Geographical Shifts In Eclipse Paths

So far we have concentrated on the spacing in time of eclipses. Next we consider some other characteristics. Flick back to Figure A-10. Imagine that saros A produced a total solar eclipse, so that the right-hand of the pair of disks may be thought of as equally well representing the Sun. Now think of the position of the Moon as it passed that position in saros B; that is, you slide it back down its inclined path until the two are aligned north—south, putting them at the same celestial longitude. In that position the center of the Moon is a little below that of the Sun, and so a total solar eclipse may still be witnessed in saros B, but its track on the Earth's surface will be displaced south from that which occurred 18.03 years earlier in saros A.

That is one distinct trend in eclipse occurrence representing a latitudinal shift. There will also tend to be an associated small shift in geographical longitude of the eclipse track because the terrestrial spin axis is tilted. However, there is another, larger, longitudinal shift, with a different origin. This was previously mentioned in Chapter 2.

The saros lasts for 6,585.32 days, indicating an excess ofjust less than one-third of a day over a round number of days. That represents almost a third of a rotation of the planet, the equivalent to 7 hours and 41 minutes. This means that the eclipse track is shifted by about 115 degrees to the west from one saros to the next.

These shifts—both north—south and east—west—were illus trated in Figure 2-2. Although the longitude movement is always from east to west, the latitudinal motion may be either from north to south or vice versa. In Figure 2-2, which is for total solar eclipses in saros sequence 136 during the twentieth century, the movement was from south to north because that sequence pertains to the descending node of the Moon's orbit. Other saronic sequences, associated with the ascending node (as in Figure A-10), demonstrate similar four-degree jumps, but from north to south.

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