee passages of the Moon.

Eclipse year: The period between two successive passages of the Sun through the same node of the Moon's orbit: 346.620 days. There are very close to 19 eclipse years in one Saros.