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It's All About Cycles
The Saros cycle is a cycle of eclipses that repeat themselves. An eclipse is an alignment of the Sun, Moon and the Earth in a straight line. For a solar eclipse to occur, there must be a new moon. For a lunar eclipse, the Moon must be a full moon.
The Moon has ascending or descending nodes. Nodes are where the path of the Sun and the Moon cross in the ecliptic. The ascending node is where the path of the Moon crosses north of the ecliptic, and the descending node is where it crosses south of the ecliptic.
A Saros cycle of eclipses occurs every 18 years 11 1/3 days. The cycle is used to predict eclipses. Any two eclipses that are separated by a Saros cycle will share the same geometry, which means they occur at the same node with the Moon. Further, the distance from the Earth is also the same, as well as the time of year. In order for an eclipse to occur, the Moon must pass through one its nodes.
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When Do Eclipses Repeat Themselves?
The Moon's orbital period around the Earth has certain periodicities from which the Saros cycle originates:
- The first is the synodic month, which is the time the Moon takes from one New Moon to another. It is equal to 29.530589 days.
- The second is the anomalistic month, which is the time from one perigee (closest approach of the Moon to the Earth) to another. It is equal to 27.55 days.
- The third is the draconic month, which is the time the Moon takes from node to node and is equal to 27.21 days.
A Saros cycle is equal to 223 synodic months, 239 anomalistic months and 242 draconic months. Following one cycle, the relative geometry of the Sun, Moon and the Earth will have exactly the same geometry. This means that the distance of the Moon from the Earth, the node where it is at and the phase of the Moon will be same. Knowing when an eclipse appears on a given date will allow one to calculate an identical eclipse exactly one Saros cycle later.
The conjunction of the Sun and the Moon shifts by about 0.5 degrees with respect to the nodes of the Moon each cycle. This results in a Saros series, which is a series of eclipses that gradually appear to change. The additional 1/3 day in the cycle translates to the Earth rotating an additional 8 hours or approximately 120 degrees each cycle. Due to this, solar eclipses shift 120 degrees westward with each successive cycle. Hence, a Saros Series will return to the exact same geographic position 3 saroses or 54 years and 34 days later.
A cycle starts with a partial eclipse and in the cycles that follow, the path of the Moon is shifted either northward (close to the descending node) or southward (close to the ascending node). Since the three lunar months are not equal to one another, it results in the termination of the eclipses and the series. A Saros series will last anywhere between 1226 to 1550 years. This is the time it takes the members to traverse the surface of the Earth from north to south or from south to north. There are usually between 70 to 80 eclipses in one series. Once a series comes to an end a new one begins.
Odd numbers are given to solar eclipses occurring near the Moon's ascending node and even numbers to those occurring near the descending mode. Between 2 and 5 solar eclipses occur each year, which results in some 40 different Saros series that are ongoing at any given time.
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The History of Documenting Cycles of Eclipses
The Saros cycle was first discovered by ancient Babylonian astronomers. Among the Romans, Pliny the Elder made a mention of the cycle but with a different name. Similarly the Greek astronomer Hipparchus also mentioned the cycle. The Babylonia word "sar" was a unit of measurement equal to 3600. However, in 1691 the British astronomer applied the word to mean the Saros cycle, which he had derived from a Byzantine dictionary dating back to the 11th century.