Geographical Shifts In Eclipse Paths

Apart from regularly repeating in time according to the saros, another effect associated with that 18.03-year cycle in eclipses is a consistent shift in the geographical location of the track of totality. There is both a small step in latitude (the north—south direction) and a larger shift in geographical longitude (the east—west direction). Let us examine the origin of these shifts.

The saros actually lasts for 6,585.32 days. Knocking off the integer number of days, there is an excess of just less than one-third of a day, representing almost one-third of a rotation of the planet. In terms of time, it is equivalent to 7 hours and 41 minutes; in terms of geographical longitude, it means that the eclipse track is shifted by about 115 degrees to the west from one saros to the next.

The latitudinal offset occurs because after one saros has elapsed the Moon takes a slightly different path across the face of the Sun. This is explained in more detail in the Appendix. From one eclipse to the next in any saronic cycle, the step in latitude is about four degrees. This may either be northwards, or southwards, depending on whether the Moon is at its descending or ascending node (passing through the ecliptic moving either south or north). These two effects (north—south and east—west shifts of eclipse tracks) are illustrated in Figure 2-2, with six twentieth-century total solar eclipses in a sequence known as saros 136 delineated. After each 18.03-year gap there is a consistent offset in the path of totality: westwards by 115 degrees, northwards by 4 degrees.

Although the tracks move from east to west from one saronic cycle to the next, the actual path followed by the lunar shadow traces across the Earth from west to east, because the Moon is

FIGURE 2-2. The six long-duration eclipses of the twentieth century were all members of the same saronic cycle. After each gap of 18.03 years, another seven-minute eclipse occurred, displaced westwards by 115 degrees and northwards by 4 degrees. From the onset of each of these tracks to its end took about five hours; that is, totality occurred at quite different times in separated locations.

FIGURE 2-2. The six long-duration eclipses of the twentieth century were all members of the same saronic cycle. After each gap of 18.03 years, another seven-minute eclipse occurred, displaced westwards by 115 degrees and northwards by 4 degrees. From the onset of each of these tracks to its end took about five hours; that is, totality occurred at quite different times in separated locations.

overtaking the Sun in the sky. For example, the eclipse of 1991 shown in Figure 2-2 started to the southwest of Hawaii, crossed the eastern Pacific, passed over Mexico and other parts of Central America, and finished over Brazil.

I have just mentioned that these were all total eclipses, which might seem unexpected: would not a mix of total, annular, and partial eclipses be anticipated? The answer is NO. The reasons for this are explored in the Appendix, but the pertinent point here is that the basic characteristics of the six eclipses in Figure 2-2 repeated; they did not comprise a random hotchpotch of partial, annular, and total eclipses. Indeed this is a particularly prominent sequence as they had the longest periods of totality (six or seven minutes) of any solar eclipses in recent centuries. Obviously something systematic happened, and this is another fundamental quality of any saros sequence. Again, we delve into this in the Appendix.

A final note on the sequence shown in Figure 2-2: it is not finished yet, with the next members being due on July 22, 2009 and August 2, 2027, each lasting for about six and a half minutes (the lengths are decreasing from a peak in 1955). It should be easy enough to extrapolate from that diagram and work out the eclipse tracks in those years, in case you want to make travel plans: the 2009 path will cross eastern Asia, while in 2027 northern Africa will be the place to be.

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