Eclipse Cycles

 

           Eclipses occur in a long series called the saros, and are about 18 years apart from one another. The word saros originated as a Babylonian word meaning ‘repetitive’. Saros has become a term that is applicable to both lunar and solar cycles of the 18-year duration. The saros lasts exactly 223 synodic months. Each series of eclipse is separated by an 18 year 11-1/3 cycle. This series of eclipses is known as a saros series. The saros also coincides closely with 19 eclipse years. Eclipses occur repeatedly, separated by a specific interval of time. This interval is called an eclipse cycle. The repetition of eclipses tends to follow a very regular pattern.

 

           Both the Sun and the Moon subtend significant angles. This means that for an eclipse to occur, they do not need to be exactly at the nodes along the Line of Nodes. In addition, an observer's position on the surface of the Earth introduces a sizable parallax of 2° in ecliptic latitude. These factors, combined with the fact that the Sun is within 18.5° of a node, make a solar eclipse possible. The Sun travels about 1° per day along the ecliptic, requiring around 37 days to travel across through the eclipse zone centred on each node. As the New Moon takes place about every 29- 1/2 day, at least one eclipse can occur in the process of each of the Sun’s node crossings. . The period during which the Sun is near a node is called an eclipse season. There are two eclipse seasons in a year.

         If the line of nodes were fixed in space, eclipse seasons will take place at the same time each year, six months apart from the previous. In actual fact, the line of nodes drifts westward slowly at a rate of about 19 degrees a year. Thus, eclipse seasons will occur every 173.3 days and two eclipse seasons will constitute to an eclipse year of 346.6 days, which is 18.6 days less than a solar year but equal to the time required for the Sun to cross the same node 2 times. In order to find a periodicity in the mechanics of solar eclipses, there must be commensurability between the synodic month and the eclipse year. Fortunately, 19 eclipse years are almost exactly equal to 223 synodic months; they differ by only 11 hours. The coincidence is all the more remarkable when compared to a period known as the anomalistic month. This is also the time required for the Moon to pass from perigee to perigee in approximately 27 1/2 days. The anomalistic month is important because the Moon's geocentric distance is the main factor that determines the annular or total nature of a solar eclipse. 239 anomalistic months are also equal to 223 synodic months to within 6 hours. This is the origin of the famous Saros cycle of 6585 1/3 days or 18 years. 11 days and 8 hours.

          Any two eclipses separated by one Saros cycle share very similar mechanical characteristics. They occur at the same node with the Moon at the same distance from Earth and at the same time of year. Because the Saros does not contain an integral number of days, its biggest drawback is that subsequent eclipses are visible from different parts of the globe. Although the 1/3 day displacement shifts the eclipse path 120° westward with each cycle, the series goes back to the same geographic region every 3 Saroses or 56 years and 34 days.

            A Saros series does not last indefinitely because the various periods are not perfectly commensurate with one another. Particularly, 19 eclipse years are 1/2 day longer than the Saros. This results in the node shifting eastward by about 0.5° with each cycle. A typical Saros series commences when New Moon takes place at about 18 degrees east of a node. If the first eclipse occurs at the Moon's descending node, the Moon's umbral shadow will pass 3500 km below the Earth resulting in a partial eclipse that may be visible from the South Polar region. On the following return, the umbra will pass about 300 km closer to the Earth and a partial eclipse of slightly larger magnitude will result. After ten or eleven Saros cycles (about 200 years), the first central eclipse will occur near the south pole of the Earth. Over the course of the next 950 years, a central eclipse will occur at each Saros but will be displaced northward by an average of 300 km. Halfway through this period, eclipses of long duration will occur near the equator. The last central eclipse of the series will occur near the North Pole. The next ten eclipses will be partial with successively smaller magnitudes. Finally, the Saros series will end some 13 centuries after it began at the opposite pole. A typical series may comprise of 70 to 80 eclipses, among which about 50 are central.

           If a Saros series begins near the ascending node, the first eclipse will be partial from the northern polar region and the previous sequence of events is reversed. Since at least two solar eclipses occur every year, there are obviously many different Saros series in progress simultaneously. For instance, during the later half of the twentieth century, there are 41 individual series and 26 of them are producing central eclipses. As old series terminate, new ones are always beginning and take their places. For instance, the total solar eclipses of 1925, 1943,1961, 1979, 1997, 2015 and 2033 are all members of Saros 120. The series began with a partial eclipse at the South Pole in 915 AD. The 2033 event is the last central eclipse of the series. Note that the paths of the last four eclipses grow progressively broader as the umbral shadow cone passes closer to the limb of the Earth. The next eclipse in the series will be a partial eclipse in 2051. Saros 120 will end with a partial eclipse near the North Pole in 2195.

 Back